Aircraft Electrical Power Generation and Distribution
Aircraft electrical power generation and distribution
This domain covers how aircraft generate, regulate, and distribute electrical power: DC generators and alternators, AC generators, constant-speed drives (CSDs), integrated drive generators (IDGs), variable-frequency (VF) generators, transformer-rectifier units, batteries, external and ground power, APU generators, bus ties, and essential or emergency buses. The exam tests system architecture, protection, and load-sharing logic.
Core rules
- A DC generator uses a commutator; a DC alternator rectifies AC internally or externally. Both need voltage regulation. Reverse-current protection prevents a battery from driving the generator.
- AC generators need constant frequency or must accept wild frequency. A CSD or IDG holds generator speed; a VF system lets frequency vary with engine speed and uses equipment that tolerates it.
- Generator control units (GCUs) regulate voltage, provide over-voltage, under-voltage, and over-frequency protection, and control bus connection. Differential protection compares current at both ends of a feeder.
- Bus architecture typically includes main, essential, and emergency buses. Load shedding removes non-essential loads to protect the remaining source. Bus ties allow transfer but should not parallel incompatible sources.
- Ground power must be checked for correct voltage, polarity, and frequency before connection. A no-break power transfer needs the sources to be synchronised.
Worked example
A 28 V DC system has a 100 A generator and a 70 A continuous load. The generator has 30 A margin for battery charging and transients. If a 40 A load is added, total demand is 110 A, so the generator is overloaded. The correct action is to shed a non-essential load or use ground power, not to increase the generator protection setting. The 100 A rating is continuous; transient capability is limited by the GCU and the generator's thermal limits.
Traps
Assuming an IDG can be paralleled without synchronising; mixing 115 V/400 Hz and 28 V DC limits; forgetting reverse-current protection; and treating a bus tie as a load-sharing device without checking source compatibility.