Aircraft General Knowledge — PPL(A)

Most light training aircraft use a horizontally-opposed, air-cooled, four-stroke piston engine. The four strokes are induction, compression, power, and exhaust. Understanding each stroke helps you understand why things go wrong.

• Induction: piston moves down, inlet valve opens, fuel/air mixture drawn in.
• Compression: both valves closed, piston moves up, mixture compressed.
• Power: spark plug fires, controlled explosion forces piston down — this is the only stroke that produces work.
• Exhaust: exhaust valve opens, piston moves up, burned gases expelled.

The magneto fires the spark plugs independently of the aircraft electrical system. Two magnetos per engine (left and right) provide redundancy. The ignition system requires NO battery power once the engine is running — this is why a piston engine can continue if the battery fails.

Carburettor icing is one of the most common causes of engine failure in light aircraft. It forms when the fuel/air mixture cools as it passes through the venturi, causing water vapour to freeze on internal surfaces.

• Can form in ambient temperatures between −10°C and +30°C — it does not need freezing conditions.
• Most likely at +10–20°C with high relative humidity, at low power settings.
• Indications on carburettor engines (fixed pitch prop): unexplained RPM loss.
• Indications on carburettor engines (constant speed prop): manifold pressure drop with no RPM change.
• Action: apply carburettor heat FULLY — expect a further RPM drop as ice melts, then RPM recovery.

The pitot-static system provides dynamic pressure (pitot) and static pressure (static) to three instruments: ASI, altimeter, and VSI. Understanding which source feeds which instrument, and what happens when each is blocked, is heavily tested.

• ASI: uses pitot and static. Pitot block: ASI reads zero (or locks if drain hole blocked). Static block: ASI over-reads in climb, under-reads in descent.
• Altimeter: uses static only. Static block: altimeter freezes at blockage altitude.
• VSI: uses static only. Static block: VSI reads zero.
• Alternate static source: provides static from the cockpit interior — slightly lower pressure, causing errors.

• Attitude Indicator (AI): suction-driven (or electric). Gyro rigidity maintains reference to horizon. Erects slowly after power is applied.
• Direction Indicator (DI / HI): suction-driven (or electric). Must be aligned to compass before flight and periodically during flight — precesses slowly.
• Turn Coordinator / Turn & Slip: electrically driven. Detects rate of turn (outer ring) and ball detects slip/skid.
• DI precession: 3°–5° per 15 minutes — reset regularly against compass on straight and level flight.

• AVGAS 100LL: dyed blue, used in most piston training aircraft.
• Mogas (ASTM D4814): may be permitted on some aircraft — check POH and EASA/CAA approval.
• Mixture control: rich mixture for take-off and climb (cooling), leaned at cruise altitude to improve economy and reduce fouling.
• Fuel primer: injects raw fuel into induction system for cold starting.
• Fuel contamination: check for water (settles to lowest point) using the fuel drain before every flight.
• Water in fuel: transparent globules at the bottom of the tester — drain until clear.