Aircraft Engine Controls

AIRCRAFT ENGINE CONTROL

Aircraft engine controls provide a means for the pilot to control and monitor the operation of the 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.

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.^[1]
• 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.

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. Some aircraft use a single gauge for all tanks, with a selector switch that can be turned to select the tank one wishes to have displayed on the shared gauge, including a setting to show the total fuel in all tanks. An example of switch settings could be "Left, Right, Fuselage, Total". This saves room on the instrument panel by negating the need for four different dedicated fuel gauges.
• 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. Some large airplanes have a fuel system that allows the flight crew to jettison or dump the fuel. When operated, the boost pumps in the fuel tanks pump the fuel to the dump chutes or jettison nozzles and overboard to atmosphere.

Propeller

If the aircraft is equipped with adjustable-pitch or constant-speed propeller(s):

• Propeller Control - Used to set the desired propeller speed. Once the pilot has set the desired propeller speed, the propeller governor maintains that propeller speed by adjusting the pitch of the propeller blades, using the engine's oil pressure to move a hydraulic piston in the propeller hub.
• Manifold Pressure Gauge - Indicates the (absolute) pressure in the engine's intake manifold. When the engine is running normally, there is a good correlation between the intake manifold pressure and the torque the engine is developing.

Cowl

Front view of open cowl flaps

Rear view of open cowl flaps

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.
• Cylinder Head Temperature Gauge - Indicates the temperature of all cylinder heads or on a single CHT system, the hottest head. 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:
1. Running too long at a high power setting.
2. Poor leaning technique.
3. Restricting the volume of cooling airflow too much.
4. Insufficient delivery of lubricating oil to the engine's moving parts.

FADEC

A full authority digital engine (or electronics) control (FADEC) is a system consisting of a digital computer, called an "electronic engine controller" (EEC) or "engine control unit" (ECU), and its related accessories that control all aspects of aircraft engine performance. FADECs have been produced for both piston engines and jet engines.

Function

True full authority digital engine controls have no form of manual override available, placing full authority over the operating parameters of the engine in the hands of the computer. If a total FADEC failure occurs, the engine fails. If the engine is controlled digitally and electronically but allows for manual override, it is considered solely an EEC or ECU. An EEC, though a component of a FADEC, is not by itself FADEC. When standing alone, the EEC makes all of the decisions until the pilot wishes to intervene.

FADEC works by receiving multiple input variables of the current flight condition including air density, throttle lever position, engine temperatures, engine pressures, and many other parameters. The inputs are received by the EEC and analyzed up to 70 times per second. Engine operating parameters such as fuel flow, stator vane position, bleed valve position, and others are computed from this data and applied as appropriate. FADEC also controls engine starting and restarting. The FADEC's basic purpose is to provide optimum engine efficiency for a given flight condition.

FADEC not only provides for efficient engine operation, it also allows the manufacturer to program engine limitations and receive engine health and maintenance reports. For example, to avoid exceeding a certain engine temperature, the FADEC can be programmed to automatically take the necessary measures without pilot intervention.

Safety

With the operation of the engines so heavily relying on automation, safety is a great concern. Redundancy is provided in the form of two or more, separate identical digital channels. Each channel may provide all engine functions without restriction. FADEC also monitors a variety of data coming from the engine subsystems and related aircraft systems, providing for fault tolerant engine control.

Engine control problems simultaneously causing loss of thrust on up to three engines have been cited as causal in the crash of an Airbus A400M aircraft at Seville Spain on 9 May 2015. Airbus Chief Strategy Officer Marwan Lahoud confirmed on 29 May that incorrectly installed engine control software caused the fatal crash. "There are no structural defects [with the aircraft], but we have a serious quality problem in the final assembly."

Applications

A typical civilian transport aircraft flight may illustrate the function of a FADEC. The flight crew first enters flight data such as wind conditions, runway length, or cruise altitude, into the flight management system (FMS). The FMS uses this data to calculate power settings for different phases of the flight. At takeoff, the flight crew advances the throttle to a predetermined setting, or opts for an auto-throttle takeoff if available. The FADECs now apply the calculated takeoff thrust setting by sending an electronic signal to the engines; there is no direct linkage to open fuel flow. This procedure can be repeated for any other phase of flight.

In flight, small changes in operation are constantly made to maintain efficiency. Maximum thrust is available for emergency situations if the throttle is advanced to full, but limitations can’t be exceeded; the flight crew has no means of manually overriding the FADEC.

Advantages

• Better fuel efficiency
• Automatic engine protection against out-of-tolerance operations
• Safer as the multiple channel FADEC computer provides redundancy in case of failure
• Care-free engine handling, with guaranteed thrust settings
• Ability to use single engine type for wide thrust requirements by just reprogramming the FADECs
• Provides semi-automatic engine starting
• Better systems integration with engine and aircraft systems
• Can provide engine long-term health monitoring and diagnostics
• Number of external and internal parameters used in the control processes increases by one order of magnitude
• Reduces the number of parameters to be monitored by flight crews
• Due to the high number of parameters monitored, the FADEC makes possible "Fault Tolerant Systems" (where a system can operate within required reliability and safety limitation with certain fault configurations)
• Saves weight

Disadvantages

• Full authority digital engine controls have no form of manual override available, placing full authority over the operating parameters of the engine in the hands of the computer.
  • If a total FADEC failure occurs, the engine fails.
  • Upon total FADEC failure, pilots have no manual controls for engine restart, throttle, or other functions.
  • Single point of failure risk can be mitigated with redundant FADECs (assuming that the failure is a random hardware failure and not the result of a design or manufacturing error, which may cause identical failures in all identical redundant components).
• High system complexity compared to hydromechanical, analogue or manual control systems
• High system development and validation effort due to the complexity
• Whereas in crisis (for example, imminent terrain contact), a non-Fadec engine can produce significantly more than its rated thrust, a FADEC engine will always operate within its limits.