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Tank selection in a pump-feed single-engine system

Aviation Mechanic Airframe (AMA) practice question 5 of 12 on Aircraft Fuel Systems. ACS element AM.II.J.K3 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONIn a single-engine pump-feed fuel system, why is fuel not drawn from both wing tanks at the same time?

  1. AThe pump would draw air from an empty tank
  2. BThe engine-driven pump has only one inlet port
  3. CFuel would siphon from one tank to the other
Show the answer and explanation

Correct answer: A. The pump would draw air from an empty tank

  • A — correct If fuel is depleted in one tank, the pump would draw air from that tank instead of fuel from the full tank, so only one tank is selected at a time.
  • B — incorrect The number of pump inlets is not the reason; the selector valve is what restricts the system to one tank at a time.
  • C — incorrect Siphoning between tanks is not the stated reason; the concern is the pump pulling air once one tank runs dry.

BACKGROUND

Low- and mid-wing single-engine aircraft cannot use gravity feed because the tanks sit below the engine, so one or more pumps move fuel to the engine. Each tank runs a line from its screened outlet to the selector valve, but the selector does not offer a BOTH position the way a gravity-feed selector does. Drawing from both tanks at once would let the pump pull air from whichever tank emptied first. Typically one electric pump and one engine-driven pump are installed, the electric pump serving as a backup and supplying fuel pressure for starting. By contrast, a high-wing gravity-feed selector can offer OFF, LEFT, RIGHT, and BOTH because no pump suction is involved.

STUDY TIP

Compare the gravity-feed selector positions with the pump-feed selector positions side by side; the difference is the whole point of the question.

KEY POINTS

  • Pump-feed systems draw from one tank at a time to avoid pulling air.
  • Gravity-feed high-wing selectors can offer a both-tanks position.
  • Pump-feed light aircraft normally carry an electric pump plus an engine-driven pump.

SOURCES

  • sources/ama-airframe/ch14.txt, "Pump Feed Systems"
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MORE ON AIRCRAFT FUEL SYSTEMS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.