Module 11A.6 — Electrical power (ATA 24)

Levels

11.6 is level 1 for A1 and level 3 for B1.1 — B1.1 candidates must understand operation well enough to troubleshoot. Topics: batteries, DC and AC generation, emergency power, voltage regulation, distribution, inverters/transformers/rectifiers, circuit protection, external power.

Typical power standards (transport aeroplanes)

Supply	Standard value
DC	28 V DC nominal (batteries 24 V nominal)
AC	115 V phase / 200 V line, 3-phase, 400 Hz constant frequency (or variable frequency about 360–800 Hz on newer types)
High-voltage DC (some new types)	±270 V DC

400 Hz allows smaller, lighter transformers and motors than 50/60 Hz.

Batteries

Type	Cell voltage	Notes
Lead-acid	2 V nominal (about 2.1 V charged)	Electrolyte sulphuric acid; state of charge by specific gravity (hydrometer; ~1.275–1.300 charged); never service in the same area/tools as NiCd
Nickel-cadmium (NiCd)	1.2 V nominal	Electrolyte potassium hydroxide — specific gravity does not change with charge, so state of charge can't be read by hydrometer; electrolyte level highest when fully charged; risk of thermal runaway (high charge current, high temperature, cell imbalance) — temperature sensors and monitoring; deep-cycling restores capacity (removes "memory")
Lithium-ion	~3.6–3.7 V	High energy density; requires battery management system, containment and venting because of thermal runaway risk

• A 24 V battery = 12 lead-acid cells or 19–20 NiCd cells.
• Capacity in ampere-hours (Ah); the battery must supply essential loads for a set time after generator loss.
• Installation: secure, ventilated box (hydrogen gas from charging), sump jar with neutraliser (sodium bicarbonate solution for lead-acid spills; boric acid solution for alkaline NiCd electrolyte, per AMM), quick-disconnect connectors, correct polarity.
• Battery charging on aircraft via battery charger (constant current/constant voltage) or TRU on the DC bus.

Key fact: A NiCd battery's state of charge cannot be determined from electrolyte specific gravity; check electrolyte level only when the battery is fully charged.