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Labyrinth versus helical oil seal in a turbine bearing housing

Aviation Mechanic Powerplant (AMP) practice question 8 of 9 on Turbine Engine Air Systems. ACS element AM.III.K.K4 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONA gas turbine engine bearing housing uses a labyrinth oil seal. How does that seal differ from the helical (thread) type seal?

  1. AThe labyrinth seal is made of carbon.
  2. BThe labyrinth seal carries thrust load.
  3. CThe labyrinth seal is pressurized.
Show the answer and explanation

Correct answer: C. The labyrinth seal is pressurized.

  • A — incorrect Carbon seals are a separate, usually spring-loaded type that rests against a mating surface; they are not the labyrinth type.
  • B — incorrect Axial thrust loads are carried by the ball bearings on the compressor or turbine shaft, not by an oil seal.
  • C — correct The two seals are very similar, differing in thread size and in the fact that the labyrinth seal is pressurized while the helical seal relies only on reverse threading.

BACKGROUND

A ball or roller bearing assembly in a gas turbine engine is carried in a bearing support housing that must take the radial and axial loads of a fast-turning rotor. That housing also contains oil seals that keep oil in its normal path of flow, and it delivers oil to the bearing, usually through spray nozzles. The oil seals may be of the labyrinth or thread (helical) type, and they may be pressurized to keep oil from leaking along the compressor shaft. The labyrinth seal is usually pressurized, while the helical seal depends solely on reverse threading to stop leakage; otherwise the two are very similar and differ mainly in thread size. A third type, the carbon seal, is spring loaded and rests against a mating surface to create a sealed bearing cavity. The rotor shaft provides machined surfaces, called lands, that fit closely to the oil seals.

STUDY TIP

Learn the three seal names together and attach one distinguishing feature to each: labyrinth (pressurized), helical (reverse threading only), carbon (spring loaded against a mating face).

KEY POINTS

  • Bearing housings contain oil seals of the labyrinth, helical (thread), or carbon type.
  • The labyrinth seal is usually pressurized; the helical seal relies solely on reverse threading.
  • Carbon seals are spring loaded and ride against a surface to form a sealed bearing cavity.

SOURCES

  • amp-powerplant ch01 "Gas Turbine Engine Bearings & Seals"
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MORE ON TURBINE ENGINE AIR 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.