EASA PPL: Aircraft General Knowledge

Aircraft General Knowledge covers the airframe, powerplant, systems and flight instruments that a PPL pilot must understand operationally. The exam favours system purpose, indications and failure consequences over deep engineering.

Airframe and powerplant

• Light aircraft structures are commonly semi-monocoque; control surfaces are actuated by cables, rods or torque tubes.
• A four-stroke engine has induction, compression, combustion and exhaust strokes. The magnetos are independent ignition sources; both are selected for normal operation.
• A carburettor meters fuel using a venturi and mixture control. Carburettor icing can occur at moderate outside temperatures with high humidity, especially at low power; carburettor heat bypasses the filter and warms the intake.
• Fuel injection avoids carburettor icing but can suffer vapour lock; fuel quantity, grade and contamination checks remain critical.

Systems and instruments

• The electrical system typically uses an engine-driven alternator and a battery; the battery supports starting and emergency loads.
• Pitot-static instruments are the airspeed indicator, altimeter and vertical speed indicator. The altimeter is set to QNH for elevation, QFE for height above a reference, or 1013.25 hPa for flight levels.
• Gyroscopic instruments depend on rigidity and precession. The attitude indicator and heading indicator are usually vacuum-driven; the turn coordinator is electrically driven in many trainers.

Worked example

At an airfield with QNH 1000 hPa, the altimeter subscale is left at 1013 hPa. The subscale is 13 hPa too high, so the altimeter over-reads by about 13 x 30 ft = 390 ft. If the airfield elevation is 500 ft, the instrument indicates about 890 ft before take-off. After setting QNH, it should read 500 ft. The same logic explains why flying from high pressure to low pressure without updating QNH makes the aircraft lower than indicated.

Traps that cost marks

Carburettor icing is not limited to freezing weather. A blocked pitot tube does not simply freeze the ASI at the last value in every configuration; cross-check with altimeter and VSI and apply pitot heat. Do not confuse QFE with QNH. A vacuum failure affects the attitude and heading indicators, not the turn coordinator if that instrument is electric.

Key points

• Four-stroke cycle: induction, compression, combustion, exhaust; magnetos are independent.
• Carburettor icing is likely at low power with humid air, not only in cold weather.
• Pitot-static instruments: ASI, altimeter, VSI; altimeter uses QNH, QFE or 1013.25 hPa.
• Gyroscopic instruments rely on rigidity and precession; vacuum failure affects specific instruments.
• Altimeter over-reads when the subscale is set higher than actual pressure: about 30 ft per hPa.
• Electrical failure leaves the battery for essential loads; know which instruments remain available.