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Alternator frequency tolerance

Aviation Mechanic Airframe (AMA) practice question 3 of 24 on Aircraft Electrical Systems. ACS element AM.II.K.K3 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONAn aircraft AC alternator should produce approximately 400 Hz. What is the lowest frequency that is still within 10 percent of that value?

  1. A390 Hz, which is 10 Hz below nominal
  2. B300 Hz, which is 100 Hz below the nominal value
  3. C360 Hz, which is 40 Hz below nominal
Show the answer and explanation

Correct answer: C. 360 Hz, which is 40 Hz below nominal

  • A — incorrect This treats the allowance as 10 Hz rather than 10 percent of 400 Hz.
  • B — incorrect A 100 Hz deviation is 25 percent of nominal, far outside the stated 10 percent allowance.
  • C — correct Ten percent of 400 Hz is 40 Hz, so the lowest frequency within tolerance is 400 minus 40, or 360 Hz.

BACKGROUND

All AC alternators must turn at a specific rpm to keep the frequency of the AC voltage within limits, and aircraft alternators should produce approximately 400 Hz. If the frequency strays more than 10 percent from that value, the electrical systems do not operate correctly. A constant-speed drive, or CSD, is used to hold the alternator at the correct speed so the frequency stays near 400 Hz even as engine rpm changes. When the CSD and the alternator are built into one unit, the assembly is called an integrated drive generator. An alternator driven directly by an APU that runs at constant rpm can hold 400 Hz without a CSD.

STUDY TIP

Work the percentage on 400 Hz before you look at the answers; 10 percent of 400 is 40 Hz, which sets the band at 360 to 440 Hz.

KEY POINTS

  • Aircraft AC alternators are designed for about 400 Hz.
  • More than 10 percent deviation upsets the electrical systems.
  • Ten percent of 400 Hz is 40 Hz, giving a low limit of 360 Hz.

SOURCES

  • sources/ama-airframe/ch09.txt "Alternator Drive"
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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.