Avionics and Electrical Systems
This domain tests basic electricity, aircraft wiring practice, and the avionics that depend on clean power and accurate static pressure.
Electrical fundamentals
- Ohm's law: V = I x R. Power: P = V x I. Series resistances add; parallel resistances use reciprocals.
- AWG numbers run opposite to size: a smaller number is a larger wire. Voltage drop rises with current and length, so long runs need larger wire.
- Circuit protection must be close to the bus, following AC 43.13-1B guidance, and sized to the wire, not the load alone.
- Bonding provides a low-resistance return path; shielding protects signal wires from interference and is normally grounded at one end.
Avionics
- Pitot pressure drives the airspeed indicator; static pressure drives the altimeter, VSI and airspeed indicator. A blocked static port leaves the altimeter frozen, the VSI at zero, and the ASI behaving like an altimeter.
- Vacuum or electrically driven gyros provide attitude and heading; solid-state AHRS and magnetometers replace them on newer aircraft.
- Transponders use 4,096 codes; Mode C reports pressure altitude referenced to 29.92 inHg. ELTs transmit on 406 MHz and 121.5 MHz.
Worked example
A 28 V bus feeds two identical 12 ohm loads in parallel. Parallel resistance = 6 ohm. Current = 28 / 6 = 4.67 A. Power = 28 x 4.67 = 130.7 W. A 5 A breaker protects the circuit; the wire must be sized for 4.67 A continuous with acceptable voltage drop.
Traps that cost marks
- Adding parallel resistances directly instead of using reciprocals.
- Sizing a breaker to the load but not the wire.
- Forgetting that 29.92 inHg, not local pressure, is used for Mode C.
- Grounding a shield at both ends and creating a ground loop.