Aeroplane Systems - Electrical Power, Avionics and Instruments
DC and AC generation
Light aeroplanes use 14 V or 28 V DC; larger types add 115 V AC at 400 Hz. A DC generator needs a reverse-current cut-out; an alternator uses rectifier diodes and is largely self-limiting. The voltage regulator holds bus voltage and overvoltage protection trips the field. Bus architecture (main, essential, avionics, split bus) decides what stays live after a failure. Lead-acid batteries fail by sulphation if left discharged and are checked by specific gravity; NiCad batteries use KOH electrolyte, can suffer thermal runaway and need capacity checks and cell balancing.
Instruments and avionics
The pitot-static group is altimeter, airspeed indicator and vertical speed indicator; blocked or leaking static lines cause errors, and a blocked static port makes the ASI behave like an altimeter. Altimeter and transponder tests are required at the intervals set in Part 43. Gyros are attitude, heading and turn indicators, now commonly replaced by AHRS and air data computers. Data buses include ARINC 429 (one-way, 32-bit words), ARINC 664/AFDX and CAN. Shields are normally grounded at one end to avoid ground loops.
Worked example
An alternator warning is on and the ammeter shows discharge. Reasoning: confirm the field breaker, then the regulator and drive belt or coupling, then alternator output. If output is present at the alternator but not at the bus, the fault is wiring or the sensing circuit; replacing the alternator would be guesswork.
Traps
Fitting 14 V components on a 28 V system; charging a NiCad with a lead-acid charger; assuming both ends of a shield must be grounded; blaming the instrument for a static leak.