AIRCRAFT SYSTEMS

Aircraft Systems Lesson by wifiCFI


Introduction

This chapter covers the primary systems found on most aircraft. 

These include: The Engine

Propeller

Induction

Ignition

Fuel

Lubrication

Cooling

Electrical

Landing Gear

Environmental Control Systems

The Powerplant (PHAK C7)

An aircraft engine, or powerplant, produces thrust to propel an aircraft. 

Reciprocating engines and turboprop engines work in combination with a propeller to produce thrust. 

Turbojet and turbofan engines produce thrust by increasing the velocity of air flowing through the engine. 

All of these powerplants also drive the various systems that support the operation of an aircraft. 

Reciprocating Engines

Most small aircraft are designed with reciprocating engines. 

The name is derived from the back-and-forth, or reciprocating, movement of the pistons that produces the mechanical energy necessary to accomplish work.

Reciprocating engines operate on the basic principle of converting chemical energy (fuel) into mechanical energy. 

This conversion occurs within the cylinders of the engine through the process of combustion. 

The two primary reciprocating engine designs are the spark ignition and the compression ignition. 

The spark ignition reciprocating engine has served as the powerplant of choice for many years. 

In an effort to reduce operating costs, simplify design, and improve reliability, several engine manufacturers are turning to compression ignition as a viable alternative. 

Often referred to as jet fuel piston engines, compression ignition engines have the added advantage of utilizing readily available and lower cost diesel or jet fuel. 

Spark and Compression Ignition

The main mechanical components of the spark ignition and the compression ignition engine are essentially the same. 

Both use cylindrical combustion chambers and pistons that travel the length of the cylinders to convert linear motion into the rotary motion of the crankshaft. 

The main difference between spark ignition and compression ignition is the process of igniting the fuel.

Spark Ignition

Spark ignition engines use a spark plug to ignite a pre-mixed fuel-air mixture.

Compression Ignition

A compression ignition engine first compresses the air in the cylinder, raising its temperature to a degree necessary for automatic ignition when fuel is injected into the cylinder. 

Cylinder Arrangements

Cylinder Arrangements include: Radial

In-line

V-type

Opposed


Radial Engine

These engines, a row or rows of cylinders are arranged in a circular pattern around the crankcase. 

The main advantage of a radial engine is the favorable power-to-weight ratio.

In-Line Engine

In-line engines have a comparatively small frontal area, but their power-to-weight ratios are relatively low. 

In addition, the rearmost cylinders of an air-cooled, in-line engine receive very little cooling air, so these engines are normally limited to four or six cylinders.

V-Type Engine

V-type engines provide more horsepower than in-line engines and still retain a small frontal area. 

Opposed Engine

The most popular reciprocating engines used on smaller aircraft. 

These engines always have an even number of cylinders, since a cylinder on one side of the crankcase “opposes” a cylinder on the other side. 

The majority of these engines are air cooled and usually are mounted in a horizontal position when installed on fixed-wing airplanes. 

Opposed-type engines have high power-to-weight ratios because they have a comparatively small, lightweight crankcase. 

In addition, the compact cylinder arrangement reduces the engine’s frontal area and allows a streamlined installation that minimizes aerodynamic drag.

Depending on the engine manufacturer, all of these arrangements can be designed to utilize spark or compression ignition and operate on either a two or four-stroke cycle.

Two Stroke Cycle

In a two-stroke engine, the conversion of chemical energy into mechanical energy occurs over a two-stroke operating cycle. 

The intake, compression, power, and exhaust processes occur in only two strokes of the piston rather than the more common four strokes. 

Because a two-stroke engine has a power stroke upon each revolution of the crankshaft, it typically has higher power-to-weight ratio than a comparable four-stroke engine. 

Due to the inherent inefficiency and disproportionate emissions of the earliest designs, use of the two-stroke engine has been limited in aviation. 

Four Stroke Cycle

Spark ignition four-stroke engines remain the most common design used in GA today.

In a four-stroke engine, the conversion of chemical energy into mechanical energy occurs over a four-stroke operating cycle. 

The intake, compression, power, and exhaust processes occur in four separate strokes of the piston in the following order: 

The intake stroke begins as the piston starts its downward travel. When this happens, the intake valve opens and the fuel-air mixture is drawn into the cylinder. 


The compression stroke begins when the intake valve closes, and the piston starts moving back to the top of the cylinder. This phase of the cycle is used to obtain a much greater power output from the fuel-air mixture once it is ignited. 


The power stroke begins when the fuel-air mixture is ignited. This causes a tremendous pressure increase in the cylinder and forces the piston downward away from the cylinder head, creating the power that turns the crankshaft.


The exhaust stroke is used to purge the cylinder of burned gases. It begins when the exhaust valve opens, and the piston starts to move toward the cylinder head once again.

In a four-cylinder engine, each cylinder operates on a different stroke.  Continuous rotation of a crankshaft is maintained by the precise timing of the power strokes in each cylinder.

The Propeller (PHAK C7)

The propeller is a rotating airfoil, subject to induced drag, stalls, and other aerodynamic principles that apply to any airfoil. 

It provides the necessary thrust to pull, or in some cases push, the aircraft through the air. 

The engine power is used to rotate the propeller, which in turn generates thrust very similar to the manner in which a wing produces lift. 

The amount of thrust produced 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.

Fixed Pitch Propeller

A propeller with fixed blade angles is a fixed-pitch propeller. 

The pitch of this propeller is set by the manufacturer and cannot be changed. 

Since a fixed-pitch propeller achieves the best efficiency only at a given combination of airspeed and rpm, the pitch setting is ideal for neither cruise nor climb. 

Thus, the aircraft suffers a bit in each performance category. 

The fixed-pitch propeller is used when low weight, simplicity, and low cost are needed.

Climb Prop

The climb propeller has a lower pitch, therefore less drag. 

Less drag results in higher rpm and more horsepower capability, which increases performance during takeoffs and climbs but decreases performance during cruising flight.

Cruise Prop

The cruise propeller has a higher pitch, therefore more drag. 

More drag results in lower rpm and less horsepower capability, which decreases performance during takeoffs and climbs but increases efficiency during cruising flight.

Fixed Pitch Propeller

In a fixed-pitch propeller, the tachometer is the indicator of engine power. 

A tachometer is calibrated in hundreds of rpm and gives a direct indication of the engine and propeller rpm. 

The instrument is color coded with a green arc denoting the maximum continuous operating rpm. 

Some tachometers have additional markings to reflect engine and/or propeller limitations.

The rpm is regulated by the throttle, which controls the fuel air flow to the engine. 

At a given altitude, the higher the tachometer reading, the higher the power output of the engine.

Adjustable Pitch Propeller

The adjustable-pitch propeller was the forerunner of the constant-speed propeller. 

It is a propeller with blades whose pitch can be adjusted on the ground with the engine not running, but which cannot be adjusted in flight.

The first adjustable-pitch propeller systems provided only two pitch settings: low and high. 

Today, most adjustable-pitch propeller systems are capable of a range of pitch settings. 

Constant Speed Propeller

A constant-speed propeller is a controllable-pitch propeller whose pitch is automatically varied in flight by a governor maintaining constant rpm despite varying air loads. 

It is the most common type of adjustable-pitch propeller. 

The main advantage of a constant-speed propeller is that it converts a high percentage of brake horsepower (BHP) into thrust horsepower (THP) over a wide range of rpm and airspeed combinations. 

A constant-speed propeller is more efficient than other propellers because it allows selection of the most efficient engine rpm for the given conditions.

An aircraft with a constant-speed propeller has two controls: the throttle and the propeller control. 

The throttle controls power output, and the propeller control regulates engine rpm. This regulates propeller rpm, which is registered on the tachometer.

Manifold Pressure Gauge

On aircraft equipped with a constant-speed propeller, power output is controlled by the throttle and indicated by a manifold pressure gauge. 

The gauge measures the absolute pressure of the fuel-air mixture inside the intake manifold and is more correctly a measure of manifold absolute pressure (MAP). 

At a constant rpm and altitude, the amount of power produced is directly related to the fuel-air mixture being delivered to the combustion chamber. 

As the throttle setting is increased, more fuel and air flows to the engine and MAP increases. 

When the engine is not running, the manifold pressure gauge indicates ambient air pressure. 

When the engine is started, the manifold pressure indication decreases to a value less than ambient pressure. 

Engine failure or power loss is indicated on the manifold gauge as an increase in manifold pressure to a value corresponding to the ambient air pressure at the altitude where the failure occurred.

Constant Speed Propeller

Once a specific rpm is selected, a governor automatically adjusts the propeller blade angle as necessary to maintain the selected rpm. 

For example, after setting the desired rpm during cruising flight, an increase in airspeed or decrease in propeller load causes the propeller blade angle to increase as necessary to maintain the selected rpm.

A reduction in airspeed or increase in propeller load causes the propeller blade angle to decrease.

High Blade Pitch = Lower RPM (more drag)

Low Blade Pitch = Higher RPM (less drag)

Single Engine Aircraft

The constant-speed propellers on almost all single-engine airplanes are of the non-feathering, oil pressure-to-increase-pitch design. 

In this design, increased oil pressure from the propeller governor drives the blade angle towards high pitch, low rpm. 

Multi-Engine Aircraft

In contrast, the constant-speed propellers installed on most multiengine airplanes are full feathering, counterweighted, oil-pressure-to-decrease-pitch designs. 

In this design, increased oil pressure from the propeller governor drives the blade angle towards low pitch, high rpm away from the feather blade angle. 

In effect, the only thing that keeps these propellers from feathering is a constant supply of high pressure engine oil. 

This is a necessity to enable propeller feathering in the event of a loss of oil pressure or a propeller governor failure.

Constant Speed Propeller When both manifold pressure and rpm need to be changed, avoid engine overstress by making power adjustments in the proper order:

When power settings are being decreased, reduce manifold pressure before reducing rpm. If rpm is reduced before manifold pressure, manifold pressure automatically increases, possibly exceeding the manufacturer’s tolerances.


When power settings are being increased, reverse the order—increase rpm first, then manifold pressure. 

Induction Systems (PHAK C7)

The induction system brings in air from the outside, mixes it with fuel, and delivers the fuel-air mixture to the cylinder where combustion occurs. 

Outside air enters the induction system through an intake port on the front of the engine cowling. 

This port normally contains an air filter that inhibits the entry of dust and other foreign objects. 

There are two types of induction systems:

The Carburetor System

Mixes the fuel and air in the carburetor before this mixture enters the intake manifold.

The Fuel Injection System

Mixes the fuel and air immediately before entry into each cylinder or injects fuel directly into each cylinder.

The Carburetor System

The float-type carburetor has several distinct disadvantages. 

First, they do not function well during abrupt maneuvers. 

Secondly, the discharge of fuel at low pressure leads to incomplete vaporization and difficulty in discharging fuel into some types of supercharged systems.  The chief disadvantage of the float-type carburetor, however, is its icing tendency. 

Since the float-type carburetor must discharge fuel at a point of low pressure, the discharge nozzle must be located at the venturi throat, and the throttle valve must be on the engine side of the discharge nozzle. 


This means that the drop in temperature due to fuel vaporization takes place within the venturi. 


As a result, ice readily forms in the venturi and on the throttle valve.

Carburetor Icing Carburetor ice is most likely to occur when temperatures are below 70 degrees Fahrenheit (°F) or 21 degrees Celsius (°C) and the relative humidity is above 80 percent. 

Carburetor Heat

Carburetor heat is an anti-icing system that preheats the air before it reaches the carburetor and is intended to keep the fuel-air mixture above freezing to prevent the formation of carburetor ice. 

Carburetor heat can be used to melt ice that has already formed in the carburetor if the accumulation is not too great.

The use of carburetor heat causes a decrease in engine power, sometimes up to 15 percent, because the heated air is less dense than the outside air that had been entering the engine. 

This enriches the mixture. 

When ice is present in an aircraft with a fixed-pitch propeller and carburetor heat is being used, there is a decrease in rpm, followed by a gradual increase in rpm as the ice melts.

Fuel Injection System

In a fuel injection system, the fuel is injected directly into the cylinders, or just ahead of the intake valve. 

The air intake for the fuel injection system is similar to that used in a carburetor system. A fuel injection system usually incorporates six basic components:  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 under pressure to the fuel-air control unit for engine starting and/or emergency use. 

After starting, the engine-driven fuel pump provides fuel under pressure from the fuel tank to the fuel-air control unit. 

This control unit, which essentially replaces the carburetor, meters fuel based on the mixture control setting and sends it to the fuel manifold valve at a rate controlled by the throttle. 

After reaching the fuel manifold valve, the fuel is distributed to the individual fuel discharge nozzles. 

The discharge nozzles, which are located in each cylinder head, inject the fuel-air mixture directly into each cylinder intake port.

Fuel Injection Advantages Reduction in evaporative icing 

Better fuel flow 

Faster throttle response 

Precise control of mixture 

Better fuel distribution 

Easier cold weather starts

Fuel Injection Disadvantages Difficulty in starting a hot engine 

Vapor locks during ground operations on hot days 

Problems associated with restarting an engine that quits because of fuel starvation

Ignition System (PHAK C7) In a spark ignition engine, the ignition system provides a spark that ignites the fuel-air mixture in the cylinders and is made up of:

Magnetos

Spark plugs

High-tension leads

An ignition switch

Magnetos

A magneto uses a permanent magnet to generate an electrical current completely independent of the aircraft’s electrical system. 

The magneto generates sufficiently high voltage to jump a spark across the spark plug gap in each cylinder. 

The system begins to fire when the starter is engaged and the crankshaft begins to turn. 

It continues to operate whenever the crankshaft is rotating.

Most standard certificated aircraft incorporate a dual ignition system with two individual magnetos, separate sets of wires, and spark plugs to increase reliability of the ignition system. 

Each magneto operates independently to fire one of the two spark plugs in each cylinder. 

The firing of two spark plugs improves combustion of the fuel-air mixture and results in a slightly higher power output. 

If one of the magnetos fails, the other is unaffected. 

The engine continues to operate normally, although a slight decrease in engine power can be expected. 

The same is true if one of the two spark plugs in a cylinder fails. 

The operation of the magneto is controlled in the flight deck by the ignition switch.  The switch has five positions:  OFF 

R (right) 

L (left) 

BOTH 

START 

With RIGHT or LEFT selected, only the associated magneto is activated. 

The system operates on both magnetos when BOTH is selected.

Oil System (PHAK C7)

The engine oil system performs several important functions:  Lubrication of the engine’s moving parts 

Cooling of the engine by reducing friction 

Removing heat from the cylinders

Providing a seal between the cylinder walls and pistons 

Carrying away contaminants

Wet Sump System Reciprocating engines use either a wet-sump or a dry-sump oil system. In a wet-sump system, the oil is located in a sump that is an integral part of the engine. 

Dry Sump System

In a dry-sump system, the oil is contained in a separate tank and circulated through the engine by pumps.

Engine Cooling System (PHAK C7)

The burning fuel within the cylinders produces intense heat, most of which is expelled through the exhaust system. 

Much of the remaining heat, however, must be removed, or at least dissipated, to prevent the engine from overheating. 

Otherwise, the extremely high engine temperatures can lead to loss of power, excessive oil consumption, detonation, and serious engine damage.

Air cooling is accomplished by air flowing into the engine compartment through openings in front of the engine cowling. 

Baffles route this air over fins attached to the engine cylinders, and other parts of the engine, where the air absorbs the engine heat. 

Expulsion of the hot air takes place through one or more openings in the lower, aft portion of the engine cowling.

Cowl Flaps

On aircraft equipped with cowl flaps, use the cowl flap positions to control the temperature. 

Cowl flaps are hinged covers that fit over the opening through which the hot air is expelled. 

If the engine temperature is low, the cowl flaps can be closed, thereby restricting the flow of expelled hot air and increasing engine temperature. 

If the engine temperature is high, the cowl flaps can be opened to permit a greater flow of air through the system, thereby decreasing the engine temperature.

Combustion System (PHAK C7)

During normal combustion, the fuel-air mixture burns in a very controlled and predictable manner. 

In a spark ignition engine, the process occurs in a fraction of a second. 

The mixture actually begins to burn at the point where it is ignited by the spark plugs.

It then burns away from the plugs until it is completely consumed. 

This type of combustion causes a smooth build-up of temperature and pressure and ensures that the expanding gases deliver the maximum force to the piston at exactly the right time in the power stroke.

Detonation

Detonation is an uncontrolled, explosive ignition of the fuel-air mixture within the cylinder’s combustion chamber. 

It causes excessive temperatures and pressures which, if not corrected, can quickly lead to failure of the piston, cylinder, or valves.

Common operational causes of detonation are:  Use of a lower fuel grade than that specified by the aircraft manufacturer.


Operation of the engine with extremely high manifold pressures in conjunction with low rpm. 


Operation of the engine at high power settings with an excessively lean mixture.


Maintaining extended ground operations or steep climbs in which cylinder cooling is reduced.

Pre-Ignition Preignition occurs when the fuel-air mixture ignites prior to the engine’s normal ignition event. 

Premature burning is usually caused by a residual hot spot in the combustion chamber, often created by a small carbon deposit on a spark plug, a cracked spark plug insulator, or other damage in the cylinder that causes a part to heat sufficiently to ignite the fuel-air charge.

Preignition causes the engine to lose power and produces high operating temperature. 

As with detonation, preignition may also cause severe engine damage because the expanding gases exert excessive pressure on the piston while still on its compression stroke.

Detonation and preignition often occur simultaneously and one may cause the other.

Fuel System (PHAK C7)

The fuel system is designed to provide an uninterrupted flow of clean fuel from the fuel tanks to the engine. 

The fuel must be available to the engine under all conditions of engine power, altitude, attitude, and during all approved flight maneuvers.  Two common classifications apply to fuel systems in small aircraft:  Gravity-feed systems 

Fuel-pump systems

Gravity Feed System The gravity-feed system utilizes the force of gravity to transfer the fuel from the tanks to the engine. 


For example, on high-wing airplanes, the fuel tanks are installed in the wings. 


This places the fuel tanks above the carburetor, and the fuel is gravity fed through the system and into the carburetor.

Fuel Pump System If the design of the aircraft is such that gravity cannot be used to transfer fuel, fuel pumps are installed. 

For example, on low-wing airplanes, the fuel tanks in the wings are located below the carburetor.

Aircraft with fuel-pump systems have two fuel pumps. 

The main pump system is engine driven with an electrically driven auxiliary pump provided for use in engine starting and in the event the engine pump fails. 

The auxiliary pump, also known as a boost pump, provides added reliability to the fuel system. 

The electrically-driven auxiliary pump is controlled by a switch in the flight deck.

Fuel Primers

Both gravity-feed and fuel-pump systems may incorporate a fuel primer into the system. 

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

The fuel tanks, normally located inside the wings of an airplane, have a filler opening on top of the wing through which they can be filled. 

A filler cap covers this opening.

The tanks are vented to the outside to maintain atmospheric pressure inside the tank. 

They may be vented through the filler cap or through a tube extending through the surface of the wing. 

Fuel tanks also include an overflow drain that may stand alone or be collocated with the fuel tank vent. 

This allows fuel to expand with increases in temperature without damage to the tank itself.

Fuel Gauges

The fuel quantity gauges indicate the amount of fuel measured by a sensing unit in each fuel tank and is displayed in gallons or pounds. 

Aircraft certification rules require accuracy in fuel gauges only when they read “empty.” 

Any reading other than “empty” should be verified.

If a fuel pump is installed in the fuel system, a fuel pressure gauge is also included. Fuel Selectors The fuel selector valve allows selection of fuel from various tanks. A common type of selector valve contains four positions: 

LEFT

RIGHT

BOTH

OFF

Selecting the LEFT or RIGHT position allows fuel to feed only from the respective tank, while selecting the BOTH position feeds fuel from both tanks. 

The LEFT or RIGHT position may be used to balance the amount of fuel remaining in each wing tank.

Fuel Strainers, Sumps, Drains

After leaving the fuel tank and before it enters the carburetor, the fuel passes through a strainer that removes any moisture and other sediments in the system. 

Since these contaminants are heavier than aviation fuel, they settle in a sump at the bottom of the strainer assembly. 

A sump is a low point in a fuel system and/or fuel tank. 

The fuel system may contain a sump, a fuel strainer, and fuel tank drains, which may be collocated. 

The fuel strainer should be drained before each flight. 

Fuel samples should be drained and checked visually for water and contaminants.

Fuel Grades

Aviation gasoline (AVGAS) is identified by an octane or performance number (grade), which designates the antiknock value or knock resistance of the fuel mixture in the engine cylinder. 

The higher the grade of gasoline, the more pressure the fuel can withstand without detonating. 

Lower grades of fuel are used in lower-compression engines because these fuels ignite at a lower temperature. 

Higher grades are used in higher-compression engines because they ignite at higher temperatures, but not prematurely. 

If the proper grade of fuel is not available, use the next higher grade as a substitute. 

Never use a grade lower than recommended. 

This can cause the cylinder head temperature and engine oil temperature to exceed their normal operating ranges, which may result in detonation.

Heating System (PHAK C7)

There are many different types of aircraft heating systems that are available depending on the type of aircraft. 

Regardless of which type or the safety features that accompany them, it is always important to reference the specific aircraft operator’s manual and become knowledgeable about the heating system.

We will discuss the following: Fuel Fired Heaters

Exhaust Heating Systems

Combustion Heater Systems


Fuel Fired Heaters

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. 

A fan blows air into a combustion chamber, and a spark plug or ignition device lights the fuel-air mixture. 

A built-in safety switch prevents fuel from flowing unless the fan is working. 

Exhaust Heating System

Exhaust heating systems are the simplest type of aircraft heating system and are used on most light aircraft. 

Exhaust heating systems are used to route exhaust gases away from the engine and fuselage while reducing engine noise. 

The exhaust systems also serve as a heat source for the cabin and carburetor. 

The risks of operating an aircraft with a defective exhaust heating system include carbon monoxide poisoning, a decrease in engine performance, and an increased potential for fire.

Combustion Heater System

Combustion heaters or surface combustion heaters are often used to heat the cabin of larger, more expensive aircraft. 

This type of heater burns the aircraft’s fuel in a combustion chamber or tube to develop required heat, and the air flowing around the tube is heated and ducted to the cabin. 

A combustion heater is an airtight burner chamber with a stainless-steel jacket. 

Fuel from the aircraft fuel system is ignited and burns to provide heat. 

Ventilation air is forced over the airtight burn chamber picking up heat, which is then dispersed into the cabin area.

Electrical System (PHAK C7) Most aircraft are equipped with either a 14- or a 28-volt direct current (DC) electrical system.  A basic aircraft 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.

Generators 

Most DC generators do not produce a sufficient amount of electrical current at low engine rpm to operate the entire electrical system. 

During operations at low engine rpm, the electrical needs must be drawn from the battery, which can quickly be depleted.

Alternators

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 (AC), which is converted to DC. 

The electrical output of an alternator is more constant throughout a wide range of engine speeds.


Bus Bars

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. 

This simplifies the wiring system and provides a common point from which voltage can be distributed throughout the system.

Fuses and Circuit Breakers

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 aircraft to replace defective or blown fuses. 

Circuit breakers have the same function as a fuse but can be manually reset, rather than replaced

The Ammeter

An ammeter is used to monitor the performance of the aircraft 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. 

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.

The Loadmeter

Another electrical monitoring indicator is a loadmeter. 

This type of gauge 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.

Hydraulic System (PHAK C7)

The hydraulic system is often used on small airplanes to operate wheel brakes, retractable landing gear, and some constant speed propellers. A basic hydraulic system consists of: A reservoir 

Pump (either hand, electric, or engine-driven)

A filter to keep the fluid clean 

A selector valve to control the direction of flow

A relief valve to relieve excess pressure 

An actuator

The hydraulic fluid is pumped through the system to an actuator or 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. 

Servos can be either single-acting or double-acting, 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.

Landing Gear (PHAK C7)

The landing gear forms the principal support of an aircraft on the surface.

Tricycle Type Landing Gear There are three advantages to using tricycle landing gear: 

It allows more forceful application of the brakes during landings at high speeds without causing the aircraft to nose over. 


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


It tends to prevent ground looping (swerving) by providing more directional stability during ground operation since the aircraft’s center of gravity (CG) is forward of the main wheels. 


The forward CG keeps the airplane moving forward in a straight line rather than ground looping.

Tailwheel Landing Gear Tailwheel landing gear airplanes have two main wheels attached to the airframe ahead of its CG that support most of the weight of the structure. 

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. 

With the CG located behind the main landing gear, directional control using this type of landing gear is more difficult while on the ground. 

This is the main disadvantage of the tailwheel landing gear.

Fixed vs Retractable

Landing gear can also be classified as either fixed or retractable. 

Fixed landing gear always remains extended and has the advantage of simplicity combined with low maintenance. 

Retractable landing gear is designed to streamline the airplane by allowing the landing gear to be stowed inside the structure during cruising flight.

Anti-ice/Deice Systems (PHAK C7)

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

These 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.

Most light aircraft have only a heated pitot tube and are not certified for flight in icing. 

These light aircraft have limited cross-country capability in the cooler climates during late fall, winter, and early spring. 

Noncertificated aircraft must exit icing conditions immediately.

Deice Boots

Inflatable deicing 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.

Heated Wing

Heat provides one of the most effective methods for preventing ice accumulation on an airfoil. 

High performance turbine aircraft often direct hot air from the compressor section of the engine to the leading edge surfaces. 

The hot air heats the leading edge surfaces sufficiently to prevent the formation of ice.

Weeping Wing

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.

Propeller Anti-Ice

Propellers are protected from icing by the 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 drives the alcohol down the leading edge of the blade. 


The boots are also grooved to help direct the flow of alcohol. 


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 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.


FAA Sources Used for this Lesson

Pilot’s Handbook of Aeronautical Knowledge (PHAK) Chapter 7

Airplane Flying Handbook (AFH) Chapter 12


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