FAA Aviation Mechanic - Powerplant (AMP) cheat sheet

Total displacement = (π/4) × bore² × stroke × number of cylinders; bore and stroke are measured in inches on most U.S. engines.

Compression ratio = (displacement of one cylinder + clearance volume) ÷ clearance volume.

Four-stroke cycle order is intake, compression, power, exhaust; one full cycle requires two crankshaft revolutions and the camshaft turns once.

Firing order balances crankshaft loading; a common four-cylinder opposed engine fires 1-3-2-4, and a six-cylinder Continental fires 1-4-5-2-3-6.

Valve clearance is checked with the piston at top dead center on the compression stroke, between the rocker arm and valve stem tip; hydraulic lifter preload for a Lycoming is typically 0.028 to 0.060 inch.

Cylinder barrels are manufactured with choke (a slightly smaller bore at the top) so piston-to-wall clearance is correct after the hotter top end expands.

Crankshaft runout is measured with a dial indicator and is typically limited to about 0.002 to 0.005 inch total indicator reading.

Magneto-to-engine timing is set on the No. 1 cylinder at top dead center of the compression stroke, then the crank is rotated back to the specified number of degrees before top center.

The gas turbine operates on the Brayton cycle: compression, constant-pressure combustion, expansion through the turbine, and constant-pressure exhaust.

N1 is the low-pressure (gas producer) compressor speed; N2 is the high-pressure compressor speed in a twin-spool engine; both are read as percent RPM.

Thrust = mass flow × (exhaust velocity − inlet velocity), plus any pressure-thrust term when exhaust pressure exceeds ambient.

Turbofan bypass ratio = bypass airflow ÷ core airflow; high-bypass designs give better propulsive efficiency at subsonic speeds.

Turbine inlet temperature is the highest temperature in the engine and is limited by blade material capability; it is monitored as EGT, TIT, or ITT depending on probe location.

Compressor stall signs are low power, rising EGT, rumble, and backfiring; recovery is by reducing power, opening bleed valves, or reducing throttle to stabilize airflow.

Creep is permanent elongation of turbine blades from temperature plus centrifugal load, checked by measuring blade length against new-part limits.

Bleed air taken from the compressor supplies cabin pressurization, anti-icing, and engine starting, and its use changes the compressor's stall margin.

Differential (leakdown) compression test: regulated air at 80 psi is applied through the spark plug hole; a reading below the manufacturer's minimum (often 60/80) requires investigation.

Air escaping at the exhaust stack indicates an exhaust valve leak; at the air intake, an intake valve leak; at the crankcase breather or oil filler, piston ring leakage.

Spectrometric oil analysis tracks wear metals such as iron, aluminum, chromium, copper, and nickel to trend internal wear before failure.

Magnetic particle inspection reveals surface and near-surface defects in ferromagnetic parts; fluorescent penetrant is used on nonmagnetic materials.

Eddy current testing detects surface cracks in bores and bolt holes; ultrasonic testing detects internal voids and flaws.

A 100-hour inspection is required for aircraft carrying persons for hire; other aircraft require an annual inspection under 14 CFR 91.409.

All airworthiness directives applicable to the engine must be complied with and the compliance recorded in the maintenance records.

Torque wrench accuracy and the use of extensions or adapters affect applied torque; an extension changes the effective lever arm and requires recalculation.

Cylinder head temperature and exhaust gas temperature are sensed by thermocouples; a two-metal junction generates a voltage proportional to temperature difference.

Manifold pressure is an absolute pressure in inches of mercury; values above 30 in Hg at sea level indicate turbocharger boost.

Standard range markings are green for normal operation, yellow for caution, red for maximum limits, and white for flap operating range.

A tachometer indicates engine or propeller RPM; in multi-engine aircraft a synchroscope is used to match propeller speeds.

Oil pressure may be indicated by a direct-reading Bourdon tube gauge or electrically by a pressure transducer and indicator.

Fuel flow indicators show gallons per hour or pounds per hour; turbine engines use mass flow because fuel density changes with temperature.

Turbine inlet temperature probes are located between the combustor outlet and the turbine inlet, the hottest measurable station.

Torque-indicating systems on turboprop engines display torque or equivalent shaft horsepower derived from engine reduction gearbox pressure or strain.

Continuous-loop detectors change resistance when heated and trigger a warning; a short or open circuit in the loop also produces a fire warning.

Thermistor and thermocouple spot detectors are used for localized overheat or fire detection in specific engine zones.

Pulling the engine fire handle closes the fuel shutoff valve, closes the bleed air valve, arms the extinguisher squibs, and discharges the agent.

Halon 1301 and Halon 1211 extinguish fire by interrupting the combustion chain reaction and leave no residue; Halon has been phased out for environmental reasons.

Carbon dioxide and dry chemical agents are not used in aircraft engine compartments in flight because of asphyxiation risk and residue damage.

Fire extinguisher bottles are checked for weight and pressure; the hydrostatic test date must be within the required interval.

Firewall shutoff valves stop fuel, hydraulic fluid, and bleed air at the firewall, isolating the engine compartment from the rest of the aircraft.

A starter is a series-wound DC motor that draws high current and produces high starting torque; the starter solenoid or relay connects battery power to it.

An alternator uses a rotating field and rectifier diodes to produce DC; a generator uses a rotating armature and commutator with brushes.

The voltage regulator controls field current to hold system voltage within limits; an overvoltage relay opens the field circuit if voltage exceeds a set value.

A starter-generator performs both cranking and generating functions and requires a cutout relay to prevent battery discharge through the unit after start.

The ignition switch stops a reciprocating engine by grounding the magneto primary circuit through the P-lead; a broken P-lead leaves the magneto hot.

Wire size is selected from current-carrying capacity and allowable voltage drop using the charts in 14 CFR 43.13-1B, accounting for bundling and temperature.

Engine-to-airframe bonding provides a low-resistance return path for starter current and prevents static discharge damage.

A wet-sump system stores oil in the engine sump; a dry-sump system uses an external tank and separate pressure and scavenge pumps.

The oil pressure relief valve bypasses excess oil when pressure exceeds a set value, limiting maximum system pressure and protecting the cooler and lines.

A full-flow oil filter incorporates a bypass valve that opens when the element is clogged, allowing unfiltered oil to reach the engine rather than starving it.

Straight mineral oil is used during break-in so rings seat; ashless dispersant oil is used afterward because additives hold contaminants in suspension.

Oil viscosity grades such as 15W-50 use the W to denote a winter (cold-flow) rating; the second number is the viscosity at operating temperature.

Oil dilution with a small percentage of gasoline thins oil for cold-weather starting in reciprocating engines and must be limited per the manufacturer.

Turbine engines use synthetic oil (MIL-PRF-23699) and may use a hot-tank or cold-tank system; a chip detector warns of metallic debris in the oil.

Oil consumption and pressure are checked after engine shutdown when the oil has drained to the sump, giving the most repeatable reading.

A magneto is a self-contained engine-driven high-voltage source containing a rotating magnet, primary coil, breaker points, condenser, and distributor.

The condenser suppresses arcing across the breaker points and speeds the collapse of the primary magnetic field, increasing secondary voltage.

An impulse coupling retards spark timing and delivers a hot spark during cranking, then disengages above a set RPM so normal timing resumes.

Magneto internal timing (E-gap) is set at the point of maximum primary current, when the breaker points just begin to open.

Spark plug gap for typical reciprocating aircraft engines is about 0.015 to 0.022 inch; fine-wire plugs wear more slowly than massive-electrode plugs.

During a magneto check the RPM drop on either magneto is limited (commonly 150 RPM maximum) and the difference between magnetos is also limited (commonly 50 RPM).

Turbine engines use high-energy exciter units and igniter plugs; ignition is used for starting and relight, not continuously in normal operation.

A float carburetor maintains a constant fuel level in the float chamber with a needle valve, and metering is accomplished by the main jet and venturi pressure drop.

The mixture control in a float carburetor moves a needle valve; idle cutoff shuts off fuel flow completely to stop the engine.

Carburetor icing is most likely with outside air temperatures between about 20 degrees F and 70 degrees F with high relative humidity; carburetor heat is the remedy.

Continuous-flow fuel injection uses an engine-driven pump, a fuel/air control unit, a distribution manifold valve, and nozzle lines to each cylinder.

Turbine fuel control units meter fuel as a function of throttle position, compressor speed, and other parameters and may be hydromechanical or FADEC-controlled.

A rich mixture at high density altitude causes rough running, plug fouling, and reduced power; leaning restores the correct fuel/air ratio.

Idle speed and idle mixture are adjusted together; changing one affects the other, so the procedure is repeated until both are within limits.

A turbocharger uses exhaust gas energy to drive a turbine that turns a compressor, raising intake manifold pressure above ambient (boost).

The wastegate bypasses exhaust gas around the turbine; closing it increases turbine speed and manifold pressure, opening it decreases boost.

Critical altitude is the highest altitude at which the engine can maintain its rated manifold pressure with the wastegate fully closed.

Overboost protection includes the wastegate controller, density controller, and an overboost valve that limits manifold pressure.

Cylinder cooling depends on baffles that force air through the fins; missing or damaged baffles cause high cylinder head temperatures and uneven cooling.

Cowl flaps control the exit area for cooling air; opening them increases airflow and cooling but also increases drag.

Exhaust mufflers often contain a heat exchanger that supplies cabin heat; a cracked muffler can admit carbon monoxide into the cabin.

Fuel system vents prevent tank collapse and vapor lock, and fuel strainer drains are checked for water and sediment during preflight.

Blade angle is the angle between the blade chord and the plane of rotation; geometric pitch is the distance the propeller would advance in one revolution if it moved through a solid medium.

A constant-speed propeller uses a governor with flyweights and a speeder spring to control oil pressure to the propeller, changing blade angle to hold selected RPM.

In a single-acting (counterweight) propeller, governor oil pressure plus centrifugal counterweights oppose each other to set blade angle.

Feathering rotates the blades to approximately zero thrust (edge-on to the airflow) to reduce drag from a failed engine on a multi-engine aircraft.

Propeller track is checked by rotating each blade past a fixed reference point; out-of-track causes vibration and is corrected within manufacturer limits.

Static balance corrects the propeller as a unit on a balancer; dynamic balance is performed on the aircraft to reduce vibration in operation.

Nick, dent, and scratch limits are published by the manufacturer; blending must stay within the allowed removal depth and area.

Composite propeller blades require a lightning protection strip and separate inspection criteria from metal blades.