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Meaning of 100 percent on a turbine tachometer

Aviation Mechanic Powerplant (AMP) practice question 5 of 12 on Engine Instrument Systems. ACS element AM.III.D.K3 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONOn a turbine engine tachometer calibrated in percent rpm, what does the 100 percent position on the indicator represent?

  1. AIdle rpm of the compressor section
  2. BMaximum rpm for engine operation
  3. CNormal cruise rpm at sea level
Show the answer and explanation

Correct answer: B. Maximum rpm for engine operation

  • A — incorrect The tachometer does indicate compressor speed, but the 100 percent mark is defined as the highest operating rpm, not an idle value.
  • B — correct The handbook defines the 100 percent position as the highest rpm at which the engine can operate.
  • C — incorrect No cruise value is assigned to the 100 percent mark, and the scale is not referenced to a particular altitude.

BACKGROUND

A tachometer for a reciprocating engine shows crankshaft revolutions per minute (rpm), calibrated in hundreds with graduations every 100 rpm. Turbine engines run at speeds high enough that percent rpm indicators are used instead, and each rotating assembly in the engine has its own percent rpm indicator. The tachometer indicates the speed of the compressor section. The 100 percent position is the highest rpm at which the engine can operate. Red lines and green arcs on these percent gauges have the same meanings as on reciprocating engine tachometers.

STUDY TIP

Remember that a turbine tachometer is a percentage of a reference speed, not an absolute rpm figure, and that 100 percent is the top of the permissible operating range.

KEY POINTS

  • Turbine tachometers are calibrated in percent rpm.
  • 100 percent is the highest rpm at which the engine can operate.
  • Each rotating assembly has its own percent rpm indicator.

SOURCES

  • FAA-H-8083-32B ch10 "Tachometer Indicator"
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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.