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Pressures compared by an EPR gauge

Aviation Mechanic Powerplant (AMP) practice question 9 of 12 on Engine Instrument Systems. ACS element AM.III.D.K7 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONWhat two pressures does a turbine engine pressure ratio (EPR) gauge compare?

  1. ACompressor discharge pressure and fuel pressure
  2. BExhaust pressure and inlet ram air pressure
  3. CBurner pressure and ambient air pressure
Show the answer and explanation

Correct answer: B. Exhaust pressure and inlet ram air pressure

  • A — incorrect Fuel pressure has its own gauge and is not one of the two pressures the ratio is built from.
  • B — correct The EPR gauge compares the total exhaust pressure with the pressure of the ram air at the inlet of the engine.
  • C — incorrect The comparison is made between the engine inlet and the engine exhaust, not against ambient air.

BACKGROUND

Turbine engines use an engine pressure ratio (EPR) indication to show the power being developed. The gauge compares the total exhaust pressure with the pressure of the ram air at the inlet of the engine, so it is a differential pressure gauge. With adjustments for temperature, altitude and other factors, the resulting ratio presents an indication of the thrust the engine is producing. EPR is a remote-sensing instrument: an engine pressure ratio transmitter contains a bellows arrangement that compares the two pressures and converts the ratio into an electric signal for the gauge, or a computer supplies the value in a digital display system. In a reciprocating engine aircraft, the comparable indication of power being developed is the manifold pressure gauge.

STUDY TIP

Anchor EPR as inlet versus exhaust: it is a ratio of two engine pressures, which is why it is classed as a differential pressure gauge.

KEY POINTS

  • EPR compares total exhaust pressure with inlet ram air pressure.
  • It is a differential pressure gauge indicating thrust developed.
  • A pressure ratio transmitter or a computer supplies the signal to the indicator.

SOURCES

  • FAA-H-8083-31B ch10 "Engine Pressure Ratio (EPR)"
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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.