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Purpose of the white slippage mark

Aviation Mechanic Powerplant (AMP) practice question 2 of 12 on Engine Instrument Systems. ACS element AM.III.D.R1 · 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 white radial line runs across the cover glass and case of an engine instrument. What does this mark allow a technician to detect?

  1. ADrift in the instrument calibration
  2. BLeakage past the pointer shaft seal
  3. CMovement of the glass on the case
Show the answer and explanation

Correct answer: C. Movement of the glass on the case

  • A — incorrect Calibration drift is found by periodic inspection and calibration of the instrument, not by an index mark on the glass.
  • B — incorrect The index mark says nothing about sealing; it only reveals relative motion between glass and case.
  • C — correct The white radial index mark, or slippage mark, shows any movement between the cover glass and the case.

BACKGROUND

Range and limit markings may be applied either to the dial face or to the cover glass of an engine instrument. When the markings appear on the cover glass, a white line is added as an index mark, often called a slippage mark, running across the joint between the glass and the case. If the glass rotates in the case, the two halves of the white line no longer align, and the technician knows the arcs and radial lines are no longer in their correct positions. That condition would cause mislocation of the other range and limit markings, so a reading taken against them could be wrong even though the instrument mechanism is sound. Instruments installed in aircraft, like those used in test cell procedures, are also inspected and calibrated periodically so that the information they give about engine operation stays accurate.

STUDY TIP

When a slippage mark is broken, treat every arc and radial line on that glass as suspect until the markings are repositioned.

KEY POINTS

  • A white radial index or slippage mark spans the cover glass and case.
  • Misalignment shows the glass has moved and the markings are mislocated.
  • Engine instruments are also inspected and calibrated periodically to stay accurate.

SOURCES

  • FAA-H-8083-32B ch10 "Engine Instruments"
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MORE ON ENGINE INSTRUMENT 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.