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Meaning of a red arc on an engine instrument

Aviation Mechanic Powerplant (AMP) practice question 1 of 12 on Engine Instrument Systems. ACS element AM.III.D.K11 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONOn an engine instrument, what does a red arc tell the technician about the range it covers?

  1. AIt may be passed through but not used
  2. BIt is the normal continuous operating range
  3. CIt is a caution range for brief use
Show the answer and explanation

Correct answer: A. It may be passed through but not used

  • A — correct A red arc marks a dangerous operating range, generally an engine propeller vibration range, that can be passed through although the engine cannot be operated in it.
  • B — incorrect The normal and safe range of operation is shown by the green arc.
  • C — incorrect A caution range is shown by the yellow arc, which is placed on the outer circumference of the cover glass or dial face.

BACKGROUND

Instrument markings, ranges of operation, and minimum and maximum limits are interpreted the same way across engine instruments, using a system of three colors. A red line or mark indicates a point beyond which a dangerous operating condition exists, and it is located radially on the cover glass or dial face; of the two red markings, the line is the more common. A red arc indicates a dangerous operating range, generally an engine propeller vibration range, and although the arc can be passed through, the engine cannot be operated in that area. The yellow arc covers a given range of operation as a caution indication and is generally placed on the outer circumference of the cover glass or dial face. The green arc shows a normal and safe range of operation.

STUDY TIP

Separate the two red markings: a red line is a limit not to be exceeded, while a red arc is a band to transit quickly and never operate in.

KEY POINTS

  • A red line marks a point beyond which a dangerous condition exists.
  • A red arc marks a dangerous range, usually propeller vibration, that may be passed through only.
  • Yellow arc is caution, green arc is the normal safe operating range.

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.