12.8 and 12.15 Helicopter electrical power and lights

Scope

• 12.8 Electrical power (ATA 24) — A 1, B1.3 3: batteries; DC and AC generation; emergency generation; voltage regulation; circuit protection; power distribution; inverters, transformers, rectifiers; external/ground power. About 8–10 questions — level 3 means detailed knowledge, including troubleshooting logic.
• 12.15 Lights (ATA 33) — A 2, B1.3 3: external (navigation, landing, taxiing), internal (cabin, cockpit, cargo), emergency.

Typical helicopter power system

Most light and medium helicopters have a 28 V DC primary system: an engine-driven starter-generator (turbine) or alternator (piston), a battery, a voltage regulator/GCU and a DC bus structure. Larger helicopters add 115/200 V AC, 400 Hz, three-phase generators driven from the main gearbox (so AC is available whenever the rotor turns, even with one engine out), plus TRUs to produce DC.

Batteries

	Lead-acid	Nickel-cadmium (Ni-Cd)
Nominal cell voltage	about 2 V	about 1.2 V
Electrolyte	Sulphuric acid + water	Potassium hydroxide (alkaline)
State of charge check	Specific gravity (fully charged about 1.275–1.300)	Not by SG or open-circuit voltage — electrolyte level only correct when fully charged
Special hazards	Hydrogen gas, acid burns	Thermal runaway (rising temperature → falling internal resistance → higher charge current); needs temperature monitoring
Separate servicing	Lead-acid and Ni-Cd must be serviced in separate areas with separate tools — acid and alkali neutralise and contaminate each other

Lithium-ion batteries are increasingly used; they have integral battery management and containment because of thermal-runaway risk.

Installation points: vented battery box with drain, secure clamping, correct terminal torque. When removing a battery, disconnect the negative (earth) lead first and reconnect it last, so a spanner touching structure cannot short the live terminal.