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matf Basic Aeronautical Knowledge - AviationEnglish.com

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Basic Aeronautical Knowledge

This section of Learning Zone is dedicated to free resources to help learners prepare for Basic Aeronautical Knowledge exams.  The material is adapted from the widely available FAA Handbook of Aeronautical Knowledge.

The trick is to find some kind of systematic approach and concentrate on the important material. The material is divided into 5 broad categories:

1) aircraft - aircraft components, systems and principles of flight
2) airmanship - aircraft performance and human performance
3) meteorology - general principles of weather formation
4) navigation - navigation methods, pilotage, dead reckoning and aircraft navigation systems
5) air law and regulations

On each of these pages you will find a short overview of the area and links into more detailed discussion of the topics that are part of the areas. At the end of the knowledge area pages you will find a link to the appropriate question banks. 

According to the aeronautics dictionary, airmanship is the combination of knowledge, skills and attitude required to operate the aircraft safely in all foreseeable situations. So, being a private pilot is basically all about airmanship.

In order to fly safely the pilot has to know about limitations to the performance of the aircraft in a number of flight situations as well as about the so called human factor - the physical and psychological effects that occur in flight and the corresponding limits to the performance of the private pilot.

The following pages cover the area of knowledge about flight planning and flight safety including weight and balance calculations and aircraft checklists as well as knowledge about the human performance factor.

Load factors describe an aircrafts operating limits. These limits and how they apply to different flight situations belong to a concept of which the pilot should have a clear understanding.

Compliance with the weight and balance limits of any airplane is critical to flight safety. The following page will provide background information into some of the reasons why weight and balance conditions are important to the safe flight of an airplane and give a basic understanding of how to do weight and balance calculations.

Aircraft Performance: This page discusses the factors that affect aircraft performance, which includes the airplane weight, atmospheric conditions, runway environment, and the fundamental physical laws governing the forces acting on an airplane.

Flight planning is at center stage of good airmanship. This page compiles the main steps in flight planning.

Aeronautical Decision Making (ADM) is a systematic approach to the mental process used by airplane pilots to consistently determine the best course of action in response to a given set of circumstances.

Aeromedical Factors As a pilot, it is important to stay aware of the mental and physical standards required for the type of flying done. This page provides information on medical certification and on aeromedical factors related to flying activities.

 Electrical system

 Airplanes are equipped with either a 14- or 28-volt direct-current electrical system. A basic airplane electrical system consists of the following components:

  • Alternator/generator
  • Battery
  • Master/battery switch
  • Alternator/generator switch
  • Bus bar, fuses, and circuit breakers
  • Voltage regulator
  • Ammeter/loadmeter 
  • Associated electrical wiring

Engine-driven alternators or generators supply electric current to the electrical system. They also maintain a sufficient electrical charge in the battery. Electrical energy stored in a battery provides a source of electrical power for starting the engine and a limited supply of electrical power for use in the event the alternator or generator fails.

Most direct current generators will not produce a sufficient amount of electrical current at low engine r.p.m. to operate the entire electrical system. Therefore, during operations at low engine r.p.m., the electrical needs must be drawn from the battery, which can quickly be depleted.

Alternators have several advantages over generators.

Alternators produce sufficient current to operate the entire electrical system, even at slower engine speeds, by producing alternating current, which is converted to direct current. The electrical output of an alternator is more constant throughout a wide range of engine speeds.

Some airplanes have receptacles to which an external ground power unit (GPU) may be connected to provide electrical energy for starting. These are very useful, especially during cold weather starting. Follow the manufacturers recommendations for engine starting using a GPU.

The electrical system is turned on or off with a master switch. Turning the master switch to the ON position provides electrical energy to all the electrical equipment circuits with the exception of the ignition system. Equipment that commonly uses the electrical system for its source of energy includes:

  • Position lights 
  • Anticollision lights
  • Landing lights
  • Taxi lights
  • Interior cabin lights
  • Instrument lights
  • Radio equipment
  • Turn indicator
  • Fuel gauges
  • Electric fuel pump
  • Stall warning system
  • Pitot heat
  • Starting motor

Many airplanes are equipped with a battery switch that controls the electrical power to the airplane in a manner similar to the master switch. In addition, an alternator switch is installed which permits the pilot to exclude the alternator from the electrical system in the event of alternator failure.

 

master-switch Basic Aeronautical Knowledge - AviationEnglish.com

Figure 1: On this master switch, the left half is for the alternator and the right half is for the battery.

With the alternator half of the switch in the OFF position, the entire electrical load is placed on the battery. Therefore, all nonessential electrical equipment should be turned off to conserve battery power.

A bus bar is used as a terminal in the airplane electrical system to connect the main electrical system to the equipment using electricity as a source of power. This simplifies the wiring system and provides a common point from which voltage can be distributed throughout the system.

 

electrical-system-schematic Basic Aeronautical Knowledge - AviationEnglish.com

Figure 2: Electrical system schematic.

Fuses or circuit breakers are used in the electrical system to protect the circuits and equipment from electrical overload. Spare fuses of the proper amperage limit should be carried in the airplane to replace defective or blown fuses. Circuit breakers have the same function as a fuse but can be manually reset, rather than replaced, if an overload condition occurs in the electrical system.

Placards at the fuse or circuit breaker panel identify the circuit by name and show the amperage limit.

An ammeter is used to monitor the performance of the airplane electrical system. The ammeter shows if the alternator/generator is producing an adequate supply of electrical power. It also indicates whether or not the battery is receiving an electrical charge.

Ammeters are designed with the zero point in the center of the face and a negative or positive indication on either side. 

 

ammeter-loadmeter Basic Aeronautical Knowledge - AviationEnglish.com

Figure 3: Ammeter and loadmeter. 

When the pointer of the ammeter on the left is on the plus side, it shows the charging rate of the battery. A minus indication means more current is being drawn from the battery than is being replaced. A full-scale minus deflection indicates a malfunction of the alternator/generator. A full-scale positive deflection indicates a malfunction of the regulator. In either case, consult the AFM or POH for appropriate action to be taken.

Not all airplanes are equipped with an ammeter. Some have a warning light that, when lighted, indicates a discharge in the system as a generator/alternator malfunction. Refer to the AFM or POH for appropriate action to be taken.

Another electrical monitoring indicator is a loadmeter. This type of gauge, illustrated on the right in figure 3, has a scale beginning with zero and shows the load being placed on the alternator/generator. The loadmeter reflects the total percentage of the load placed on the generating capacity of the electrical system by the electrical accessories and battery. When all electrical components are turned off, it reflects only the amount of charging current demanded by the battery.

A voltage regulator controls the rate of charge to the battery by stabilizing the generator or alternator electrical output. The generator/alternator voltage output should be higher than the battery voltage. For example, a 12-volt battery would be fed by a generator/alternator system of approximately 14 volts.The difference in voltage keeps the battery charged.

 

 

 

Hydraulic systems

 

 

 

There are multiple applications for hydraulic use in airplanes, depending on the complexity of the airplane.

 

For example, hydraulics are often used on small airplanes to operate wheel brakes, retractable landing gear, and some constant-speed propellers. On large airplanes, hydraulics are used for flight control surfaces, wing flaps, spoilers, and other systems.

 

A basic hydraulic system consists of a reservoir, pump (either hand, electric, or engine driven), a filter to keep the fluid clean, selector valve to control the direction of flow, relief valve to relieve excess pressure, and an actuator.

 

The hydraulic fluid is pumped through the system to an actuator or servo. Servos can be either single-acting or double-acting servos based on the needs of the system.

 

This means that the fluid can be applied to one or both sides of the servo, depending on the servo type, and therefore provides power in one direction with a single-acting servo. A servo is a cylinder with a piston inside that turns fluid power into work and creates the power needed to move an aircraft system or flight control. The selector valve allows the fluid direction to be controlled. This is necessary for operations like the extension and retraction of landing gear where the fluid must work in two different directions. The relief valve provides an outlet for the system in the event of excessive fluid pressure in the system. Each system incorporates different components to meet the individual needs of different aircraft.

 

A mineral-based fluid is the most widely used type for small airplanes. This type of hydraulic fluid, which is a kerosene-like petroleum product, has good lubricating properties, as well as additives to inhibit foaming and prevent the formation of corrosion. It is quite stable chemically, has very little viscosity change with temperature, and is dyed for identification. Since several types of hydraulic fluids are commonly used, make sure your airplane is serviced with the type specified by the manufacturer. Refer to the AFM, POH, or the Maintenance Manual.

 

 

 

hydraulic-system Basic Aeronautical Knowledge - AviationEnglish.com

Figure 4: Basic hydraulic system.

 

 

 

Landing gear

 

 

 

The landing gear forms the principal support of the airplane on the surface. The most common type of landing gear consists of wheels, but airplanes can also be equipped with floats for water operations, or skis for landing on snow.

 

 

 

landing-gear-supports Basic Aeronautical Knowledge - AviationEnglish.com

Figure 5: The landing gear supports the airplane during the takeoff run, landing, taxiing, and when parked.

 

 

 

The landing gear on small airplanes consists of three wheels - two main wheels, one located on each side of the fuselage, and a third wheel, positioned either at the front or rear of the airplane. Landing gear employing a rear-mounted wheel is called a conventional landing gear. Airplanes with conventional landing gear are often referred to as tailwheel airplanes. When the third wheel is located on the nose, it is called a nosewheel, and the design is referred to as a tricycle gear. A steerable nosewheel or tailwheel permits the airplane to be controlled throughout all operations while on the ground.

 

 

 

Tricycle landing gear airplanes

 

A tricycle gear airplane has three main advantages:

 

  1. It allows more forceful application of the brakes during landings at high speeds without resulting in the airplane nosing over.

     

  2. It permits better forward visibility for the pilot during takeoff, landing, and taxiing.

     

  3. It tends to prevent ground looping (swerving) by providing more directional stability during ground operation since the airplane´s center of gravity (CG) is forward of the main wheels. The forward CG, therefore, tends to keep the airplane moving forward in a straight line rather than ground looping.

     

Nosewheels are either steerable or castering. Steerable nosewheels are linked to the rudders by cables or rods, while castering nosewheels are free to swivel. In both cases, you steer the airplane using the rudder pedals.

 

However, airplanes with a castering nosewheel may require you to combine the use of the rudder pedals with independent use of the brakes.

 

 

 

Tailwheel landing gear airplanes

 

On tailwheel airplanes, two main wheels, which are attached to the airframe ahead of its center of gravity, support most of the weight of the structure, while a tailwheel at the very back of the fuselage provides a third point of support. This arrangement allows adequate ground clearance for a larger propeller and is more desirable for operations on unimproved fields.

 

 

 

tailwheel-landing-gear Basic Aeronautical Knowledge - AviationEnglish.com

Figure 6: Tailwheel landing gear.

 

 

 

The main drawback with the tailwheel landing gear is that the center of gravity is behind the main gear. This makes directional control more difficult while on the ground. If you allow the airplane to swerve while rolling on the ground at a speed below that at which the rudder has sufficient control, the center of gravity will attempt to get ahead of the main gear. This may cause the airplane to ground loop.

 

Another disadvantage for tailwheel airplanes is the lack of good forward visibility when the tailwheel is on or near the surface. Because of the associated hazards, specific training is required in tailwheel airplanes.

 

 

 

Fixed and retractable landing gear

 

Landing gear can also be classified as either fixed or retractable. A fixed gear always remains extended and has the advantage of simplicity combined with low maintenance. A retractable gear is designed to streamline the airplane by allowing the landing gear to be stowed inside the structure during cruising flight.

 

 

 

fixed-retractable-gear Basic Aeronautical Knowledge - AviationEnglish.com

Figure 7: Fixed and retractable gear airplanes.

 

 

 

Brakes

 

Airplane brakes are located on the main wheels and are applied by either a hand control or by foot pedals (toe or heel). Foot pedals operate independently and allow for differential braking. During ground operations, differential braking can supplement nosewheel/tailwheel steering.

 

 

 

Autopilot

 

 

 

Autopilots are designed to control the aircraft and help reduce the pilot´s workload. The limitations of the autopilot depend on the complexity of the system. The common features available on an autopilot are altitude and heading hold. More advanced systems may include a vertical speed and/or indicated airspeed hold mode. Most autopilot systems are coupled to navigational aids.

 

An autopilot system consists of servos that actuate the flight controls. The number and location of these servos depends on the complexity of the system. For example, a single-axis autopilot controls the aircraft about the longitudinal axis and a servo actuates the ailerons. A three-axis autopilot controls the aircraft about the longitudinal, lateral, and vertical axes; and three different servos actuate the ailerons, the elevator, and the rudder.

 

The autopilot system also incorporates a disconnect safety feature to automatically or manually disengage the system. Autopilots can also be manually overridden. Because autopilot systems differ widely in their operation, refer to the autopilot operating instructions in the AFM or POH.

 

 

 

Ice control systems

 

 

 

Ice control systems installed on aircraft consist of anti-ice and de-ice equipment. Anti-icing equipment is designed to prevent the formation of ice, while de-icing equipment is designed to remove ice once it has formed. Ice control systems protect the leading edge of wing and tail surfaces, pitot and static port openings, fuel tank vents, stall warning devices, windshields, and propeller blades. Ice detection lighting may also be installed on some airplanes to determine the extent of structural icing during night flights. Since many airplanes are not certified for flight in icing conditions, refer to the AFM or POH for details.

 

 

 

Airfoil ice control

 

Inflatable de-icing boots consist of a rubber sheet bonded to the leading edge of the airfoil. When ice builds up on the leading edge, an engine-driven pneumatic pump inflates the rubber boots. Some turboprop aircraft divert engine bleed air to the wing to inflate the rubber boots. Upon inflation, the ice is cracked and should fall off the leading edge of the wing. De-icing boots are controlled from the cockpit by a switch and can be operated in a single cycle or allowed to cycle at automatic, timed intervals. It is important that de-icing boots are used in accordance with the manufacturer´s recommendations. If they are allowed to cycle too often, ice can form over the contour of the boot and render the boots ineffective.

 

 

 

de-icing-boots Basic Aeronautical Knowledge - AviationEnglish.com

Figure 8: De-icing boots on the leading edge of the wing.

 

 

 

Many de-icing boot systems use the instrument system suction gauge and a pneumatic pressure gauge to indicate proper boot operation. These gauges have range markings that indicate the operating limits for boot operation. Some systems may also incorporate an annunciator light to indicate proper boot operation.

 

Proper maintenance and care of de-icing boots is important for continued operation of this system. They need to be carefully inspected prior to a flight.

 

Another type of leading edge protection is the thermal anti-ice system installed on airplanes with turbine engines. This system is designed to prevent the buildup of ice by directing hot air from the compressor section of the engine to the leading edge surfaces. The system is activated prior to entering icing conditions. The hot air heats the leading edge sufficiently to prevent the formation of ice.

 

An alternate type of leading edge protection that is not as common as thermal anti-ice and de-icing boots is known as a weeping wing. The weeping-wing design uses small holes located in the leading edge of the wing. A chemical mixture is pumped to the leading edge and weeps out through the holes to prevent the formation and buildup of ice.

 

 

 

Windscreen ice control

 

There are two main types of windscreen anti-ice systems. The first system directs a flow of alcohol to the windscreen. By using it early enough, the alcohol will prevent ice from building up on the windshield.

 

The rate of alcohol flow can be controlled by a dial in the cockpit according to procedures recommended by the airplane manufacturer.

 

Another effective method of anti-icing equipment is the electric heating method. Small wires or other conductive material is imbedded in the windscreen.

 

The heater can be turned on by a switch in the cockpit, at which time electrical current is passed across the shield through the wires to provide sufficient heat to prevent the formation of ice on the windscreen. The electrical current can cause compass deviation errors; in some cases, as much as 40°. The heated windscreen should only be used during flight. Do not leave it on during ground operations, as it can overheat and cause damage to the windscreen.

 

 

 

Propeller ice control

 

Propellers are protected from icing by use of alcohol or electrically heated elements. Some propellers are equipped with a discharge nozzle that is pointed toward the root of the blade. Alcohol is discharged from the nozzles, and centrifugal force makes the alcohol flow down the leading edge of the blade. This prevents ice from forming on the leading edge of the propeller.

 

Propellers can also be fitted with propeller anti-ice boots. The propeller boot is divided into two sections—the inboard and the outboard sections. The boots are grooved to help direct the flow of alcohol, and they are also imbedded with electrical wires that carry current for heating the propeller. The prop anti-ice system can be monitored for proper operation by monitoring the prop anti-ice ammeter. During the preflight inspection, check the propeller boots for proper operation. If a boot fails to heat one blade, an unequal blade loading can result, and may cause severe propeller vibration.

 

 

 

prop-anti-ice Basic Aeronautical Knowledge - AviationEnglish.com

Figure 9: Prop ammeter and anti-ice boots.

 

 

 

Other ice control systems

 

Pitot and static ports, fuel vents, stall-warning sensors, and other optional equipment may be heated by electrical elements. Operational checks of the electrically heated systems are to be checked in accordance with the AFM or POH.

 

Operation of aircraft anti-icing and de-icing systems should be checked prior to encountering icing conditions. Encounters with structural ice require immediate remedial action. Anti-icing and de-icing equipment is not intended to sustain long-term flight in icing conditions.

 

 

Pressurized airplanes

 

 

 

When an airplane is flown at a high altitude, it consumes less fuel for a given airspeed than it does for the same speed at a lower altitude. In other words, the airplane is more efficient at a high altitude. In addition, bad weather and turbulence may be avoided by flying in the relatively smooth air above the storms. Because of the advantages of flying at high altitudes, many modern general aviation-type airplanes are being designed to operate in that environment. It is important that pilots transitioning to such sophisticated equipment be familiar with at least the basic operating principles.

 

A cabin pressurization system accomplishes several functions in providing adequate passenger comfort and safety. It maintains a cabin pressure altitude of approximately 8,000 feet at the maximum designed cruising altitude of the airplane, and prevents rapid changes of cabin altitude that may be uncomfortable or cause injury to passengers and crew. In addition, the pressurization system permits a reasonably fast exchange of air from the inside to the outside of the cabin. This is necessary to eliminate odors and to remove stale air.

 

 

 

standard-atmospheric-pressu Basic Aeronautical Knowledge - AviationEnglish.com

Figure 10: Standard atmospheric pressure chart.

 

 

 

Pressurization of the airplane cabin is an accepted method of protecting occupants against the effects of hypoxia. Within a pressurized cabin, occupants can be transported comfortably and safely for long periods of time, particularly if the cabin altitude is maintained at 8,000 feet or below, where the use of oxygen equipment is not required. The flight crew in this type of airplane must be aware of the danger of accidental loss of cabin pressure and must be prepared to deal with such an emergency whenever it occurs.

 

In the typical pressurization system, the cabin, flight compartment, and baggage compartments are incorporated into a sealed unit that is capable of containing air under a pressure higher than outside atmospheric pressure. On aircraft powered by turbine engines, bleed air from the engine compressor section is used to pressurize the cabin. Superchargers may be used on older model turbine powered airplanes to pump air into the sealed fuselage. Piston-powered airplanes may use air supplied from each engine turbocharger through a sonic venturi (flow limiter). Air is released from the fuselage by a device called an outflow valve. The outflow valve, by regulating the air exit, provides a constant inflow of air to the pressurized area.

 

 

 

pressurization-system Basic Aeronautical Knowledge - AviationEnglish.com

Figure 11: High performance airplane pressurization system.

 

 

 

To understand the operating principles of pressurization and air-conditioning systems, it is necessary to become familiar with some of the related terms and definitions, such as:

 

  • Aircraft altitude—the actual height above sea level at which the airplane is flying.

     

  • Ambient temperature—the temperature in the area immediately surrounding the airplane.

     

  • Ambient pressure—the pressure in the area immediately surrounding the airplane.

     

  • Cabin altitude—used to express cabin pressure in terms of equivalent altitude above sea level.

     

  • Differential pressure—the difference in pressure between the pressure acting on one side of a wall and the pressure acting on the other side of the wall. In aircraft air-conditioning and pressurizing systems, it is the difference between cabin pressure and atmospheric pressure.

     

The cabin pressure control system provides cabin pressure regulation, pressure relief, vacuum relief, and the means for selecting the desired cabin altitude in the isobaric and differential range. In addition, dumping of the cabin pressure is a function of the pressure control system. A cabin pressure regulator, an outflow valve, and a safety valve are used to accomplish these functions.

 

The cabin pressure regulator controls cabin pressure to a selected value in the isobaric range and limits cabin pressure to a preset differential value in the differential range. When the airplane reaches the altitude at which the difference between the pressure inside and outside the cabin is equal to the highest differential pressure for which the fuselage structure is designed, a further increase in airplane altitude will result in a corresponding increase in cabin altitude. Differential control is used to prevent the maximum differential pressure, for which the fuselage was designed, from being exceeded. This differential pressure is determined by the structural strength of the cabin and often by the relationship of the cabin size to the probable areas of rupture, such as window areas and doors.

 

The cabin air pressure safety valve is a combination pressure relief, vacuum relief, and dump valve. The pressure relief valve prevents cabin pressure from exceeding a predetermined differential pressure above ambient pressure. The vacuum relief prevents ambient pressure from exceeding cabin pressure by allowing external air to enter the cabin when ambient pressure exceeds cabin pressure. The cockpit control switch actuates the dump valve. When this switch is positioned to ram, a solenoid valve opens, causing the valve to dump cabin air to atmosphere.

 

The degree of pressurization and the operating altitude of the aircraft are limited by several critical design factors. Primarily the fuselage is designed to withstand a particular maximum cabin differential pressure.

 

Several instruments are used in conjunction with the pressurization controller. The cabin differential pressure gauge indicates the difference between inside and outside pressure. This gauge should be monitored to assure that the cabin does not exceed the maximum allowable differential pressure. A cabin altimeter is also provided as a check on the performance of the system.

 

In some cases, these two instruments are combined into one. A third instrument indicates the cabin rate of climb or descent. A cabin rate-of-climb instrument and a cabin altimeter are illustrated in Figure 12.

 

 

 

cabin-pressurization-instru Basic Aeronautical Knowledge - AviationEnglish.com

Figure 12: Cabin pressurization instruments.

 

 

 

Decompression is defined as the inability of the airplane´s pressurization system to maintain its designed pressure differential. This can be caused by a malfunction in the pressurization system or structural damage to the airplane. Physiologically, decompressions fall into two categories; they are:

 

  • Explosive Decompression—Explosive decompression is defined as a change in cabin pressure faster than the lungs can decompress; therefore, it is possible that lung damage may occur. Normally, the time required to release air from the lungs without restrictions, such as masks, is 0.2 seconds. Most authorities consider any decompression that occurs in less than 0.5 seconds as explosive and potentially dangerous.

     

  • Rapid Decompression—Rapid decompression is defined as a change in cabin pressure where the lungs can decompress faster than the cabin; therefore, there is no likelihood of lung damage.

     

During an explosive decompression, there may be noise, and for a split second, one may feel dazed. The cabin air will fill with fog, dust, or flying debris. Fog occurs due to the rapid drop in temperature and the change of relative humidity. Normally, the ears clear automatically. Air will rush from the mouth and nose due to the escape of air from the lungs, and may be noticed by some individuals.

 

The primary danger of decompression is hypoxia.

 

Unless proper utilization of oxygen equipment is accomplished quickly, unconsciousness may occur in a very short time. The period of useful consciousness is considerably shortened when a person is subjected to a rapid decompression. This is due to the rapid reduction of pressure on the body—oxygen in the lungs is exhaled rapidly. This in effect reduces the partial pressure of oxygen in the blood and therefore reduces the pilot´s effective performance time by one-third to one-fourth its normal time. For this reason, the oxygen mask should be worn when flying at very high altitudes (35,000 feet or higher). It is recommended that the crewmembers select the 100 percent oxygen setting on the oxygen regulator at high altitude if the airplane is equipped with a demand or pressure demand oxygen system.

 

Another hazard is being tossed or blown out of the airplane if near an opening. For this reason, individuals near openings should wear safety harnesses or seatbelts at all times when the airplane is pressurized and they are seated.

 

Another potential hazard during high altitude decompressions is the possibility of evolved gas decompression sicknesses. Exposure to wind blasts and extremely cold temperatures are other hazards one might have to face.

 

Rapid descent from altitude is necessary if these problems are to be minimized. Automatic visual and aural warning systems are included in the equipment of all pressurized airplanes.

 

 

 

Oxygen systems

 

Most high altitude airplanes come equipped with some type of fixed oxygen installation. If the airplane does not have a fixed installation, portable oxygen equipment must be readily accessible during flight. The portable equipment usually consists of a container, regulator, mask outlet, and pressure gauge. Aircraft oxygen is usually stored in high pressure system containers of 1,800 — 2,200 pounds per square inch (p.s.i.). When the ambient temperature surrounding an oxygen cylinder decreases, pressure within that cylinder will decrease because pressure varies directly with temperature if the volume of a gas remains constant. If a drop in indicated pressure on a supplemental oxygen cylinder is noted, there is no reason to suspect depletion of the oxygen supply, which has simply been compacted due to storage of the containers in an unheated area of the aircraft. High pressure oxygen containers should be marked with the p.s.i. tolerance (i.e., 1,800 p.s.i.) before filling the container to that pressure. The containers should be supplied with aviation oxygen only, which is 100 percent pure oxygen. Industrial oxygen is not intended for breathing and may contain impurities, and medical oxygen contains water vapor that can freeze in the regulator when exposed to cold temperatures. To assure safety, oxygen system periodic inspection and servicing should be done.

 

An oxygen system consists of a mask and a regulator that supplies a flow of oxygen dependent upon cabin altitude. Regulators approved for use up to 40,000 feet are designed to provide zero percent cylinder oxygen and 100 percent cabin air at cabin altitudes of 8,000 feet or less, with the ratio changing to 100 percent oxygen and zero percent cabin air at approximately 34,000 feet cabin altitude. Regulators approved up to 45,000 feet are designed to provide 40 percent cylinder oxygen and 60 percent cabin air at lower altitudes, with the ratio changing to 100 percent at the higher altitude.

 

Pilots should avoid flying above 10,000 feet without oxygen during the day and above 8,000 feet at night.

 

 

 

oxygen-regulator Basic Aeronautical Knowledge - AviationEnglish.com

Figure 13: Oxygen system regulator.

 

 

 

Pilots should be aware of the danger of fire when using oxygen. Materials that are nearly fireproof in ordinary air may be susceptible to burning in oxygen. Oils and greases may catch fire if exposed to oxygen, and cannot be used for sealing the valves and fittings of oxygen equipment. Smoking during any kind of oxygen equipment use is prohibited. Before each flight, the pilot should thoroughly inspect and test all oxygen equipment. The inspection should include a thorough examination of the aircraft oxygen equipment, including available supply, an operational check of the system, and assurance that the supplemental oxygen is readily accessible. The inspection should be accomplished with clean hands and should include a visual inspection of the mask and tubing for tears, cracks, or deterioration; the regulator for valve and lever condition and positions; oxygen quantity; and the location and functioning of oxygen pressure gauges, flow indicators and connections. The mask should be donned and the system should be tested. After any oxygen use, verify that all components and valves are shut off.

 

 

 

Masks

 

There are numerous types of oxygen masks in use that vary in design detail. It would be impractical to discuss all of the types on this page. It is important that the masks used be compatible with the particular oxygen system involved. Crew masks are fitted to the user´s face with a minimum of leakage. Crew masks usually contain a microphone. Most masks are the oronasal type, which covers only the mouth and nose.

 

Passenger masks may be simple, cup-shaped rubber moldings sufficiently flexible to obviate individual fitting. They may have a simple elastic head strap or the passenger may hold them to the face.

 

All oxygen masks should be kept clean. This reduces the danger of infection and prolongs the life of the mask. To clean the mask, wash it with a mild soap and water solution and rinse it with clear water. If a microphone is installed, use a clean swab, instead of running water, to wipe off the soapy solution. The mask should also be disinfected. A gauze pad that has been soaked in a water solution of Merthiolate can be used to swab out the mask. This solution should contain one-fifth teaspoon of Merthiolate per quart of water.

 

Wipe the mask with a clean cloth and air dry.

 

 

 

Diluter demand oxygen systems

 

Diluter demand oxygen systems supply oxygen only when the user inhales through the mask. An automix lever allows the regulators to automatically mix cabin air and oxygen or supply 100 percent oxygen, depending on the altitude. The demand mask provides a tight seal over the face to prevent dilution with outside air and can be used safely up to 40,000 feet. A pilot who has a beard or mustache should be sure it is trimmed in a manner that will not interfere with the sealing of the oxygen mask. The fit of the mask around the beard or mustache should be checked on the ground for proper sealing.

 

 

 

Pressure demand oxygen systems

 

Pressure demand oxygen systems are similar to diluter demand oxygen equipment, except that oxygen is supplied to the mask under pressure at cabin altitudes above 34,000 feet. Pressure demand regulators also create airtight and oxygen-tight seals, but they also provide a positive pressure application of oxygen to the mask face piece that allows the user´s lungs to be pressurized with oxygen. This feature makes pressure demand regulators safe at altitudes above 40,000 feet.

 

Some systems may have a pressure demand mask with the regulator attached directly to the mask, rather than mounted on the instrument panel or other area within the flight deck. The mask-mounted regulator eliminates the problem of a long hose that must be purged of air before 100 percent oxygen begins flowing into the mask.

 

 

 

Continous flow oxygen system

 

Continuous flow oxygen systems are usually provided for passengers. The passenger mask typically has a reservoir bag, which collects oxygen from the continuous flow oxygen system during the time when the mask user is exhaling. The oxygen collected in the reservoir bag allows a higher aspiratory flow rate during the inhalation cycle, which reduces the amount of air dilution. Ambient air is added to the supplied oxygen during inhalation after the reservoir bag oxygen supply is depleted. The exhaled air is released to the cabin.

 

 

 

mask Basic Aeronautical Knowledge - AviationEnglish.com

Figure 14: Continuous flow mask and rebreather bag.

 

 

 

Servicing of oxygen systems

 

Certain precautions should be observed whenever aircraft oxygen systems are to be serviced. Before servicing any aircraft with oxygen, consult the specific aircraft service manual to determine the type of equipment required and procedures to be used. Oxygen system servicing should be accomplished only when the aircraft is located outside of the hangars. Personal cleanliness and good housekeeping are imperative when working with oxygen. Oxygen under pressure and petroleum products create spontaneous results when they are brought in contact with each other.

 

Service people should be certain to wash dirt, oil, and grease (including lip salves and hair oil) from their hands before working around oxygen equipment. It is also essential that clothing and tools are free of oil, grease, and dirt. Aircraft with permanently installed oxygen tanks usually require two persons to accomplish servicing of the system. One should be stationed at the service equipment control valves, and the other stationed where he or she can observe the aircraft system pressure gauges. Oxygen system servicing is not recommended during aircraft fueling operations or while other work is performed that could provide a source of ignition. Oxygen system servicing while passengers are on board the aircraft is not recommended.

Cumulonimbus (Cb) clouds, in which Thunderstorms are found, form when three conditions are met:

  • There must be a deep layer of unstable air.
  • The air must be warm and moist.
  • A trigger mechanism must cause the warm moist air to rise:
    • Heating of the layer of air close to the surface.
    • Rising ground forcing the air upwards (orographic uplift).
    • A front forcing the air upwards.
Thunderstormlifecycle.jpg

A Cumulonimbus Cloud develops in three distinct phases:

  1. Building Phase. A pocket of warm air begins to ascend as a result of one of the triggers mentioned above. As the moist air rises, it becomes saturated, cloud forms, and the latent heat released as the moisture condenses further warms the air and it continues to rise. The air within the cloud is warmer than the air outside it and more air is drawn into the cloud from the base as well as the sides. The cloud grows in height rapidly, faster than many aircraft can climb, extending from the surface to a great height, sometimes as far as the Tropopause. as the temperature of the rising air drops below freezing point, the water droplets become super cooled and join together to become larger and larger.
  2. Mature Phase. As the top of the cloud reaches great heights, precipitation begins to fall. The falling rain, snow and/or ice (Hail) cools the surrounding air creating downdrafts. The friction between ice particles descending through the cloud, and ice particles being carried aloft by the updrafts, creates a static charge in the cloud with the top of the cloud having a positive charge and the bottom of the cloud having a negative charge. Eventually the difference in potential is so great that powerful electrical discharges (Lightning) occur, accompanied by Thunder. The top of the cloud begins to flatten out and Cirrus like cloud, consisting of ice crystals, spreads out creating a distinctive anvil shape.
  3. Dissipation Phase. The cooling effect of the downdrafts on the air beneath the cloud reduces the strength of the updrafts and until the the updrafts eventually stop and the lower cloud begins to dissipate. the upper cloud will then linger for some time afterwards.

The active cycle of the Cb cell lasts little more than an hour but many Thunderstorms contain several active Cb cells in various stages of development meaning that a storm can last several hours and extend over a large area. The active cells are often embedded in a larger cloud mass consisting of the remains of decayed cells as well as other cloud types at various levels. This can make the active cells very difficult to detect visually and appropriate use of Radar is required to safely avoid active weather.

Source: www.skybrary.aero

The following is a sample from Leading Edge: Volume 1 - Basic Aeronautical Knowledge, a self study workbook for English as a Second Language Learners who are preparing for initial flight training.  Some activities have been omitted.

Airplane design characteristics  

Experienced pilots note that each airplane handles differently - that is, each resists or responds to control pressures in its own way. A training type airplane is quick to respond to control applications, while a transport airplane usually feels heavy on the controls and responds to control pressures more slowly. These features can be designed into an airplane to facilitate the particular purpose the airplane is to fulfill by considering certain stability and maneuvering requirements.

In the following section, we summarize the more important aspects of an airplane's stability; its maneuvering and controllability qualities; how they are analyzed; and their relationship to various flight conditions. In brief, the basic differences between stability, maneuverability, and controllability are as follows:

  • Stability - The inherent quality of an airplane to correct for conditions that may disturb its equilibrium, and to return or to continue on the original flightpath. It is primarily an airplane design characteristic.
  • Maneuverability - The quality of an airplane that permits it to be maneuvered easily and to withstand the stresses imposed by maneuvers. It is governed by the airplane's weight, inertia, size and location of flight controls, structural strength, and powerplant. It too is an airplane design characteristic.
  • Controllability - The capability of an airplane to respond to the pilot's control, especially with regard to flightpath and attitude. It is the quality of the airplane's response to the pilot's control application when maneuvering the airplane, regardless of its stability characteristics.

Basic concepts of stabiity

The flightpaths and attitudes in which an airplane can fly are limited only by the aerodynamic characteristics of the airplane, its propulsive system, and its structural strength. These limitations indicate the maximum performance and maneuverability of the airplane. If the airplane is to provide maximum utility, it must be safely controllable to the full extent of these limits without exceeding the pilot's strength or requiring exceptional flying ability. If an airplane is to fly straight and steady along any arbitrary flightpath, the forces acting on it must be in static equilibrium. The reaction of any body when its equilibrium is disturbed is referred to as stability. There are two types of stability; static and dynamic. Static will be discussed first, and in this discussion the following definitions will apply:

  • Equilibrium - All opposing forces acting on the airplane are balanced; (i.e., steady, unaccelerated flight conditions).
  • Static Stability - The initial tendency that the airplane displays after its equilibrium is disturbed.
  • Positive Static Stability - The initial tendency of the airplane to return to the original state of equilibrium after being disturbed.
  • Negative Static Stability - The initial tendency of the airplane to continue away from the original state of equilibrium after being disturbed.
  • Neutral Static Stability - The initial tendency of the airplane to remain in a new condition after its equilibrium has been disturbed. 

stability Basic Aeronautical Knowledge - AviationEnglish.com

Static stability

Stability of an airplane in flight is slightly more complex than just explained, because the airplane is free to move in any direction and must be controllable in pitch, roll, and direction. When designing the airplane, engineers must compromise between stability, maneuverability, and controllability; and the problem is compounded because of the airplane's three-axis freedom. Too much stability is detrimental to maneuverability, and similarly, not enough stability is detrimental to controllability. In the design of airplanes, compromise between the two is key.

Dynamic stability

Static stability has been defined as the initial tendency that the airplane displays after being disturbed from its trimmed condition. Occasionally, the initial tendency is different or opposite from the overall tendency, so distinction must be made between the two. Dynamic stability is the overall tendency that the airplane displays after its equilibrium is disturbed. The curves of figure 2 represent the variation of controlled functions versus time. It is seen that the unit of time is very significant. If the time unit for one cycle or oscillation is above 10 seconds' duration, it is called a "long-period" oscillation (phugoid) and is easily controlled. In a longitudinal phugoid oscillation, the angle of attack remains constant when the airspeed increases and decreases. To a certain degree, a convergent phugoid is desirable but is not required. The phugoid can be determined only on a statically stable airplane, and this has a great effect on the trimming qualities of the airplane. If the time unit for one cycle or oscillation is less than one or two seconds, it is called a "short-period" oscillation and is normally very difficult, if not impossible, for the pilot to control. This is the type of oscillation that the pilot can easily "get in phase with" and reinforce.

damped_undamped Basic Aeronautical Knowledge - AviationEnglish.com

Figure 2: Damped versus undamped stability.

A neutral or divergent, short-period oscillation is dangerous because structural failure usually results if the oscillation is not damped immediately. Short-period oscillations affect airplane and control surfaces alike and reveal themselves as "porpoising" in the airplane, or as in "buzz" or "flutter" in the control surfaces. Basically, the short-period oscillation is a change in angle of attack with no change in airspeed. A short-period oscillation of a control surface is usually of such high frequency that the airplane does not have time to react. Logically, the Code of Federal Regulations require that short-period oscillations be heavily damped (i.e., die out immediately). Flight tests during the airworthiness certification of airplanes are conducted for this condition by inducing the oscillation in the controls for pitch, roll, or yaw at the most critical speed (i.e., at VNE, the never-exceed speed). The test pilot strikes the control wheel or rudder pedal a sharp blow and observes the results. 

Longitudinal stability (pitching)

In designing an airplane, a great deal of effort is spent in developing the desired degree of stability around all three axes. But longitudinal stability about the lateral axis is considered to be the most affected by certain variables in various flight conditions.

Longitudinal stability is the quality that makes an airplane stable about its lateral axis. It involves the pitching motion as the airplane's nose moves up and down in flight. Alongitudinally unstable airplane has a tendency to dive or climb progressively into a very steep dive or climb, or even a stall. Thus, an airplane with longitudinal instability becomes difficult and sometimes dangerous to fly.

Static longitudinal stability or instability in an airplane, is dependent upon three factors:

  1. Location of the wing with respect to the center of gravity;
  2. Location of the horizontal tail surfaces with respect to the center of gravity; and
  3. The area or size of the tail surfaces.

In analyzing stability, it should be recalled that a body that is free to rotate will always turn about its center of gravity.

To obtain static longitudinal stability, the relation of the wing and tail moments must be such that, if the moments are initially balanced and the airplane is suddenly nosed up, the wing moments and tail moments will change so that the sum of their forces will provide an unbalanced but restoring moment which, in turn, will bring the nose down again.

Similarly, if the airplane is nosed down, the resulting change in moments will bring the nose back up.

The center of lift, sometimes called the center of pressure, in most unsymmetrical airfoils has a tendency to change its fore and aft position with a change in the angle of attack. The center of pressure tends to move forward with an increase in angle of attack and to move aft with a decrease in angle of attack. This means that when the angle of attack of an airfoil is increased, the center of pressure (lift) by moving forward, tends to lift the leading edge of the wing still more. This tendency gives the wing an inherent quality of instability.

Figure 3 shows an airplane in straight-and-level flight. The line CG-CL-T represents the airplane's longitudinal axis from the center of gravity (CG) to a point T on the horizontal stabilizer. The center of lift (or center of pressure) is represented by the point CL.

longitudinal_stability Basic Aeronautical Knowledge - AviationEnglish.com

Figure 3: Longitudinal stability.

Most airplanes are designed so that the wing's center of lift (CL) is to the rear of the center of gravity. This makes the airplane "nose heavy" and requires that there be a slight downward force on the horizontal stabilizer in order to balance the airplane and keep the nose from continually pitching downward.

Compensation for this nose heaviness is provided by setting the horizontal stabilizer at a slight negative angle of attack. The downward force thus produced, holds the tail down, counterbalancing the "heavy" nose. It is as if the line CG-CL-T was a lever with an upward force at CL and two downward forces balancing each other, one a strong force at the CG point and the other, a much lesser force, at point T (downward air pressure on the stabilizer). Applying simple physics principles, it can be seen that if an iron bar were suspended at point CL with a heavy weight hanging on it at the CG, it would take some downward pressure at point T to keep the "lever" in balance.

Even though the horizontal stabilizer may be level when the airplane is in level flight, there is a downwash of air from the wings. This downwash strikes the top of the stabilizer and produces a downward pressure, which at a certain speed will be just enough to balance the "lever." The faster the airplane is flying, the greater this downwash and the greater the downward force on the horizontal stabilizer (except "T" tails).

speed_downwash Basic Aeronautical Knowledge - AviationEnglish.com

Figure 4: Effect of speed on downwash.

In airplanes with fixed position horizontal stabilizers, the airplane manufacturer sets the stabilizer at an angle that will provide the best stability (or balance) during flight at the design cruising speed and power setting.

power_pitch Basic Aeronautical Knowledge - AviationEnglish.com

Figure 5: Reduced power allows pitch down.

If the airplane's speed decreases, the speed of the airflow over the wing is decreased. As a result of this decreased flow of air over the wing, the downwash is reduced, causing a lesser downward force on the horizontal stabilizer. In turn, the characteristic nose heaviness is accentuated, causing the airplane's nose to pitch down more. This places the airplane in a nose-low attitude, lessening the wing's angle of attack and drag and allowing the airspeed to increase. As the airplane continues in the nose-low attitude and its speed increases, the downward force on the horizontal stabilizer is once again increased.

Consequently, the tail is again pushed downward and the nose rises into a climbing attitude. As this climb continues, the airspeed again decreases, causing the downward force on the tail to decrease until the nose lowers once more. However, because the airplane is dynamically stable, the nose does not lower as far this time as it did before. The airplane will acquire enough speed in this more gradual dive to start it into another climb, but the climb is not so steep as the preceding one.

After several of these diminishing oscillations, in which the nose alternately rises and lowers, the airplane will finally settle down to a speed at which the downward force on the tail exactly counteracts the tendency of the airplane to dive. When this condition is attained, the airplane will once again be in balanced flight and will continue in stabilized flight as long as this attitude and airspeed are not changed.

A similar effect will be noted upon closing the throttle. The downwash of the wings is reduced and the force at T in figure 3 is not enough to hold the horizontal stabilizer down. It is as if the force at T on the lever were allowing the force of gravity to pull the nose down. This, of course, is a desirable characteristic because the airplane is inherently trying to regain airspeed and reestablish the proper balance.

Power or thrust can also have a destabilizing effect in that an increase of power may tend to make the nose rise. The airplane designer can offset this by establishing a "high thrustline" wherein the line of thrust passes above the center of gravity.

thrust_line_stability Basic Aeronautical Knowledge - AviationEnglish.com

Figure 6: Thrust line affects longitudinal stability.

power_stability Basic Aeronautical Knowledge - AviationEnglish.com

Figure 7: Power changes affect longitudinal stability.

In this case, as power or thrust is increased a moment is produced to counteract the down load on the tail. On the other hand, a very "low thrust line" would tend to add to the nose-up effect of the horizontal tail surface.

It can be concluded, then, that with the center of gravity forward of the center of lift, and with an aerodynamic tail-down force, the result is that the airplane always tries to return to a safe flying attitude.

A simple demonstration of longitudinal stability may be made as follows: Trim the airplane for "hands off" control in level flight. Then momentarily give the controls a slight push to nose the airplane down. If, within a brief period, the nose rises to the original position and then stops, the airplane is statically stable. Ordinarily, the nose will pass the original position (that of level flight) and a series of slow pitching oscillations will follow. If the oscillations gradually cease, the airplane has positive stability; if they continue unevenly, the airplane has neutral stability; if they increase, the airplane is unstable.

Lateral stability (rolling)

Stability about the airplane's longitudinal axis, which extends from nose to tail, is called lateral stability. This helps to stabilize the lateral or rolling effect when one wing gets lower than the wing on the opposite side of the airplane. There are four main design factors that make an airplane stable laterally: dihedral, keel effect, sweepback, and weight distribution.

The most common procedure for producing lateral stability is to build the wings with a dihedral angle varying from one to three degrees. In other words, the wings on either side of the airplane join the fuselage to form a slight V or angle called "dihedral," and this is measured by the angle made by each wing above a line parallel to the lateral axis.

The basis of rolling stability is, of course, the lateral balance of forces produced by the airplane's wings. Any imbalance in lift results in a tendency for the airplane to roll about its longitudinal axis.

Stated another way, dihedral involves a balance of lift created by the wings' angle of attack on each side of the airplane's longitudinal axis.

If a momentary gust of wind forces one wing of the airplane to rise and the other to lower, the airplane will bank. When the airplane is banked without turning, it tends to sideslip or slide downward toward the lowered wing.

dihedral_stability Basic Aeronautical Knowledge - AviationEnglish.com

Figure 8: Dihedral for lateral stability.

Since the wings have dihedral, the air strikes the low wing at much greater angle of attack than the high wing. This increases the lift on the low wing and decreases lift on the high wing, and tends to restore the airplane to its original lateral attitude (wings level)-that is, the angle of attack and lift on the two wings are again equal.

The effect of dihedral, then, is to produce a rolling moment tending to return the airplane to a laterally balanced flight condition when a sideslip occurs.

The restoring force may move the low wing up too far, so that the opposite wing now goes down. If so, the process will be repeated, decreasing with each lateral oscillation until a balance for wings-level flight is finally reached.

Conversely, excessive dihedral has an adverse effect on lateral maneuvering qualities. The airplane may be so stable laterally that it resists any intentional rolling motion. For this reason, airplanes that require fast roll or banking characteristics usually have less dihedral than those designed for less maneuverability.

The contribution of sweepback to dihedral effect is important because of the nature of the contribution.

In a sideslip, the wing into the wind is operating with an effective decrease in sweepback, while the wing out of the wind is operating with an effective increase in sweepback. The swept wing is responsive only to the wind component that is perpendicular to the wing's leading edge. Consequently, if the wing is operating at a positive lift coefficient, the wing into the wind has an increase in lift, and the wing out of the wind has a decrease in lift. In this manner, the swept back wing would contribute a positive dihedral effect and the swept forward wing would contribute a negative dihedral effect.

During flight, the side area of the airplane's fuselage and vertical fin react to the airflow in much the same manner as the keel of a ship. That is, it exerts a steadying influence on the airplane laterally about the longitudinal axis.

Such laterally stable airplanes are constructed so that the greater portion of the keel area is above and behind the center of gravity.

keel_stability Basic Aeronautical Knowledge - AviationEnglish.com

Figure 9: Keel area for lateral stability.

Thus, when the airplane slips to one side, the combination of the airplane's weight and the pressure of the airflow against the upper portion of the keel area (both acting about the CG) tends to roll the airplane back to wings-level flight.

Vertical stability (yawing)

Stability about the airplane's vertical axis (the sideways moment) is called yawing or directional stability. Yawing or directional stability is the more easily achieved stability in airplane design. The area of the vertical fin and the sides of the fuselage aft of the center of gravity are the prime contributors which make the airplane act like the well known weathervane or arrow, pointing its nose into the relative wind.

In examining a weathervane, it can be seen that if exactly the same amount of surface were exposed to the wind in front of the pivot point as behind it, the forces fore and aft would be in balance and little or no directional movement would result.

Consequently, it is necessary to have a greater surface aft of the pivot point that forward of it.

Similarly in an airplane, the designer must ensure positive directional stability by making the side surface greater aft than ahead of the center of gravity.

fin_stability Basic Aeronautical Knowledge - AviationEnglish.com

Figure 10: Fuselage and fin for vertical stability.

To provide more positive stability aside from that provided by the fuselage, a vertical fin is added. The fin acts similar to the feather on an arrow in maintaining straight flight.

If an airplane is flying in a straight line, and a sideward gust of air gives the airplane a slight rotation about its vertical axis (i.e., the right), the motion is retarded and stopped by the fin because while the airplane is rotating to the right, the air is striking the left side of the fin at an angle. This causes pressure on the left side of the fin, which resists the turning motion and slows down the airplane's yaw. In doing so, it acts somewhat like the weathervane by turning the airplane into the relative wind. The initial change in direction of the airplane's flightpath is generally slightly behind its change of heading. Therefore, after a slight yawing of the airplane to the right, there is a brief moment when the airplane is still moving along its original path, but its longitudinal axis is pointed slightly to the right.

The airplane is then momentarily skidding sideways, and during that moment (since it is assumed that although the yawing motion has stopped, the excess pressure on the left side of the fin still persists) there is necessarily a tendency for the airplane to be turned partially back to the left. That is, there is a momentary restoring tendency caused by the fin.

This restoring tendency is relatively slow in developing and ceases when the airplane stops skidding. When it ceases, the airplane will be flying in a direction slightly different from the original direction. In other words, it will not of its own accord return to the original heading; the pilot must reestablish the initial heading.

A minor improvement of directional stability may be obtained through sweepback. Sweepback is incorporated in the design of the wing primarily to delay the onset of compressibility during high-speed flight.

In lighter and slower airplanes, sweepback aids in locating the center of pressure in the correct relationship with the center of gravity. A longitudinally stable airplane is built with the center of pressure aft of the center of gravity.

Because of structural reasons, airplane designers sometimes cannot attach the wings to the fuselage at the exact desired point. If they had to mount the wings too far forward, and at right angles to the fuselage, the center of pressure would not be far enough to the rear to result in the desired amount of longitudinal stability. By building sweepback into the wings, however, the designers can move the center of pressure toward the rear. The amount of sweepback and the position of the wings then place the center of pressure in the correct location.

The contribution of the wing to static directional stability is usually small. The swept wing provides a stable contribution depending on the amount of sweepback, but the contribution is relatively small when compared with other components.

Free directional oscillations (Dutch roll)

Dutch Roll is a coupled lateral/directional oscillation that is usually dynamically stable but is objectionable in an airplane because of the oscillatory nature. The damping of the oscillatory mode may be weak or strong depending on the properties of the particular airplane.

Unfortunately all air is not smooth. There are bumps and depressions created by gusty updrafts and downdrafts, and by gusts from ahead, behind, or the side of the airplane.

The response of the airplane to a disturbance from equilibrium is a combined rolling/yawing oscillation in which the rolling motion is phased to precede the yawing motion. The yawing motion is not too significant, but the roll is much more noticeable.

When the airplane rolls back toward level flight in response to dihedral effect, it rolls back too far and sideslips the other way. Thus, the airplane overshoots each time because of the strong dihedral effect. When the dihedral effect is large in comparison with static directional stability, the Dutch Roll motion has weak damping and is objectionable. When the static directional stability is strong in comparison with the dihedral effect, the Dutch Roll motion has such heavy damping that it is not objectionable. However, these qualities tend toward spiral instability.

The choice is then the least of two evils-Dutch Roll is objectionable and spiral instability is tolerable if the rate of divergence is low. Since the more important handling qualities are a result of high static directional stability and minimum necessary dihedral effect, most airplanes demonstrate a mild spiral tendency. This tendency would be indicated to the pilot by the fact that the airplane cannot be flown "hands off" indefinitely.

In most modern airplanes, except high-speed swept wing designs, these free directional oscillations usually die out automatically in a very few cycles unless the air continues to be gusty or turbulent. Those airplanes with continuing Dutch Roll tendencies usually are equipped with gyro stabilized yaw dampers. An airplane that has Dutch Roll tendencies is disconcerting, to say the least. Therefore, the manufacturer tries to reach a medium between too much and too little directional stability. Because it is more desirable for the airplane to have "spiral instability" than Dutch Roll tendencies, most airplanes are designed with that characteristic.

Spiral instability

Spiral instability exists when the static directional stability of the airplane is very strong as compared to the effect of its dihedral in maintaining lateral equilibrium. When the lateral equilibrium of the airplane is disturbed by a gust of air and a sideslip is introduced, the strong directional stability tends to yaw the nose into the resultant relative wind while the comparatively weak dihedral lags in restoring the lateral balance. Due to this yaw, the wing on the outside of the turning moment travels forward faster than the inside wing and as a consequence, its lift becomes greater. This produces an overbanking tendency which, if not corrected by the pilot, will result in the bank angle becoming steeper and steeper. At the same time, the strong directional stability that yaws the airplane into the relative wind is actually forcing the nose to a lower pitch attitude. Then, the start of a slow downward spiral which has begun, if not counteracted by the pilot, will gradually increase into a steep spiral dive.

Usually the rate of divergence in the spiral motion is so gradual that the pilot can control the tendency without any difficulty.

All airplanes are affected to some degree by this characteristic although they may be inherently stable in all other normal parameters. This tendency would be indicated to the pilot by the fact that the airplane cannot be flown "hands off" indefinitely.

Much study and effort has gone into development of control devices (wing leveler) to eliminate or at least correct this instability. Advanced stages of this spiral condition demand that the pilot be very careful in application of recovery controls, or excessive loads on the structure may be imposed.

Of the in-flight structural failures that have occurred in general aviation airplanes, improper recovery from this condition has probably been the underlying cause of more fatalities than any other single factor. The reason is that the airspeed in the spiral condition builds up rapidly, and the application of back elevator force to reduce this speed and to pull the nose up only "tightens the turn," increasing the load factor. The results of the prolonged uncontrolled spiral are always the same; either in-flight structural failure, crashing into the ground, or both. The most common causes on record for getting into this situation are: loss of horizon reference, inability of the pilot to control the airplane by reference to instruments, or a combination of both.

Source: adapted from FAA manuals

Language development activities

LESSON: AIRCRAFT STABILITY AND HANDLING

Prepared by: Michael Egerton

Objectives:

  • Students will be able to understand, pronounce, and use the words in the vocabulary list below.
  • Students will be able to demonstrate comprehension of the passage by answering questions which require basic understanding and interpretation of the content.  

Pronunciation: L

Directions: Practice by saying the word pairs and example sentences, then speak the words from this text. Also distinguish light and dark L sounds.

Word Pairs lame-fame, last-fast, lap-flap, least-feast

Example sentence: Lou flew the yellow plane to London.

Examples from this text: Reveal - Developing - Facilitate - Neutral - Lateral - Balanced - Recalled - Variables - Involves - Fulfill - Detrimental - Complex - Initial - Control - Utility - Following - Analyzed - Limitations - Ability - Horizontal - Noticeable - Indefinitely - Generally - Coupled - Tolerable - Prolonged - Underlying - Lags - General - Imbalance - Finally - Reestablish - Slight - Lever - Destabilizing - Original - Keel - Establishing - Concluded -

Comprehension

Read the text at http://aviationenglish.com/basic-aeronautical-knowledge/aeronautics

  1. Explain lateral stability.
  2. Explain longitudinal stability.
  3. Explain vertical stability.
  4. Explain spiral stability and instability.
  5. What are the effects of stability at higher altitudes?

Email This email address is being protected from spambots. You need JavaScript enabled to view it. for the answers.

Focus on Grammar

Words that describe actions are called adverbs. In the sentence "Bob eats quickly", the adverb quickly describes how Bob eats.

Directions: Identify how the adverbs are used in the sentences below.

  1. However, because the airplane is dynamically stable, the nose does not lower as far this time as it did before.

  2. After several of these diminishing oscillations, in which the nose alternately rises and lowers, the airplane will finally settle down to a speed at which the downward force on the tail exactly counteracts the tendency of the airplane to dive.

  3. This, of course, is a desirable characteristic because the airplane is inherently trying to regain airspeed and reestablish the proper balance.

  4. When this condition is attained, the airplane will once again be in balanced flight and will continue in stabilized flight as long as this attitude and airspeed are not changed.

  5. This means that when the angle of attack of an airfoil is increased, the center of pressure (lift) by moving forward, tends to lift the leading edge of the wing still more.

  6. This makes the airplane nose heavy and requires that there be a slight downward force on the horizontal stabilizer in order to balance the airplane and keep the nose from continually pitching downward.

  7. As a result of this decreased flow of air over the wing, the downwash is reduced, causing a lesser downward force on the horizontal stabilizer.

  8. It is as if the line CG-CL- T was a lever with an upward force at CL and two downward forces balancing each other, one a strong force at the CG point and the other, a much lesser force, at point T (downward air pressure on the stabilizer).

  9. It can be concluded, then, that with the center of gravity forward of the center of lift, and with an aerodynamic tail-down force, the result is that the airplane always tries to return to a safe flying attitude.

  10. In other words, the wings on either side of the airplane join the fuselage to form a slight V or angle called dihedral, and this is measured by the angle made by each wing above a line parallel to the lateral axis.

  11. The most common procedure for producing lateral stability is to build the wings with a dihedral angle varying from one to three degrees.

  12. When the airplane is banked without turning, it tends to sideslip or slide downward toward the lowered wing.

  13. Conversely, excessive dihedral has an adverse effect on lateral maneuvering qualities.

  14. If so, the process will be repeated, decreasing with each lateral oscillation until a balance for wings-level flight is finally reached.
     
  15. If, within a brief period, the nose rises to the original position and then stops, the airplane is statically stable.
     
  16. Then momentarily give the controls a slight push to nose the airplane down.
     
  17. Ordinarily, the nose will pass the original position (that of level flight) and a series of slow pitching oscillations will follow.
     
  18. if they continue unevenly, the airplane has neutral stability;

  19. If the oscillations gradually cease, the airplane has positive stability;
     
  20. The phugoid can be determined only on a statically stable airplane, and this has a great effect on the trimming qualities of the airplane.
     
  21. If the time unit for one cycle or oscillation is above 10 seconds' duration, it is called a long-period oscillation (phugoid) and is easily controlled.

  22. If the time unit for one cycle or oscillation is less than one or two seconds, it is called a short-period oscillation and is normally very difficult, if not impossible, for the pilot to control.
     
  23. A neutral or divergent, short-period oscillation is dangerous because structural failure usually results if the oscillation is not damped immediately.

  24. This is the type of oscillation that the pilot can easily get in phase with and reinforce.

  25. Occasionally, the initial tendency is different or opposite from the overall tendency, so distinction must be made between the two.
     
  26. A training type airplane is quick to respond to control applications, while a transport airplane usually feels heavy on the controls and responds to control pressures more slowly.
     
  27. Experienced pilots note that each airplane handles differently- that is, each resists or responds to control pressures in its own way.
     
  28. If the airplane is to provide maximum utility, it must be safely controllable to the full extent of these limits without exceeding the pilot's strength or requiring exceptional flying ability.
     
  29. Too much stability is detrimental to maneuverability, and similarly, not enough stability is detrimental to controllability.

  30. Stability of an airplane in flight is slightly more complex than just explained, because the airplane is free to move in any direction and must be controllable in pitch, roll, and direction.
     
  31. Short-period oscillations affect airplane and control surfaces alike and reveal themselves as porpoising in the airplane, or as in buzz or flutter in the control surfaces.
     
  32. To obtain static longitudinal stability, the relation of the wing and tail moments must be such that, if the moments are initially balanced and the airplane is suddenly nosed up, the wing moments and tail moments will change so that the sum of their forces will provide an unbalanced but restoring moment which, in turn, will bring the nose down again.
     
  33. The center of pressure tends to move forward with an increase in angle of attack and to move aft with a decrease in angle of attack.
     
  34. The airplane may be so stable laterally that it resists any intentional rolling motion.

  35. This tendency would be indicated to the pilot by the fact that the airplane cannot be flown hands off indefinitely.

  36. Dutch Roll is a coupled lateral/directional oscillation that is usually dynamically stable but is objectionable in an airplane because of the oscillatory nature.

  37. When the lateral equilibrium of the airplane is disturbed by a gust of air and a sideslip is introduced, the strong directional stability tends to yaw the nose into the resultant relative wind while the comparatively weak dihedral lags in restoring the lateral balance.

  38. Such laterally stable airplanes are constructed so that the greater portion of the keel area is above and behind the center of gravity.
     
  39. A longitudinally stable airplane is built with the center of pressure aft of the center of gravity.
     
  40. The airplane is then momentarily skidding sideways, and during that moment( since it is assumed that although the yawing motion has stopped, the excess pressure on the left side of the fin still persists) there is necessarily a tendency for the airplane to be turned partially back to the left.
     
  41. Occasionally, the initial tendency is different or opposite from the overall tendency, so distinction must be made between the two.

  42. Conversely, excessive dihedral has an adverse effect on lateral maneuvering qualities. 

Vocabulary Practice

FILL IN THE BLANK

Directions: Use the word bank to identify the word that best completes the sentence. You might need to change the form of the word.

arbitrary

discussion

facilitate

analyzed

exceeding

maneuvering

dynamic

extent

fulfill

respond

summarize

ability

can

transport

following

     
  1. The text _______________d the basic concepts of stability.
  2. They had a _______________ about negative static stability.
  3. You must _______________ the procedures to fly safely.
  4. He has the _______________ to be a great pilot.
  5. She's a very _______________ woman; she never seems to relax!
  6. I _______________ read and write in English.
  7. A fundamental skill is _______________ the aircraft.
  8. He _______________ed his dream of becoming an airline pilot.
  9. After the earthquake, the government measured the _______________ of the damage.
  10. The controller didn’t _______________ to her request for help.
  11. The scientist _______________ the results.
  12. He _______________ the speed limit by twenty miles an hour!
  13. Shuttle buses _______________ people from the terminal to the aircraft.
  14. Blood is necessary for the _______________ of oxygen to all the cells in the human body.

MATCHING

Directions: Write the word that matches the definition.

  1. _______ To give a short description of the main points.

    2. _______ Debate and conversation about a subject.

    3. _______ Decided by feeling, not logic, thought or reason.

    4. _______ To do something the way it is written or defined.

    5. _______ Skill or talent.

    6. _______ Having energy and movement.

    7. _______ Have the ability to do something.

    8. _______ To move with great skill.

    9. _______ Satisfy or complete.

    10. _______ The amount, size or reach of something.

    11. _______ To answer with words or actions.

    12. _______ To examine carefully.

    13. _______ To go beyond the limit.

    14. _______ To make it easier to do something.

    15. _______ The movement of a particular object.

What to do next:

For more free resources to help you improve your Basic Aeronautical Knowledge and your Aviation English visit http://aviationenglish.com/learning-zone/basic-aeronautical-knowledge/ call +852 81799295 or email This email address is being protected from spambots. You need JavaScript enabled to view it. to join a course with Aviation English Asia Ltd

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