Electrical and Electronic Systems, Instruments and Avionics
Power, distribution and bonding
Aircraft DC systems usually use a 28 V bus, with batteries for starting and emergency power and alternators/generators for normal supply. Power = volts × amps. Circuit breakers protect wiring; bonding gives a low-resistance return and lightning path; shielding controls electromagnetic interference. Avionics need stable power and cooling. Redundancy may be dual buses, dual channels or standby instruments.
Instruments and avionics fundamentals
Pitot-static instruments: airspeed uses pitot and static; altimeter uses static; VSI uses static. A blocked pitot with open static makes airspeed act like an altimeter; a blocked static makes airspeed, altitude and VSI unreliable. Gyroscopic instruments use rigidity and precession; AHRS uses solid-state sensors. EFIS displays ADC and AHRS data; a disagreement requires cross-check with standby instruments.
Worked example
A 28 V alternator rated 70 A can supply 28 × 70 = 1,960 W. If the continuous load is 45 A, the margin is 70 − 45 = 25 A (700 W). After adding a 10 A avionics load, the load is 55 A; margin is 15 A. That is acceptable only if the alternator rating is valid at the actual temperature and rpm, and if the battery is not required to carry part of the load.
Traps
Do not use battery voltage to prove alternator output; do not confuse a circuit breaker with a fuse; do not troubleshoot a static leak by adjusting the altimeter; and do not assume a single EFIS display is sufficient after a sensor disagree.