Aircraft engine controls
Aircraft engine controls provide a means for the pilot to control and monitor the operation of his aircraft's powerplant. This article describes controls used with a basic internal-combustion engine driving a propeller. Some optional or more advanced configurations are described at the end of the article. Jet turbine engines use different operating principles and have their own sets of controls and sensors.
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Basic Controls and Indicators
- Master Switch - Most often actually two separate switches, the Battery Master and the Alternator Master. The Battery Master activates a relay (sometimes called the battery contactor) which connects the output of the battery to the aircraft's main electrical bus. The alternator master activates the alternator by applying power to the alternator field circuit. These two switches provide electrical power to all the systems in the aircraft.
- Throttle - Sets the desired engine power level. The Throttle controls the volume of fuel/air mixture delivered to the cylinders.
- Pitch Control - Adjusts the Constant Speed Unit, which turn adjusts the propeller pitch & regulates the engine load as necessary to maintain the set R.P.M.
- Mixture Control - Sets the amount of fuel added to the intake airflow. At higher altitudes the air pressure (and therefor the oxygen level) declines so the fuel volume must also be reduced to give the correct air/fuel mixture. This process is known as "leaning".
- Ignition Switch - Activates the magnetos by opening the grounding or 'p-lead' circuit; with the p-lead ungrounded the magneto is free to send its high-voltage output to the spark plugs. In most aircraft the ignition switch also applies power to the starter motor during engine start. In piston aircraft engines, the battery does not generate the spark for combustion. This is accomplished using devices called magnetos. Magnetos are connected to the engine by gearing. When the crankshaft turns, it turns the magnetos which mechanically generate voltage for spark. In the event of an electrical failure, the engine will continue to run. The Ignition Switch has the following positions:
- Off - Both magnetos are connected to electrical ground.
- Left - The left magneto is connected to its spark plugs. The right magneto is grounded.
- Right - The right magneto is connected to its spark plugs. The left magneto is grounded.
- Both - Both magnetos are connected to their spark plugs. This is the normal operating configuration.
- Start - The pinion gear on the starter motor is engaged with the flywheel and the starter motor runs to turn the engine over.
- Tachometer - A gauge to indicate engine speed in revolutions per minute (R.P.M.). It is a measure of engine power in aircraft with “fixed pitch” propellers.
- Manifold Pressure Gauge – Used to measure the pressure in the induction manifold. It is a measure of engine power in aircraft using a controllable pitch propeller.
- Oil Temperature Gauge - Indicates the operating temperature of the engine.
- Oil Pressure Gauge - Indicates the supply pressure of the engine lubricant.
- Exaust Gas Temperature Gauge – Indicates the temperature of the exaust gas just after compustion. Used to set the air/fuel mixture (leaning) correctly.
- Carburetor Heat Control - Controls the application of heat to the carburetor venturii area to remove or prevent the formation of ice in the throat of the carburetor. Fuel injected engines do not have this control as there is no carburetor.
Fuel
- Fuel Primer Pump - A manual pump to add a small amount of fuel at the cylinder intakes to assist in starting a cold engine. Fuel injected engines do not have this control. For fuel injected engines, a fuel boost pump is used to prime the engine prior to start.
- Fuel Quantity Gauge - Indicates the amount of fuel remaining in the identified tank. One per fuel tank.
- Fuel Select Valve - Connects the fuel flow from the selected tank to the engine.
If the aircraft is equipped with a fuel pump:
- Fuel Pressure Gauge - Indicates the supply pressure of fuel to the carburetor (or in the case of a fuel injected engine, to the fuel controller.)
- Fuel Boost Pump Switch - Controls the operation of the auxiliary electric fuel pump to provide fuel to the engine before it starts or in case of failure of the engine powered fuel pump.
Propeller
If the aircraft is equipped with adjustable-pitch (constant-speed) propeller:
- Propeller Control - Sets the desired R.P.M. of the propeller governor, which turn regulates the engine load as necessary to maintain the set R.P.M. Once the R.P.M. is set by the pilot, the governor adjusts engine load by increasing or decreasing propeller pitch by using engine oil pressure to move a piston in the propeller hub.
- Manifold Pressure Gauge - Indicates the absolute pressure in the engine intake manifold, between the throttle and the cylinders.
Cowl
If the aircraft is equipped with adjustable Cowl Flaps:
- Cowl Flap Position Control - Cowl Flaps are opened during high power/low airspeed operations like takeoff to maximize the volume of cooling airflow over the engine's cooling fins. They are closed during normal cruise flight to maintain normal engine operating temperature and minimize drag.
- Cylinder Head Temperature Gauge - Indicates the temperature of the cylinder with the least cooling airflow. A Cylinder Head Temperature Gauge has a much shorter response time than the oil temperature gauge, so it can alert the pilot to a developing cooling issue more quickly. Engine overheating may be caused by:
- Running too long at a high power setting.
- Running too lean on the fuel mixture.
- Restricting the volume of cooling airflow too much.
- Insufficient delivery of lubricating oil to the engine's moving parts.
Related topics
References
- Private Pilot Manual; Jeppesen Sanderson; ISBN 0-88487-238-6 (hardcover, 1999)