Pilot's Handbook of Aeronautical Knowledge

Chapter 7: Aircraft Systems

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Aircraft systems include the engine, propeller, induction, and ignition systems. They also include the fuel, lubrication, cooling, electrical, landing gear, and environmental control systems.

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Reciprocating Engines

Reciprocating engines feature a back-and-forth movement of the pistons that produces energy. Combustion within the cylinders turns chemical energy (fuel) into mechanical energy. Spark ignition systems are common, while compression systems utilizing diesel or jet fuel are starting to appear.

Within combustion chambers, pistons convert linear motion into rotary motion. Spark ignition systems use spark plugs to create combustion. Compression systems compress fuel until temperature rises to achieve automatic combustion.

Radial engines arrange cylinders around the crankcase, and they have a favorable power-to-weight ratio. Horizontally opposed engines have high power-to-weight ratios, and they permit a smaller frontal area. V-type engines have a power advantage, also with a small frontal area.

With a two-stroke engine, the intake, compression, ignition, and exhaust processes happen within only two engine strokes of the piston. Because they have a power-stroke on each cycle, they offer a power advantage, but also excessive emissions, although they have been improved with recent advances.

A four-stroke engine converts chemical energy to mechanical energy via a four-stroke piston operating cycle: Intake (down), compression (up), power/ignition (down), exhaust (up). Each cylinder/piston operates in rapid sequence to rotate the crankshaft with continuous power.

Diesel engines for small aircraft have been pioneered by Thielert Aircraft Engines (TAE), beginning in 2001. Their use of Jet-A fuel permits for a reliable engine with lower operational costs.

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Propeller

The amount of thrust produced by the propeller depends on the shape of the airfoil, the angle of attack (AOA) of the propeller blade, and the revolutions per minute (RPM) of the engine. The twisted angle-of-incidence (pitch angle) from the root to the tip permits uniform thrust across the entire area of the propeller.

The pitch on a fixed-pitch propeller is set by the manufacturer and cannot be changed. It is most efficient at one combination of airspeed and RPM. A climb propeller has a lower pitch and less drag, increasing takeoff and climb performance. A cruise propeller has high pitch, more drag and less horsepower capability — it is most efficient in cruising flight.

The RPM of the propeller and crankshaft are identical, unless the propeller is mounted on a shaft that is geared to the crankshaft.

In a fixed-pitch propeller, the tachometer is the indicator of engine power. As operating altitude increases, the engine may output less power for a given tachometer setting when compared to the same setting at sea level. A decrease in air density reduces power output, and at higher altitudes the throttle setting must be increased to maintain the same power setting as at lower altitudes.

An adjustable-pitch propeller, often referred to as a ground-adjustable propeller, can have its pitch angle changed on the ground but not in flight.

A constant-speed propeller (a type of adjustable-pitch propeller) utilizes a governor so that its pitch angle automatically changes during flight. The governor maintains a single RPM, regardless of air load. Constant-speed propellers are very efficient, converting a substantial amount of brake horsepower to thrust horsepower across a range of RPM and airspeed combinations. An aircraft with a constant-speed propeller has a throttle to set engine power and a propeller control to set engine/propeller RPM. High airspeeds/propeller loads cause the blade angle to increase in order to maintain the RPM. At lower airspeeds/propeller loads, the blade angle decreases.

When a constant-speed propeller encounters the high or low pitch-stop, changes in engine power will change the propeller RPM. This is common when landing, which is done at low speed.

Power is set with the throttle and monitored via a manifold pressure gauge, which measures the the absolute pressure of the fuel-air mixture. When the engine is not running (or if it fails in flight), the manifold pressure gauge will indicate the ambient air pressure.

If manifold pressure is exceeded for a given RPM, pressure within the cylinders could exceed operational constraints, eventually leading to engine failure. The POH will outline manifold pressure settings for a range of RPM settings. Pilots should reduce power before reducing RPM, and increase RPM before increasing power.

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Induction System

The induction system combines air with fuel for purposes of combustion within the cylinder. External air filters are designed to eliminate contaminants, while an alternate source of air usually comes from inside the engine cowling.

Engines with carburetors can have either float- type or pressure-type carburetors. The vast majority of small aircraft have float-type carburetors. Fuel and air are mixed in an airstream and then sent to combustion chambers for ignition. A float within the float chamber restricts how much fuel enters the carburetor. Flow of fuel is regulated by the throttle.

Float-type carburetors don't function well during abrupt maneuvers, or with some supercharged systems. They are most prone to carburetor icing at the venturi and throttle valve. Pressure carburetors do not carry the same risk.

As altitude increases, the fuel-air mixture becomes more rich, creating a risk of engine roughness due to spark-plug fouling. The fuel mixture is leaned with the mixture control, which increases/reduces fuel flow. An exhaust gas temperature (EGT) gauge helps the pilot set the mixture properly and for better fuel economy.

Carburetor-ice buildup leads to a loss of power and can cause the engine to stop. Carburetor ice is most common in temperatures below 70° F (21° C) and relative humidity above 80%, but it can occur with even higher temperatures and lower humidity. The first indication of carburetor icing in an aircraft with a fixed-pitch propeller is a decrease in engine RPM. Engine roughness can follow. With constant-speed propellers, a loss in manifold pressure is evident, but not RPM, since propeller pitch is automatically adjusted. The greatest risk of carburetor ice is during low power settings during descent.

Carburetor heat warms the air before it reaches the carburetor. It is best used as a preventive measure, rather than for removing ice only after it has formed. Carburetor heat also may be used as an alternate air intake if the air filter is clogged or airframe ice has formed.

If the throttle is closed during operations, carburetor heat should be applied, and the throttle should be opened periodically to keep the engine warm, which also will ensure that applied carburetor heat will be effective.

Application of carburetor heat can reduce engine power by as much as 15%, since heated air enriches the mixture. If ice is present when carburetor heat is applied, a drop in RPM will be followed by an increase in RPM, with the engine running more smoothly. For constant-speed propellers, there will be a decrease/increase in manifold pressure instead of RPM.

Application of carburetor heat after ice has formed can lead to engine roughness lasting several seconds to a few minutes, as water is introduced to the fuel system. Carburetor heat should remain applied until the engine is running smoothly again.

Some aircraft are equipped with a carburetor air temperature gauge. The outside air temperature (OAT) gauge also is useful for detecting carburetor icing conditions.

In a fuel injection system, fuel is injected directly into the cylinders, or just ahead of the intake valve. The system usually incorporates an engine-driven fuel pump, a fuel-air control unit, a fuel manifold (fuel distributor), discharge nozzles, an auxiliary fuel pump, and fuel pressure/flow indicators.

The auxiliary fuel pump provides fuel for engine starting or emergency use. Under power, the engine-driven fuel pump then provides the fuel. The fuel-air control unit meters fuel based on the mixture control setting and sends it to the fuel manifold based on the throttle setting. Fuel is distributed to the discharge nozzles in each cylinder head. The system is less susceptible to icing, since there is no carburetor, but impact icing is still possible.

Fuel-injection systems permit a reduction in evaporative icing, better fuel flow, faster throttle response, a precise control of mixture, better fuel distribution, and easier cold-weather starts.

Fuel-injection system drawbacks include difficulty in starting a hot engine, vapor locks during ground operations on hot days, and problems associated with restarting an engine that quit because of fuel starvation.

Supercharger and turbocharger systems compress the intake air to increase its density.

A supercharger relies on an engine-driven air pump or compressor. A turbocharger gets its power from the exhaust stream that runs through a turbine, which in turn spins the compressor. Both systems have a manifold pressure gauge.

When induction air entering the engine is pressurized, the aircraft's service ceiling can be increased. Higher operational altitudes afford the aircraft higher true airspeeds and the ability to avoid some kinds of adverse weather.

Because turbochargers are powered by an engine's exhaust gases, they recover energy from hot exhaust gases that would otherwise be lost. Critical altitude is the maximum altitude at which a turbocharged engine can produce its rated horsepower. Above the critical altitude, power output begins to decrease like it does for a normally aspirated engine.

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Ignition System

The ignition system comprises magnetos, spark plugs, high-tension leads, and an ignition switch.

A magneto uses a permanent magnet to generate an electrical current completely independent of the aircraft's electrical system. High voltage jumps a spark across the spark plug gap in each cylinder. It continues to operate whenever the crankshaft is rotating.

A dual-ignition system utilizes two individual magnetos (with sets of wires and spark plugs) to increase reliability of the ignition system. If one of the magnetos fails, the other is unaffected. The firing of two spark plugs in normal operation improves combustion of the fuel-air mixture and results in slightly higher power output. A malfunctioning dual-ignition system can be detected during a pre-takeoff check.

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Oil System

The engine oil system:

• lubricates the engine's moving parts
• cools the engine by reducing friction
• removes heat from the cylinders
• provides a seal between the cylinder walls and pistons
• carries away contaminants

In a wet-sump system, the oil is located in a sump that is an integral part of the engine. An oil pump draws oil from the sump and routes it to the engine. After the oil passes through the engine, it returns to the sump.

In a dry-sump system, the oil is contained in a separate tank and circulated through the engine by pumps. Oil is routed through the engine and then pumped from the various locations in the engine back to the oil tank by scavenge pumps.

The oil pressure gauge provides a direct indication of the oil system operation. The oil temperature gauge measures the temperature of oil, which changes more slowly than oil pressure.

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Engine Cooling Systems

Most engine heat is expelled through the exhaust system, but remaining heat also must be removed. Most small aircraft are air-cooled. The oil system cools the engine's internal components. The air intake and baffles direct air over cooling fins on the cylinders, which increase the area exposed to the airflow.

Air cooling is less effective during ground operations, as well as during slow-speed flight with high power settings. High-speed descents can shock-cool the engine.

Overheating the engine can lead to oil loss, detonation, loss of power, and permanent engine damage. High engine temperatures can be reduced by increasing airspeed, enriching the mixture, and/or reducing power.

The oil temperature gauge can be the only indication of engine temperature in some small aircraft. It is an indirect and delayed measure of engine temperature. The cylinder head temperature (CHT) gauge is a direct measure of engine temperature.

Some aircraft are equipped with cowl flaps to manage engine temperature. Cowl flaps are closed if the engine is running cool, which traps warm air and warms the engine. Conversely, cowl flaps are opened if the engine is running hot.

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Exhaust Systems

Exhaust systems vent combustion gasses. They also provide cabin heat and window defrost. Heat is captured via a shroud around the muffler. Air within the shroud is heated by the muffler, which then can be vented to the cabin. To avoid carbon monoxide poisoning, the exhaust system must be in good condition. Carbon monoxide is tasteless and odorless.

Some exhaust systems include an exhaust gas temperature gauge probe. The EGT gauge on the flight deck measures the temperature of exhaust gases, which can be used to regulate the fuel/air mixture. The operating handbook should be consulted for leaning procedures.

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Starting Systems

A direct-cranking electric starter system uses electricity to operate the starter and a starter motor. The starter rotates the engine, via the flywheel, at a speed that allows the engine to begin operating. Power is normally provided by the battery, but it can be provided by an external source.

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Combustion

Detonation is an uncontrolled, explosive ignition of the fuel-air mixture within the cylinder's combustion chamber. If not corrected, excessive temperatures and pressures can lead to overheating, roughness, loss of power, and engine damage. Detonation is characterized by high cylinder head temperatures and is most likely to occur when operating at high power settings.

Common causes of detonation include:

• Use of a lower fuel grade than that specified
• Operation with high manifold pressures and low RPM
• Operation with high RPM and an overly lean mixture
• Extended ground operations
• Extended climbs

To avoid detonation, pilots should monitor engine instruments, use cowl flaps when available, and avoid prolonged steep climbs.

Pre-ignition occurs when the fuel-air mixture ignites prior to the engine's normal ignition event. Typically, this is caused by a residual hot spot in the combustion chamber, often created by a small carbon deposit on a spark plug. It also can be caused by a cracked spark plug insulator or other damage in the cylinder. Pre-ignition causes the engine to lose power, produces high operating temperatures, and can lead to engine damage.

Detonation and pre-ignition often occur simultaneously, and one condition may cause the other to occur. Pilots should always use the correct fuel and operate the engine within its established limits.

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Full Authority Digital Engine Control (FADEC)

A FADEC system uses a digital computer and other components to control the engine and propeller. These systems are becoming more common in smaller aircraft. A FADEC system monitors each cylinder and adjusts ignition timing as necessary. FADEC systems eliminate the need for magnetos, carburetor heat, mixture controls, and engine priming. The system is controlled via a single throttle.

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Turbine Engines

See Pilot's Handbook of Operational Knowledge, Chapter 7.

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Airframe Systems: Fuel

A gravity-feed system utilizes gravity to transfer the fuel from the tanks to the engine. These are typical on high-wing aircraft, where the fuel tanks are placed above the carburetor. An aircraft with a fuel-pump system has two fuel pumps — an engine-driven pump and an auxiliary pump, which is used for starting and emergencies. The auxiliary pump is sometimes referred to as the "boost pump."

The fuel primer is used to draw fuel from the tanks to vaporize fuel directly into the cylinders prior to starting the engine.

Fuel tanks are normally found in the wings. They are vented to the outside to maintain atmospheric pressure inside the tank. Fuel vents can be found in the filler cap or a tube extending through the surface of the wing. An overflow drain may be co-located with the venting system. The overflow drain allows fuel to expand with increases in temperature without damage to the tank itself.

Fuel gauges display fuel quantity in either gallons or pounds. Aircraft certification rules require gauges to be accurate when they read "empty." All other readings should be verified on the ground via examination of the fuel quantity in each tank.

Aircraft with fuel pumps also will have a fuel pressure gauge. Consult each aircraft's operating handbook for proper interpretation.

Fuel selectors permit (or restrict) flow of fuel from one or more fuel tanks. Running for prolonged periods on one tank causes an unbalanced fuel load between tanks.

Vapor lock is a condition in which fuel vapors form in the fuel line between the tank and the engine. The pressure of the vapor is high enough that it prevents liquid fuel from flowing to the engine, resulting in fuel starvation. It is most likely to form during extended ground operations on hot days. The risk can be minimized by engaging the boost pump to maintain positive pressure. Running a tank completely dry may allow air to enter the fuel system, causing vapor lock. Consult the operating handbook regarding use of the boost pump when switching tanks.

Between the fuel tank and the carburetor, a fuel strainer removes contaminants (moisture and sediments) from the fuel. These contaminants then are heavier than fuel and settle at the bottom of the strainer assembly, where they can be removed via a fuel strainer drain (also called a "sump drain" or "sump point").

Aviation gasoline (AVGAS) comes in various fuel grades. Higher fuel grades can withstand greater pressure without detonating. The grade is considered an "antiknock value" or "knock resistance" of the fuel. If the proper grade of fuel is not available, the next highest grade can be used as a substitute. Using a lower grade of fuel than recommended can result in engine overheating and detonation.

Dyes are added to help identify the type and grade of fuel. Aircraft with reciprocating engines use AVGAS 80 (red), AVGAS 100 (green), and AVGAS 100LL (blue). Aircraft with turbine engines use JET A, JET A-1, and JET-B (all clear in color), which has a distinct kerosene odor. Some aircraft engines have been modified per FAA guidelines to operate on automobile gasoline (MOGAS).

Water is the principal fuel contaminant and should be completely removed from the system before flight operations. Water may give fuel a cloudy appearance, or it may separate and settle to the bottom of the sample jar. Fuel tanks should be filled after each flight or after the last flight of the day to prevent moisture condensation within the tank.

Ice formation in the aircraft fuel system results from the presence of water in the fuel system. Free water can be drained from a fuel tank through the sump drains, which are provided for that purpose. Free water, frozen on the bottom of reservoirs, such as the fuel tanks and fuel filter, may render water drains useless and can later melt releasing the water into the system thereby causing engine malfunction or stoppage. Entrained water — water in solution with petroleum fuels — constitutes a relatively small part of the total potential water in a particular system. Water in suspension may freeze and form ice crystals of sufficient size such that fuel screens, strainers, and filters may be blocked. The use of anti-icing additives for some aircraft has been approved as a means of preventing problems with water and ice in AVGAS.

During refueling, static electricity is formed by the flow of fuel through the hose and nozzle. Aircraft also build up static electricity in flight. To guard against the possibility of static electricity igniting fuel fumes, a ground wire should be attached to the aircraft before the fuel cap is removed from the tank. By bonding the aircraft and refueler to each other, the static differential charge is equalized.

A fuel fired heater is a small mounted or portable space-heating device. The fuel is brought to the heater by using piping from a fuel tank, or taps into the aircraft's fuel system. Exhaust heating systems route exhaust gases away from the engine and fuselage, reduce engine noise, and heat the cabin and carburetor. Exhaust heating systems should be thoroughly inspected to eliminate the risk of carbon monoxide poisoning.

Combustion heaters (or surface combustion heaters) are often used to heat the cabin of larger, more expensive aircraft. Bleed air heating systems are used on turbine-engine aircraft.

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Airframe Systems: Electrical

Most aircraft are equipped with either a 14- or a 28-volt direct current (DC) electrical system, which includes 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 and maintain the battery's charge. The battery provides power to the starting system and is an emergency backup to the alternator or generator.

Direct-current (DC) generators do not provide sufficient power during operations at low engine RPM. Thus, electrical needs are drawn from the battery, which can quickly be depleted. Alternators produce sufficient current to operate the entire electrical system, even at slower engine speeds. Alternating current (AC) is converted to direct current (DC), and power supply is consistent throughout all engine speeds.

Some aircraft use a ground power unit (GPU) for starting, particularly during cold weather.

When activated, the master switch sends power to all electrical components, except for the ignition system. These include:

• Position lights
• Anti-collision 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

The alternator half of the master switch permits the pilot to exclude the alternator from the electrical system in the event of alternator failure. When the alternator half of the master switch in the "Off" position, the entire electrical load is placed on the battery.

A bus bar is used as a terminal in the aircraft electrical system to connect the main electrical system to the equipment using electricity as a source of power. Fuses or circuit breakers are used in the electrical system to protect the circuits and equipment from electrical overload.

An ammeter is used to monitor the performance of the aircraft electrical system. When the pointer of the ammeter 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, while a full-scale positive deflection indicates a malfunction of the regulator.

A loadmeter has a scale beginning with zero and shows the load being placed on the alternator or generator.

A voltage regulator controls the rate of charge to the battery. The generator/alternator voltage output should be higher than the battery voltage, which keeps the battery charged.

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Airframe Systems: Hydraulic

On small aircraft, hydraulic systems are often used to operate wheel brakes, retractable landing gear, and some constant-speed propellers. Large aircraft use hydraulics for flight control surfaces, wing flaps, spoilers, and other systems. Hydraulic fluid is pumped through the system to an actuator or servo, which is a cylinder with a piston inside that turns fluid power into work, creating the power needed to move an aircraft system or flight control. Servos can be either single-acting or double-acting, based on the needs of the system. A mineral-based (petroleum) hydraulic fluid is the most widely used type for small aircraft.

Landing gear can consist of wheels, floats, or skis. Conventional gear employs a rear wheel, while tricycle gear features a nosewheel. Some nosewheels are steerable for ground operations. A non-steerable nosewheel is "castering."

Tailwheel aircraft permit larger propellers with more ground clearance. They are preferable for operations on rough fields. Directional control during ground operations is more difficult. A swerve on the ground may be difficult to recover with limited rudder authority, resulting in a ground loop.

Tricycle gear permits more forceful application of braking without causing the aircraft to nose over. It also permits better forward visibility during ground operations and lowers the risk of ground loop, since the CG is forward of the main wheels.

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Airframe Systems: Pressurized and Oxygen Systems

Aircraft flown at high altitudes consume less fuel for a given airspeed than they do for the same speed at a lower altitude. They also can fly above adverse weather and turbulence.

Cabin pressurization systems typically maintains a cabin pressure altitude of approximately 8,000 feet at the maximum designed cruising altitude of an aircraft. They also eliminate odors and to remove stale air. Flight crew must be aware of the danger of accidental loss of cabin pressure and be prepared to deal with such an emergency.

• Aircraft altitude is the aircraft's actual height above sea level.
• Ambient temperature is the temperature in the area immediately surrounding the aircraft.
• Ambient pressure is the pressure in the area immediately surrounding the aircraft.
• Cabin altitude is cabin pressure in terms of equivalent altitude above sea level.
• Differential pressure is 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 — the difference between cabin pressure and atmospheric pressure.

When an aircraft 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 aircraft 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. The cabin differential pressure gauge indicates the difference between inside and outside pressure. The cabin altimeter provides a check on the performance of the system.

Decompression can be caused by a malfunction in the pressurization system or structural damage to the aircraft. Explosive decompression is a change in cabin pressure faster than the lungs can decompress, possibly resulting in lung damage. Rapid decompression is a change in cabin pressure in which the lungs decompress faster than the cabin, which has a greater "period of useful consciousness." Pilots should use oxygen when flying above 35,000 MSL in pressurized aircraft. Rapid descent from altitude is necessary in order to minimize a flight emergency caused by decompression.

Flight crews are required to use supplemental oxygen after 30 minutes exposure to cabin pressure altitudes from 12,501 to 14,000 feet. Supplemental oxygen is required immediately upon exposure to cabin pressure altitudes above 14,000 feet. Passengers must have supplemental oxygen above 15,000 feet cabin pressure altitude.

At night, especially when fatigued, high-altitude effects may occur as low as 5,000 feet. Pilots are encouraged to use supplemental oxygen above 10,000 feet cabin altitude during the day and above 5,000 feet at night. If the aircraft does not have a fixed installation, portable oxygen equipment must be readily accessible during flight.

Pilots should be aware of the danger of fire when using oxygen. Materials that are nearly fireproof in ordinary air may be susceptible to combustion in oxygen.

There are a wide variety of oxygen masks and portable oxygen systems. A pulse oximeter is a device that measures the amount of oxygen in an individual's blood, in addition to heart rate.

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Airframe Systems: Anti-Ice and De-Ice Systems

Anti-icing equipment is designed to prevent the formation of ice, while de-icing equipment is designed to remove ice once it has formed.

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.

A thermal anti-ice system directs hot air from the compressor section of the engine to the leading edge surfaces.

The weeping-wing design uses small holes located in the leading edge of the wing to prevent the formation and build-up of ice. An antifreeze solution is pumped to the leading edge and weeps out through the holes.

Two common windscreen anti-ice systems include the application of alcohol before ice has formed, and the application of heat via wires in the windscreen. The heating system can cause compass deviation errors.

Propellers are protected from icing by the use of alcohol or electrically heated elements. Pitot and static ports, fuel vents, stall-warning sensors, and other optional equipment may be heated by electrical elements.

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