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Absolute pressure gauge on a stopped engine

Aviation Mechanic General (AMG) practice question 16 of 20 on Physics for Aviation. ACS element AM.I.J.K3 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONA mechanic connects an absolute pressure (psia) gauge to the oil system of a piston engine that is not running. On a standard day at sea level, what will the gauge indicate?

  1. AZero, because the oil pump is not turning
  2. BApproximately 29.92 psi
  3. CApproximately 14.7 psi
Show the answer and explanation

Correct answer: C. Approximately 14.7 psi

  • A — incorrect A gauge pressure (psig) instrument would read zero with the pump stopped. An absolute gauge includes atmospheric pressure in its reading and does not drop to zero.
  • B — incorrect 29.92 is standard atmospheric pressure expressed in inches of mercury, not psi. In psi the standard value is 14.7.
  • C — correct Absolute pressure equals gauge pressure plus atmospheric pressure. With the pump not turning there is no gauge pressure, so a psia instrument reads atmospheric pressure, 14.7 psi on a standard day.

BACKGROUND

Gauge pressure (psig) is a reading that shows pressure over and above ambient; an oil, fuel, or hydraulic pressure gauge reads zero when its pump is not turning. Absolute pressure (psia) includes atmospheric pressure in the reading, so absolute pressure equals gauge pressure plus atmospheric pressure. If a psia instrument were connected to an engine oil system, it would read atmospheric pressure with the engine stopped, which is why psia gauges are not used for that purpose. Absolute gauges do make sense for manifold pressure, which can range from below to above atmospheric. Standard day atmospheric pressure is 14.7 psi, 29.92 inches of mercury, 760 millimeters of mercury, or 1013.2 millibars.

STUDY TIP

Remember that psia = psig + atmospheric pressure, and that only a psig gauge reads zero when the system is unpressurized.

KEY POINTS

  • Absolute pressure = gauge pressure + atmospheric pressure
  • A psia gauge on a stopped system reads atmospheric pressure
  • Standard sea level pressure is 14.7 psi or 29.92 in Hg

SOURCES

  • FAA-H-8083-30B ch05 "Gauge Pressure"
  • FAA-H-8083-30B ch05 "Absolute Pressure"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-30B Aviation Maintenance Technician Handbook – General, 14 CFR part 43 and part 65. Regulations and handbooks are revised; confirm details against the current edition.