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Hot battery bus hazard

Aviation Mechanic Airframe (AMA) practice question 12 of 24 on Aircraft Electrical Systems. ACS element AM.II.K.R2 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONWhy does a hot battery bus remain energized when the battery switch is off?

  1. AIt is fed by the inverter at all times
  2. BIt is held on by the battery relay contacts
  3. CIt is connected directly to the battery
Show the answer and explanation

Correct answer: C. It is connected directly to the battery

  • A — incorrect Inverters supply the AC buses from DC power; they are not the source that keeps this DC bus alive.
  • B — incorrect The battery relay feeds the main battery bus when the battery switch is closed, which is not the hot bus path.
  • C — correct The hot battery bus has a direct connection to the aircraft battery and is always hot if a charged battery is installed.

BACKGROUND

Power distribution on a multiengine aircraft is arranged around several buses with different sources. The hot battery bus shows a direct connection to the aircraft battery, and this bus is always hot if there is a charged battery in the aircraft. Items powered by it include basics such as entry door lighting and the aircraft clock, along with loads critical to flight safety such as fire extinguishers, fuel shutoffs, and fuel pumps. During a massive system failure, the hot battery bus is the last bus on the aircraft that should fail. If the battery switch is closed and the battery relay activates, battery power is also connected to the main battery bus and the isolation bus.

STUDY TIP

Before working on any circuit, find out which bus feeds it; a hot battery bus circuit stays live with the battery switch off, so the battery itself has to be disconnected.

KEY POINTS

  • The hot battery bus connects directly to the battery.
  • It stays energized whenever a charged battery is installed.
  • It feeds loads that must never lose power, and fails last.

SOURCES

  • sources/ama-airframe/ch09.txt "Power Distribution on Multiengine Aircraft"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.