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Blocked output of a nonpositive-displacement fuel pump

Aviation Mechanic Powerplant (AMP) practice question 18 of 20 on Engine Fuel and Fuel Metering Systems. ACS element AM.III.I.K13 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONThe output port of a nonpositive-displacement fuel pump becomes blocked while the pump is being driven. What happens?

  1. AThe pressure relief valve opens at once
  2. BThe pumping element stops turning
  3. CPressure rises and flow drops to zero
Show the answer and explanation

Correct answer: C. Pressure rises and flow drops to zero

  • A — incorrect That is what happens in a positive displacement pump, where negligible slippage causes pressure to increase instantaneously until the relief valve opens.
  • B — incorrect The pumping element continues moving; it is internal slippage, not a stopped element, that causes flow to cease.
  • C — correct Without a positive internal seal against slippage, pressure rises and output falls to zero while the pumping element keeps moving, because fuel slips back inside the pump.

BACKGROUND

Main fuel pumps must deliver a continuous supply of fuel at the proper pressure at all times during engine operation. Engine-driven pumps are described as either nonpositive-displacement or positive displacement, and which type is used depends on where in the fuel system it sits. A nonpositive-displacement pump, such as a centrifugal type, produces continuous flow but has no positive internal seal against slippage, so its output varies considerably as pressure varies. If its output port is blocked, pressure rises and output decreases to zero even though the pumping element keeps moving. In a positive displacement pump, slippage is negligible, so blocking the output makes pressure increase instantaneously until the pressure relief valve opens.

STUDY TIP

Decide first which kind of pump the question describes, then apply the slippage rule. Centrifugal boost stages slip and stall out, while gear main stages build pressure until the relief valve lifts.

KEY POINTS

  • Nonpositive-displacement pumps have no internal seal against slippage, so output varies with pressure.
  • Blocking the outlet of such a pump raises pressure and drops flow to zero while the element keeps turning.
  • A positive displacement pump instead builds pressure instantly until its relief valve opens.

SOURCES

  • amp-powerplant/ch02.txt "Main Fuel Pumps (Engine Driven)"
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MORE ON ENGINE FUEL AND FUEL METERING 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.