FAA Aviation Mechanic - Powerplant (AMP) key terms

A calculated average pressure that would have to act on the piston throughout the power stroke to produce the measured torque; used to compare the work output of engines of different sizes.

The ratio of the total cylinder volume with the piston at bottom dead center to the volume remaining at top dead center; higher ratios generally yield more power and greater efficiency.

A small hydraulic unit in the valve train that uses engine oil pressure to automatically take up clearance as the engine warms and parts wear, keeping valve lash near zero.

The manufacturer-specified sequence in which the cylinders deliver their power strokes, chosen to balance loads on the crankshaft and smooth out engine operation.

A self-contained, engine-driven ignition device that generates its own high-voltage current without a battery, using a rotating magnet and coil to fire the spark plugs.

A spring-loaded device on a magneto that retards the spark timing and provides a hot spark at low cranking speeds, then becomes inoperative as engine speed rises.

The temperature of the exhaust gases measured at the inlet to the turbine section; a primary limit that protects turbine nozzles and blades from heat damage.

A disruption of smooth airflow through the compressor, often from too high an angle of attack or distorted inlet air, causing a bang, vibration, and possible flameout.

A turbine engine start in which combustion occurs but internal temperatures rise above safe limits, usually because too much fuel was introduced before adequate airflow.

A turbine start in which the engine lights off but fails to accelerate to self-sustaining idle speed, typically due to low starter power, weak ignition, or insufficient fuel.

Compressed air tapped from the compressor for uses such as cabin pressurization, anti-icing, and engine starting; excessive bleed reduces power available from the engine.

An unintended extinguishing of combustion in a turbine engine, resulting in loss of thrust; causes include fuel interruption, compressor stall, or flameout from heavy rain or icing.

Spool speeds of a turbine engine expressed as a percentage of design rpm; N1 is the low-pressure compressor or fan and N2 the high-pressure compressor, each with its own limits.

An internal visual inspection performed by inserting a flexible optical instrument through a spark plug hole or inspection port to examine cylinder walls, valves, and turbine blades without disassembly.

A check of each cylinder's ability to hold pressure, made by admitting air and reading the leakage; low readings indicate worn rings, valves, or head gaskets.

A mandatory directive issued by the FAA to correct an unsafe condition found in a product; compliance is required and must be documented in the maintenance records.

A notice from the manufacturer recommending inspections, modifications, or part replacements; unlike an AD, compliance is normally voluntary unless made mandatory by the FAA.

An instrument that displays engine rotational speed, typically as rpm or percent; for turbine engines it may be a dual indicator showing both N1 and N2 speeds.

An instrument that shows the absolute pressure in the induction system of a reciprocating engine, used with rpm to set power output and to detect induction leaks.

An instrument that displays exhaust gas temperature, typically using thermocouples; it helps set mixture, monitor engine health, and detect a lean or misfiring cylinder.

A fire detection device that senses the rate of temperature rise rather than a fixed value, producing a warning when a rapid increase indicates a localized fire.

A valve installed in fuel, oil, hydraulic, or bleed-air lines at the firewall that lets the crew cut off flammable fluid flow to the engine compartment in an emergency.

A halogenated extinguishing agent that stops combustion chemically by interrupting the chain reaction; it leaves no residue and is effective in concentrations safe for occupied spaces.

A device that controls the output of a generator or alternator by varying field current, keeping system voltage within limits as electrical load and engine speed change.

A heavy-duty electromagnetic switch that connects battery power to the starter motor when the cockpit start switch is engaged, keeping high current out of the flight deck.

A measure of a fluid's resistance to flow; oil viscosity must be low enough to circulate when cold and high enough to maintain a protective film when hot.

A system in which oil is stored in a separate tank and pumped to and from the engine by pressure and scavenge pumps, common on aircraft engines for reliable oil supply in all attitudes.

A spring-loaded valve that opens to bypass excess oil back to the sump or inlet, preventing dangerously high pressure when the oil is cold or the engine is at high rpm.

A narrowed section in the carburetor throat where airflow speeds up and pressure drops; this low pressure draws fuel out of the discharge nozzle into the airstream.

A carburetor that uses a float-operated needle valve to maintain a constant level of fuel in the bowl, feeding fuel through a metering jet into the venturi airflow.

The proportion by weight of fuel to air entering the engine; approximately 15 parts air to 1 part fuel is chemically correct, while richer or leaner mixtures change power and temperature.

A small pump in a carburetor that delivers a brief extra charge of fuel when the throttle is opened quickly, preventing a lean hesitation or stumble during acceleration.

An exhaust-driven compressor that packs more air into the induction system, allowing a reciprocating engine to maintain power at high altitude; it consists of a turbine and a compressor on one shaft.

A valve that controls how much exhaust gas bypasses the turbocharger turbine, thereby regulating the compressor speed and the manifold pressure delivered to the engine.

An adjustable opening in the engine cowling that controls the flow of cooling air around the cylinders, helping the pilot keep cylinder head temperatures within limits in all phases of flight.

A control unit that senses engine rpm and adjusts the pitch of a constant-speed propeller through oil pressure to hold the selected speed as load changes.

A propeller whose blade angle is automatically varied in flight to maintain a selected engine rpm, converting engine power into thrust efficiently over a wide range of speeds.

Rotating the blades of an inoperative engine's propeller to a high pitch, nearly edge-on to the airflow, so it stops windmilling and produces minimum drag.

The angle between the propeller blade's chord line and its plane of rotation; increasing it increases the load on the engine and the thrust produced at a given rpm.