Aircraft Engine Controls - Basic Controls and Indicators

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 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 power level. The throttle controls the mass flow-rate of air (in fuel-injected engines) or air/fuel mixture (in carburetted engines) delivered to the cylinders.
• Propeller Control - Adjusts the Constant Speed Unit, which in turn adjusts the propeller pitch and 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 therefore 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:
  1. Off - Both magneto p-leads are connected to electrical ground. This disables both magnetos, no spark is produced.
  2. Right - The left magneto p-lead is grounded, and the right is open. This disables the left magneto and enables the right magneto only.
  3. Left - The right magneto p-lead is grounded, and the left is open. This disables the right magneto and enables the left magneto only.
  4. Both - This is the normal operating configuration, both p-leads are open enabling both magnetos.
  5. Start - The pinion gear on the starter motor is engaged with the flywheel and the starter motor runs to turn the engine over. In most cases, only the left magneto is active (the right p-lead is grounded) due to timing differences between the magnetos at low RPMs.
• Tachometer - A gauge to indicate engine speed in revolutions per minute (RPM) or percentage of maximum.
• Manifold Pressure (MP) Gauge - Indicates the absolute pressure in the intake manifold.
• Oil Temperature Gauge - Indicates the engine oil temperature.
• Oil Pressure Gauge - Indicates the supply pressure of the engine lubricant.
• Exhaust Gas Temperature (EGT) Gauge - Indicates the temperature of the exhaust gas just after combustion. Used to set the fuel/air mixture (leaning) correctly.
• Cylinder Head Temperature (CHT) Gauge - Indicates the temperature of at least one of the cylinder heads. Used to set the fuel/air mixture.
• Carburetor Heat Control - Controls the application of heat to the carburetor venturi area to remove or prevent the formation of ice in the throat of the carburetor as well as bypassing the air filter in case of impact icing.
• Alternate Air - Bypasses the air filter on a fuel-injected engine.