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Emery cloth prohibited for seating brushes

Aviation Mechanic Powerplant (AMP) practice question 6 of 13 on Engine Electrical Systems. ACS element AM.III.F.K1 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONA mechanic needs to seat a generator brush that is not making good contact with the commutator. Why must emery cloth not be used for this job?

  1. AIt contains conductive material that causes arcing.
  2. BIt is too fine to shape the brush face correctly.
  3. CIt leaves an oily film on the commutator bar surfaces.
Show the answer and explanation

Correct answer: A. It contains conductive material that causes arcing.

  • A — correct The handbook warns that emery cloth and similar abrasives must never be used to seat brushes or smooth commutators because they contain conductive materials that cause arcing between the brushes and the commutator bars.
  • B — incorrect Fineness is not the objection; the recommended material is extra-fine 000 grit sandpaper or finer, so a fine abrasive is exactly what the procedure calls for.
  • C — incorrect Oil on a generator is traced to failed engine oil seals during inspection, not to the abrasive used for seating brushes.

BACKGROUND

Brushes that do not make good contact with the commutator or slip ring are seated by sliding a strip of extra-fine 000 grit or finer sandpaper under the brush with the rough side toward the carbon. The sandpaper is pulled only in the direction of armature rotation, and the brush is lifted when the paper is drawn back. Emery cloth and similar abrasives are prohibited for this work and for smoothing commutators because they contain conductive materials that cause arcing between brushes and commutator bars. After sanding, all carbon dust must be cleaned thoroughly from the generator, and the brushes are re-inspected after a short run to be sure no grit has embedded itself.

STUDY TIP

Tie the ban on emery cloth to one word, conductive, and remember that the approved abrasive is 000 grit sandpaper or finer.

KEY POINTS

  • Seat brushes with extra-fine 000 grit sandpaper or finer.
  • Sand only in the direction of armature rotation.
  • Emery cloth is conductive and causes arcing at the commutator.

SOURCES

  • amp-powerplant/ch04.txt "DC Generator Maintenance"
Practice the Aviation Mechanic Powerplant (AMP) test →Timed 100-question exam, by ACS area, or review your misses. Free.
MORE ON ENGINE ELECTRICAL SYSTEMS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.