Aircraft Systems (Private Pilot)

In this section, we will discuss all of the other systems on the airplane.

  • Under the Hood (Engine), Propeller

  • Oil system

  • Engine Cooling

 

Under the Hood (Engine)

In aviation aircraft engines are also known as, the power plant, no matter what type of fixed wing aircraft. There are different types of aircraft engines, from reciprocating, turboprop, turbojet and even turbofans. They all produce thrust that propels the aircraft, along with driving various systems to support the aircraft.

Depending on the type of airplane you are training in, a Cessna or a Piper aircraft along with most small aircraft, it is designed with a reciprocating (or piston) engine.

Reciprocating simples means the back and forth movement of the pistons.

The most common type of engine found in smaller aircraft today, is the horizontally opposed engine. They are designed with even number of cylinders, four or six cylinders, which are air cooled, and is a direct drive to the propeller.

  • Remember, always refer to the aircraft POH for the proper engine information.

    • Continental or Lycoming 4 cylinder engine

    • Air cooled

    • Cessna 172 is 160 Horse Power (HP), direct drive.

    • 2500 rpm

In small aircraft spark ignition four stroke engines are the most common found. This engines a build with cylinders, crankcase, and the accessory housing. Other parts of the engine are a part of the cylinder, such as the intake/exhaust valves, spark plugs, and the pistons.

The stroke operating cycle is:

  • Intake – Fuel/air mixture enters the cylinder

  • Compression –Piston moves up compressing the Fuel/air mixture

  • Power - Fuel/air mixture combustion from the spark plug firing.

  • Exhaust - burnt gases exiting the cylinders

 

Propellers:

Propellers are designed with either two or more blades, all attached at the center point at the central hub. The blades of the propeller are basically wings or airfoils, with cambers and chord lines. On a single engine aircraft the rotation of the propeller is clockwise, as seen from the pilots’ position.

 

The propeller provides thrust by pulling the airplane. The amount of thrust produced by the propeller depends on the airfoil shape and angle of attack of the propellers blades and the engines revolutions per minute (rpm).

Aircraft propellers are designed with the blade twisted. The highest pitch is located at the hub of the propeller. The blade angle becomes smaller as the pitch changes going towards the tip of the blade. Blade angle is measured in inches, which also determines pitch. Propellers are 74 inches in length and have a pitch of 48 inches. The 48 inches of pitch means; that when the propeller makes one full revolution it has moved forward 48 inches. 

So, why the twist? The reason is so the blade produces the same lift from the hub to the tip. While the propeller is spinning at any given rpm. With that said, the tip of the blade has to travel a much further distance than the root of the blade, when they are rotating.

On small aircraft, there are two types of propellers:

  • Fixed-Pitch Propellers - A Fixed-Pitch Propeller is designed with a fixed blade angles and the pitch is set by the manufacture of the propeller.

There are two types of fixed-pitch propellers, a climb and a cruise propeller. A climb prop has a lower pitch and produces less drag, where as a cruise prop has a high pitch and creates more drag.

The tachometer indicates engine power; it is read in hundreds of rpm. RPM is regulated by the throttle, which controls fuel/air flow to the engine.

  • Adjustable-Pitch Propeller - Also known as a Constant-Speed Propeller. The blade pitch can be adjusted during flight, while maintaining a constant rpm.

On aircraft that are equipped with a constant-speed propeller, the power is controlled by the throttle and is indicated on the manifold pressure gauge. The manifold pressure gauge measures pressure of the fuel/air mixture in the intake manifold.

When aircraft is parked and the engine is not operating the manifold pressure gauge will indicate ambient air pressure such as 29.92” Hg, this is another way on reading the pressure of the day, you verify it be listing to the ATIS.

Carburetors and Fuel injection:

Carburetors are a float-type chamber system, as air passes through the venturi chamber, the air speeds up and mixes with the vaporized fuel prior to entering the cylinders. This fuel/air mixture is then inducted into the cylinders where it is ignited by the spark plugs.

The throttle controls the amount of air inducted in the engine, while the mixture controls the amount fuel is mixed with the air.

Carburetor or induction icing is likely to form when: relative humidity above 80%, OAT is between -7˚C (20˚F) – 21˚C (70˚F).

Carburetor heat acts like an anti-icing system; it heats the air before

it inters the carburetor. It is also used to melt any ice that has formed

inside of the carburetor. When applying carburetor heat, the engine

power will decrease or run rough, about 15%. This is due to the

heated air being less dense than the outside air that was entering

the carburetor. 

Fuel injection systems are designed with a fuel pump to supply

fuel to the injectors in to each cylinder. Fuel injection is different

than carbureted systems; the fuel injection does not mix air with the

fuel.

The injectors spray fuel outside of the cylinder head at the intake manifold, which means that the fuel is vaporized and then mixed with the air prior to entering the cylinder.  With fuel injection system, aircraft use an electric-driven pump as well as a backup electric pump, to ensure the fuel continues to flow in case the engine-driven pump fails.

 

Oil System

The oil system performs many different functions:

  • Lubricates the engines moving parts.

  • Helps cool the internal parts of the engine

  • Removes heat from the cylinders

  • Carries away contaminates

There two types of oil systems for reciprocating engines, wet-sump or a dry-sump. A wet-sump design is when the oil is located an integral part of the engine. Wet-sump system use oil pumps to draw oil from a sump and sends it to the engine.

A dry-sump system is where the oil is located in an external or separate oil tank from the engine. Oil pumps are also used and recycles the oil, by pumping it from the tank to the engine and is return back to the oil tank.

The oil pressure gauge is indicated in pounds per square inch (PSI). The normal operating range is in the green are.

Always check the oil level by using the dipstick during the preflight operation. If the oil level is indicating low on the dipstick, simply add oil.

Always refer to the AFM/POH or any placards for proper oil type, weight, and the proper maximum and minimum oil quantity.

TIP:

Always make sure the airplane is parked on a level surface, to get an accurate oil dipstick indication.

 

Engine Cooling

Most, small aircraft are air cooled by the air intakes in the front of the cowling. Where others are liquid cooled.  On air-cooled airplanes, the outside airflow enters the large openings, baffles route and circulate the air over fin's attached to the cylinders and around the exterior of the cylinders and engine, before exiting out of the bottom through the cowl flaps.

Avoid operating an engine at higher than temperatures, this can cause a loss of power, oil consumption and will also lead to permanent damage such as scoring of the cylinder walls, damage to pistons and rings and burning of and warping the valves.

The following systems are a part of airframe, these include:

  • Fuel

  • Electrical

  • Hydraulic

  • Oxygen system

Fuel System   

The fuel system provides fuel under all conditions to the engine during all flight maneuvers, altitude, and attitude and power settings. There are two types of fuel systems in small aircraft, they are:

  • Gravity feed

  • Fuel-pump system

Gravity Feed:

This system simply uses the force of gravity to supply fuel from the tanks to the engine. The gravity feed systems is used in high-winged aircraft, such as Cessna’s. 

If for any reason the gravity feed system cannot be used do to design, then a fuel pump needs to be used.

Fuel-pump system:

The fuel-pump system is design with two pumps, one is the main engine driven pump and the other is an electrical driven auxiliary pump. An example is aircraft with low-wings, such as Piper aircraft, where the fuel tank is located in the wings below the carburetor, if equipped.

The auxiliary pump serves two purposes, one is for engine starting and the other is as a backup for, in case the engine driven pump fails.

Fuel Tanks:

The fuel tanks are normally inside of each wing of the airplane.

Each fuel tank is vented by a small hole in the fuel cap or by a

tube coming out of the wing, from the outside to maintain

atmospheric pressure in the tank. Each tank has an overflow

drain to allow the fuel to expand and does not cause damage to

the fuel tank.

TIP:

It is normal to see fuel coming from the overflow drain on hot days.

Fuel selector valve allows a selection of fuel from different tanks. The selector is located inside the aircraft and is operated by the pilot. There are four positions to select from on the fuel sector, “Left”, “Right”, “Both” and “Off”. When selected to ether “Left” or “Right” fuel feeds only from that tank selected. When selected to “Both”, fuel is flowing from both tanks, and when set to “Off”, no fuel is flowing.

Pilot’s need to be aware of fuel consumption and monitor each tank to ensure one tank does not run out of fuel. An empty tank can stop the engine from operating and cause an aircraft to be unbalanced.

Fuel gauges measure fuel in gallons or pounds, as well as the amount fuel in the tank.

TIP:

Never rely on fuel gages, they are only accurate when the read “empty”, during the preflight inspection always visually inspect the fuel level inside the tank.

Fuel types:

In aviation airplanes with reciprocating engines use aviation gasoline also known as AVGAS, it is identified by octane. Kind of like the gas that goes in your car.

Fuel (AVGAS) has a weight, 6 pounds per gallon and must be factored in, when doing weight and balance calculations.

AVGAS is classified in a few different types, AVGAS 80, 100, 100LL.  Grade 100 and 100LL is the same grade; the only difference is the ‘LL’, which means "Low Lead". For turbine engine aircraft, they use JETA, JETA-1, and JETB. Jet fuel smells like and is kerosene.

TIP:

Never use a fuel grade lower than recommended. It may cause the oil temperature, and cylinder head temperature to exceed their limits. 

Pre-ignition and detonation:

  • Pre-ignition - Is the ignition of fuel-air prior to the spark plug firing. Pre-ignition is caused by some other ignition source such as an overheated spark plug tip, carbon deposits in the combustion chamber, and, rarely, a burned exhaust valve.
     

  • Detonation - Is the spontaneous combustion of the end gas (remaining fuel/air mixture) in the chamber.

During every preflight inspection pilots should drain the fuel strainer and any fuel sumps in to a separated container and discard of it, to remove any dirt, water or contaminates. Be aware that while draining the sumps water may not appear right away.

  • Remember, water weighs more than fuel so water will bubble and settle to the bottom of the container. Water is a milky or cloudy looking bubble.

Draining the fuel sumps eliminates in water droplets lift in the fuel tanks or fuel lines. If water droplets are present in either, they may freeze and form ice crystals in flight, causing a blocking in fuel lines, screens, filters and can enter the carburetor causing carburetor icing.

 

Electrical system

In aviation, most airplanes electrical systems are built to either have a 14V (volt) or 28V direct current (DC) system.

The components of the electrical systems of aircraft are:

  • Alternator / generator

  • Master switch

  • Battery

  • Bus bar

  • Fuses / circuit breakers

  • Voltage regulator

  • Ammeter and associated electrical wiring.

Alternator and generators:

Alternators or generators are engine-driven components that supply electrical current to the aircraft electrical system; as well maintain an electrical change on the battery.

Alternators provide enough power to operate all the electrical systems even during slower engine speeds. The alternator produces alternating current (AC) current, which is converted into direct current (DC).  Whereas most DC generators do not produce enough current at lower engine speeds (rpm).

The Master Switch can be turned “On” or “Off” and supplies electrical power to operate multiple circuits except the ignition system. The multiple circuits are: exterior/interior lighting, radio equipment, turn indicator, fuel gauges, electric fuel pump, stall warning, pitot heat and starter.

Battery:

Electrical energy that is storied in the battery provides power to start the engine, as well as a limited source of electrical power to operator curtain systems in case the alternator or generator fails.

 

Aircraft batteries come in either a 12V (volt) or 24V.

Bus bar:

In the aircraft you are flying for training such as a Cessna 150 or 172, they are designed with two bus bars.  One bus bar is the primary bus and the other is the avionics bus.

Bus bars are used as a terminal to simplify and organize the connections of the main electrical system to the different equipment using electrical power. 

Fuses / circuit breakers:

Fuses and or circuit breakers are designed to protect electrical systems from overloads. Circuit breakers and fuses are very similar in how the work, an advantage to the circuit breaker is that they can be reset.

Placards are used to identify what equipment the circuit breaker is tired to. 

The number stamped on each circuit breaker is to identify the amperage limit.

TIP:

If a circuit break is tripped, it is okay to reset it after about a two minute cool down.

If the breaker trips a second time, it is a good idea not to rest it.

 

Hydraulic Systems

Hydraulic systems are used in aircraft to operate multiple external systems, depending on the complexity of the aircraft design. In small aircraft Hydraulics are used to operate the wheel brakes, retractable landing gear and some constant-speed propellers.

Large aircraft use hydraulics for flight control surfaces, flaps, spoilers, along with other systems.

Hydraulic systems can be a very basic system design, with a reservoir, pump, filters, selector valve, actuator, and a pressure relief valve.

 

Anti-Ice and Deice Systems

Anti-ice equipment is to prevent ice from forming, and deicing equipment is used to remove ice after it has formed. Anti-Ice and Deice Systems are used to protect the leading edge of the wings, pitot/static port openings, fuel tanks, propeller blades and many more components.

On smaller aircraft the only anti-Ice and deice System is the heated pitot tube which is electrically heated. This limits the aircraft in cooler climate areas during fall, winter, and spring months.

 

References

Private Pilot ACS PA.I.G.K1c, PA.I.G.K1e - PA.I.G.K1I

PHAK (CH.7)

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