Aircraft General Knowledge: Airframe, Systems, Powerplant and Instruments
Aircraft General Knowledge is the paper that tests whether you understand the machine you fly: its structure, how its systems move and feed the engine, how the powerplant converts fuel into thrust, and what the flight instruments are really telling you. NCAA PPL questions in this domain are practical: they ask why a semi-monocoque fuselage carries load, what happens when the pitot head ices, or why a magneto check is run before take-off.
Airframe and flight controls
- A semi-monocoque fuselage uses skin to carry much of the load; formers and stringers stiffen it against buckling, while frames carry concentrated loads.
- Primary controls are ailerons, elevator and rudder; flaps and trim are secondary. The wing spar carries bending, ribs hold the aerofoil shape.
- Control surfaces are mass-balanced to delay flutter.
Systems and powerplant
- The four-stroke cycle is induction, compression, combustion and exhaust. Mixture is leaned using EGT.
- Magnetos are independent of the battery; the dead-cut check proves both are off.
- Carburettor icing can form between roughly 0 °C and 20 °C with high humidity; carb heat melts it but reduces power.
- A constant-speed propeller governor changes blade angle with oil pressure.
- Fuel sump drains remove water; the electric pump is used for take-off.
Instruments
- Pitot-static instruments are ASI, altimeter and VSI. A blocked static port affects all three.
- Gyro instruments: attitude indicator uses rigidity, heading indicator uses precession and needs periodic alignment, turn coordinator shows rate of turn.
- The magnetic compass suffers variation (geographic), deviation (aircraft) and dip errors.
Worked example. You climb at constant IAS with the pitot tube blocked by ice and the static port clear. Trapped pitot pressure stays fixed while static pressure falls with altitude, so the differential pressure rises: the ASI overreads and behaves like an altimeter. The trap is to increase speed; instead use attitude and GPS, apply pitot heat, and descend to warmer air.
Traps that cost marks: thinking a static blockage affects only the altimeter; mixing QNH (altitude) with QFE (height); leaving carb heat on for take-off; assuming the engine stops when the master switch is off; and forgetting that compass acceleration error is opposite in the Southern Hemisphere.