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Using external power on the aircraft

Aviation Mechanic Airframe (AMA) practice question 11 of 24 on Aircraft Electrical Systems. ACS element AM.II.K.R1 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONBefore external power can start the engine or power the aircraft electrical loads, what condition must exist?

  1. AThe battery must be disconnected first
  2. BThe battery master switch must be closed
  3. CThe alternator master switch must be closed
Show the answer and explanation

Correct answer: B. The battery master switch must be closed

  • A — incorrect External power can charge the aircraft battery, so the battery is left connected rather than removed from the circuit.
  • B — correct For external power to start the engine or power electrical loads, the battery master switch must be closed.
  • C — incorrect The alternator master switch controls the alternator field circuit, not the path from the external power receptacle to the bus.

BACKGROUND

Many aircraft have an external power circuit that connects a ground power source to the aircraft, which allows engine starting and maintenance work without discharging the battery. The system typically consists of a plug in the fuselage, an electrical solenoid that connects external power to the bus, and the related wiring. The external power receptacle connects to the external power solenoid through a reverse polarity diode, which prevents an accidental reversed connection from reaching the aircraft systems. External power can be used to charge the aircraft battery or to power the aircraft electrical loads. For external power to start the engine or power those loads, the battery master switch must be closed.

STUDY TIP

Follow the current path from the receptacle through the solenoid to the bus, and remember the master switch is what ties the bus to the rest of the aircraft.

KEY POINTS

  • External power reaches the bus through a solenoid and a reverse polarity diode.
  • The battery master switch must be closed for external power to feed loads.
  • External power can also be used to charge the aircraft battery.

SOURCES

  • sources/ama-airframe/ch09.txt "External Power Circuit"
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MORE ON AIRCRAFT ELECTRICAL SYSTEMS

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.