Private Pilot | Lesson 3 - Aircraft Engines

TABLE OF CONTENTS:
3.1 AIRCRAFT IGNITION SYSTEMS
3.2 STARTING THE ENGINE
3.3 CARBURETOR ICING
3.4 CARBURETOR HEAT
3.5 ENGINE TEMPERATURE REGULATION
3.6 ABNORMAL COMBUSTION
3.7 FUEL/AIR MIXTURE
3.8 AVIATION FUEL AND ENGINE FUEL PUMPS
3.9 CONSTANT-SPEED PROPELLER

3.1 Aircraft Ignition Systems

Generally, all General Aviation aircraft have dual ignition systems. There are essentially two main benefits to having a dual ignition source system, the first benefit is the increased safety and reliability of the dual-source system - it allows for one source to fail completely while not losing the entire ignition system completely.

The other benefit of a dual ignition system is that it will provide improved engine performance.

However, it is critical that, in the event of a power failure after becoming airborne, the most important thing to do is to immediately establish and maintain the best glide airspeed.

DO NOT maintain altitude at the expense of airspeed or a stall/lose of control/spin could result.

3.2 Starting the Engine

For engine starts, it is best and safest to follow the manufacturer's recommend starting procedure - found in the aircraft's POH. After the engine starts, the throttle should be adjusted to the proper RPM and the engine gauges, especially the oil pressure, should be immediately checked to ensure proper engine start and fluid circulation.

When starting an airplane engine by hand (commonly called, "hand-propping"), it is extremely important that a competent pilot be at the controls of the aircraft.

3.3 Carburetor Icing

Most General Aviation training aircraft are equipped with carbureted engines. The operating principle of float-type carburetors is the difference in air pressure between the venturi throat and the air inlet. For this reason (the drop in pressure), carburetor-equipped engines are susceptible to induction icing.

Induction icing is the formation of ice in and around the venturi throat of the carburetor, restricting airflow into the carburetor, and, if left unchecked, finally rendering the carburetor useless as ice build-up completely seals the venturi throat. Carburetor ice is likely to form when outside air temperature is between 20°F and 70°F and there is visible moisture or high humidity.

The first indication of carburetor ice on airplanes with fixed-pitch propellers and float-type carburetors is a loss of RPM.

When carburetor heat is applied to eliminate carburetor ice in an airplane equipped with a fixed-pitch propeller, there will be a further decrease in RPM (due to the less dense hot air entering the engine) followed by a gradual increase in RPM as the ice melts. There may also be "pops" and "sputters" as the engine ingests the melting ice and water.

3.4 Carburetor Heat

Selecting Carburetor Heat allows warm to enter the fuel/air mixture flowing to the combustion chamber. Carburetor heat also enriches the fuel/air mixture, because warm air is less dense than cold air.

Since the warmer air has less density than the relatively cooler air that had previously been flowing to the engine, the fuel/air mixture (ratio) becomes richer in fuel since there is less air for the same amount of fuel.

Also, the application of carburetor heat will decrease engine power output (sometimes by as much as 15%) and increases operating temperature. However, the use of carburetor hear should be as a preventative measure to avoid the uncontrolled power loss that can occur through induction icing.

3.5 Engine Temperature Regulation

Most aircraft engines are cooled by the flow of air over the cylinders and, in large part, by circulating oil through the system to reduce friction and absorb heat from the internal engine parts.

Excessively high engine temperature either in the air or on the ground will cause loss of power, excessive oil consumption, and excessive wear on the internal engine.

Engine oil and cylinder head temperatures can exceed their normal operating range because of (among other causes):

• Operating with too much power
• Climbing too steeply, and at too low an airspeed, in hot weather
• Operating with too lean a mixture
• Using fuel that has a lower-than-specified octane rating
• The oil level being too low

Excessively high engine temperatures may be reduced by reversing any of the above situation, so, reducing power, climbing less steeply (increasing airspeed), enriching the mixture, or using a higher octane fuel, etc.

3.6 Abnormal Combustion

There are two main types of abnormal combustion that occur in piston powered general aviation aircraft:

Detonation and Pre-ignition.

Detonation occurs when the fuel/air mixture explodes instead of burning evenly. Detonation is usually caused by using a lower-than-specified grade (octane) of aviation fuel or by excessive engine temperature. Detonation causes many engine problems, including excessive wear and higher than normal operating temperatures.

If you suspect detonation during climbout after takeoff, lower the nose slightly - this will increase cooling and decrease the engine's workload.

Pre-ignition is the uncontrolled firing of the fuel/air charge in advance of the normal spark ignition. Pre-ignition is usually caused by a residual hot-spot on the cylinder wall, or by a small carbon deposit on a spark plug.

3.7 Fuel/Air Mixture

In order to maintain optimum engine performance, the fuel/air mixture must be monitored and adjusted while in flight. As you climb to higher altitudes, the fuel/air mixture must be leaned to decrease the fuel flow in order to keep the fuel/air mixture constant and compensate for the decreased air density. As you descend, you must enrich the fuel/air mixture in order to keep the mix constant and compensate for the returning air density.

If you descend from high altitudes to lower altitudes without enriching the mixture, the mixture will become leaner because the air is denser at lower altitudes. Failing to remember to enrich the mixture could lead to engine fuel starvation due to too lean a fuel/air mixture.

Similarly, when operating at a high-altitude airport, you may eliminate engine roughness by leaning the mixture - the elevation of the airport may be great enough that the factory "full rich" setting produces to rich a fuel/air mixture.

3.8 Aviation Fuel and Engine Fuel Pumps

Aviation gasoline, or AVGAS, is identified by an octane rating. So, 100LL gas is 100 octane Low Lead gasoline. When several grades of fuel are available, use of the next-higher-than-specified (octane) grade of fuel is better than using the next-lower-than-specified grade of fuel. This will prevent the possibility of detonation, or running the engine too hot. Also, filling the fuel tanks at the end of the day prevents moisture condensation by eliminating the airspace in the tanks.

In an airplane equipped with fuel pumps, the auxiliary electric fuel pump is used in the event the engine-driven fuel pump fails.

3.9 Constant-Speed Propeller

Most general aviation training aircraft have fixed-pitch propellers. However, many general aviation aircraft have constant-speed propeller systems. The advantage of a constant-speed propeller (or, a "controllable-pitch propeller") is that it permits the pilot to select the blade angle for the most efficient performance.

Constant-speed propeller airplanes have both throttle and propeller controls.

• The throttle controls power output, which is registered on the manifold pressure gauge.
• The propeller control regulates engine revolutions per minute (RPM), which are registered on the tachometer.

To avoid over-stressing cylinders, excessively high manifold pressure should not be used with low RPM settings.

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