NCAA AME Powerplant Exam Practice Test & Question Bank cheat sheet

Otto cycle (constant-volume combustion) models the reciprocating engine; Brayton cycle (constant-pressure combustion) models the gas turbine.

Thermal efficiency of an ideal Otto cycle = 1 − 1/r^(k−1), where r is compression ratio and k is the specific heat ratio (≈1.4 for air).

Four-stroke cycle order: intake, compression, power (combustion), exhaust - two crankshaft revolutions per cycle; a four-stroke cylinder fires once every two revolutions.

Four-stroke power impulse duration ≈ 180° of crankshaft rotation; in a four-cylinder engine, power impulses overlap so one fires every 180°.

Thrust in a turbojet equals mass flow × (exhaust velocity − inlet velocity) plus the pressure-thrust term (Pe − Pamb) × Ae.

Horsepower = torque (lb-ft) × RPM ÷ 5252; brake mean effective pressure (BMEP) relates torque to displacement: BMEP (psi) = 150.8 × torque (lb-ft) ÷ displacement (in³).

Specific fuel consumption (SFC) = fuel flow ÷ power (piston) or ÷ thrust (turbine); lower SFC means better efficiency.

Detonation is uncontrolled explosive combustion after normal ignition; preignition is ignition before the spark - both raise cylinder temperature and can damage the engine.

Stroke = crankshaft throw × 2; displacement of one cylinder = π/4 × bore² × stroke; total displacement = that value × number of cylinders.

Compression ratio = (cylinder volume at BDC + clearance volume) ÷ clearance volume; higher ratio raises power and heat.

Horizontally opposed air-cooled engines dominate light aircraft; radial engines use an odd number of cylinders (typically 5, 7, or 9) so every other cylinder fires on alternate revolutions.

Valve clearance (lash) is measured cold between rocker arm and valve stem; insufficient clearance holds valves open and causes compression loss and burned valves.

Valve timing is set by camshaft lobe position and gear/chain alignment; a camshaft rotating at half crankshaft speed gives one valve cycle per two crank revolutions.

Firing order is arranged to balance loads and smooth torque; the Lycoming four-cylinder order is 1-3-2-4 (Continental four-cylinder is 1-3-2-4 as well; six-cylinder Continental is 1-4-5-2-3-6).

Piston rings: compression rings seal combustion gas; oil control ring scrapes oil from the cylinder wall - ring end gap and side clearance must be within limits.

Cylinder head and barrel are matched and torqued to specification; through-bolts and hold-down nuts must be torqued in the prescribed sequence to avoid distortion.

Magneto timing: the magneto must be internally timed (E-gap) before installation; E-gap is the point of maximum magnetic flux change and strongest spark.

Impulse coupling retards spark at low RPM for easy starting and automatically returns to normal timing above roughly 200-300 RPM.

Two magnetos provide redundancy; the right magneto typically fires the right-side spark plugs and the left magneto the left-side plugs - a dead magneto shows a large RPM drop on the corresponding check.

Carburetor heat bypasses the air filter and uses exhaust-heated air; it enriches the mixture and reduces power, so it is applied for ice and then removed.

Cooling: cylinder head temperature is the limiting factor; baffles and cowl flaps direct airflow - cowl flaps open in climb, closed in cruise/descent for drag and temperature control.

Dry-sump lubrication uses a separate oil tank, pressure pump, and scavenge pump; wet-sump stores oil in the crankcase - most aircraft use dry sump.

Oil pressure is regulated by a pressure-relief (bypass) valve; oil temperature is controlled by a vernatherm or oil cooler bypass valve.

Direct-crank vs. geared starters: check starter duty cycle limits and never engage a starter while the engine is running (sheared drive or damaged ring gear).

Float-type carburetor: venturi creates low pressure that draws fuel through the main metering jet; the float chamber maintains a constant fuel level relative to the venturi.

Mixture ratio by weight: stoichiometric gasoline ≈ 15:1 air-to-fuel; best power ≈ 12:1 to 13:1; best economy ≈ 16:1 to 18:1.

Idle and off-idle circuits supply fuel at low airflow; the accelerator pump enriches during rapid throttle opening; the economizer/power valve enriches at high power.

Carburetor icing is most likely at 20-70°F (−7 to 21°C) with high humidity; it can occur with the throttle partially closed even at above-freezing outside air temperature.

Fuel injection meters fuel by volume (or mass) directly to each cylinder; it avoids carburetor icing and gives more even mixture distribution.

Continuous-flow injection uses a fuel control unit, fuel/air control unit (servo), and flow divider; the mixture control adjusts fuel pressure relative to air metering.

Idle cutoff and mixture leaning: leaning increases exhaust gas temperature until peak EGT, then further leaning lowers EGT; peak EGT is the reference point for setting mixture.

Fuel grade and detonation: use the minimum octane specified in the POH; automotive fuel requires a supplemental type certificate or specific approval.

Geometric pitch is the theoretical forward distance per revolution; effective pitch accounts for slip, so effective pitch = geometric pitch − slip.

Blade angle is measured between the blade chord and the plane of rotation; higher blade angle (coarser) suits high speed, lower angle (finer) suits takeoff and climb.

Fixed-pitch propellers are matched to airframe/engine; a climb prop has lower pitch (higher RPM), a cruise prop has higher pitch (lower RPM).

Constant-speed propeller uses a governor with flyweights and a speeder spring: flyweights move out with increasing RPM and reduce blade angle to hold selected RPM.

Oil pressure to the propeller hub increases blade angle (coarsens) on most single-engine installations; loss of oil pressure drives blades to low pitch (high RPM) - a fail-safe toward high RPM.

Feathering rotates blades to zero thrust (edge-on to the relative wind) on multiengine aircraft; unfeathering accumulator stores oil pressure to drive blades out of feather.

Propeller unbalance and track: blade tracking checks tip path in the same plane; dynamic balance reduces vibration - check spinner and blade condition before balancing.

Propeller vibration: a wood propeller must not be operated in a restricted RPM range; check the POH for the range and never leave the throttle in that band.

Turbojet sections in order: inlet, compressor, diffuser, combustion chamber, turbine, exhaust nozzle; the turbine drives the compressor through the shaft.

Centrifugal compressor: air moves outward through the impeller, achieving one pressure rise per stage; axial compressor: air moves along the axis through alternating rotor and stator stages.

Compressor stall/surge: airflow separates from blades when back pressure rises or airflow is disturbed; symptoms include bang, rumble, and high EGT - reduce power and allow recovery.

Turbine inlet temperature (TIT) or exhaust gas temperature (EGT) is the limiting parameter; exceeding limits damages turbine blades and nozzles.

Combustion chamber types: can (individual cans), annular (single continuous ring), and can-annular (cans within an annular casing).

Turboprop uses a gas generator plus a reduction gearbox to drive the propeller; free-turbine designs let the power turbine turn independently of the gas generator.

Turbofan: fan driven by the low-pressure turbine; high bypass ratio increases propulsive efficiency and reduces specific fuel consumption.

Thrust reversers redirect fan or exhaust flow forward; they are used only on the ground and must be stowed before flight - check annunciation and interlocks.

Fuel system: engine-driven fuel pump, fuel control unit (FCU) or hydromechanical unit, fuel/oil heat exchanger, fuel nozzles, and fuel flow divider.

Fuel control schedules fuel flow against compressor speed, turbine temperature, and inlet conditions; the fuel shutoff valve is the primary means of stopping fuel flow.

Turbine lubrication is dry-sump: pressure pump, scavenge pumps, oil cooler, and chip detector; oil also cools bearings and seals.

Ignition: high-energy capacitor-discharge exciter and igniter plugs; typically two igniters, with one used for starting and both for continuous ignition in icing or turbulence.

Starting sequence: starter rotates the N1/N2 compressor to a minimum speed, ignition energizes, fuel is introduced, light-off occurs, then the starter disengages at self-sustaining speed.

Hot start: fuel introduced too early or too much fuel - EGT exceeds limits; hung start: engine fails to accelerate to idle; both require aborting the start and motoring to clear fuel.

Bleed air is used for cabin pressurization, anti-ice, and starter air (air-turbine starter); excessive bleed reduces engine power.

Active clearance control and cooling air: compressor bleed air cools turbine blades and controls tip clearance - verify bleed valve operation during inspection.

Compression check: differential test compares cylinder pressure to regulated air pressure; a cylinder reading below the manufacturer's minimum (commonly 60/80) or excessive leakage requires investigation.

Leakage location diagnosis: air from the oil filler = rings; air from the exhaust = exhaust valve; air from the intake = intake valve; air from the crankcase breather = rings or crankcase.

Oil analysis and spectrometric oil analysis program (SOAP) detect wear metals; a chip detector or magnetic plug with significant metal requires inspection.

Engine run-up: check magneto drop (typically 50-150 RPM per mag, max 175 RPM difference), carburetor heat drop, propeller cycle, and idle.

Cylinder head temperature and EGT limits are in the POH; leaning to peak EGT in cruise is a standard procedure - never exceed the engine's maximum continuous limits.

Turbine borescope inspection checks compressor and turbine blades for cracks, erosion, and foreign object damage without disassembly.

Torque and safetying: use calibrated torque wrenches, follow the maintenance manual, and safety wire or cotter pins per AC 43.13-1B practices.

Return to service requires an approved maintenance manual procedure, a logbook entry, and a signature with certificate number by the authorized person.

Engine mounts must allow controlled movement and isolate vibration; check for cracks, loose bolts, and deteriorated shock mounts at inspection.

Fire detection: thermocouple or resistance-type detectors in engine compartments; the detector circuit triggers a warning light and often an aural warning.

Fire extinguishing agents: Halon 1301 (or replacement agents) for engine compartments; CO2 and dry chemical are used in ground/hangar settings - never use water on an electrical or fuel fire.

Firewall and shutoff: firewall seals and stainless steel firewall protect the cabin; fuel, oil, and hydraulic shutoff valves are located for emergency isolation.

Fuel system installation: lines must be supported, fire-sleeved in fire zones, and routed away from exhaust and hot surfaces; check for chafing and leaks.

Electrical bonding and shielding: bonding straps carry static and fault current; shielded ignition leads reduce radio interference.

Exhaust systems: cracks, leaking gaskets, and loose clamps allow carbon monoxide into the cabin - inspect at every annual/100-hour.

Cowl and baffle installation affects cooling; missing baffle seals cause hot cylinders and reduced engine life.

14 CFR Part 43 defines maintenance, preventive maintenance, rebuilding, and alteration; Part 43 Appendix A lists preventive maintenance tasks an owner/pilot may perform.

Annual inspection (Part 91) is required every 12 calendar months; 100-hour inspection applies to aircraft used for hire or flight instruction (with exceptions for certain operations).

Airworthiness Directives (ADs) are mandatory; compliance must be recorded, and repetitive ADs must be tracked by time or calendar limit.

Maintenance records must include a description of work, date, signature, certificate number, and total time in service; major repairs and alterations require FAA Form 337.

Return to service: an A&P mechanic may approve for return to service after maintenance; an IA is required for annual inspections and major repairs/alterations.

Service difficulty reports (SDR) and malfunction reports document failures; the FAA uses them for fleet safety analysis.

Safety: lockout/tagout, fuel vapor control, proper grounding during fueling, and use of PPE when handling solvents and hydraulic fluid.

Weight and balance: any alteration affecting weight or balance requires recalculation and an updated equipment list and weight-and-balance record.