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Seal material required for phosphate ester fluids

Aviation Mechanic Airframe (AMA) practice question 15 of 15 on Hydraulic and Pneumatic Systems. ACS element AM.II.F.R4 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONPetroleum oil hydraulic system seals of neoprene or Buna-N must be replaced with seals of what material when the system uses phosphate ester-based fluid?

  1. Aneoprene rated for high temperature
  2. Bbutyl rubber or ethylene-propylene
  3. CBuna-N with Teflon backup rings
Show the answer and explanation

Correct answer: B. butyl rubber or ethylene-propylene

  • A — incorrect Neoprene is one of the petroleum-fluid seal materials that is not compatible with phosphate ester-based fluid, regardless of temperature rating.
  • B — correct Seals for phosphate ester-based fluids are made from butyl rubber or ethylene-propylene elastomers.
  • C — incorrect Backup rings are unaffected by system fluid, but the Buna-N seal itself remains incompatible with phosphate ester-based fluid.

BACKGROUND

Most seals are made from synthetic materials that are compatible with the hydraulic fluid used, and seals used for MIL-H-5606 hydraulic fluid are not compatible with phosphate ester-based fluids. Seals for systems that use MIL-H-5606 are made of neoprene or Buna-N, while seals for phosphate ester-based fluids are made from butyl rubber or ethylene-propylene elastomers. Servicing a hydraulic system with the wrong fluid could result in leaks and system malfunctions, and petroleum oil hydraulic system seals of neoprene or Buna-N are specifically called out as requiring replacement with butyl rubber or ethylene-propylene elastomers when phosphate ester-based fluid is used. When gaskets, seals, and hoses are replaced, positive identification should be made to ensure that they are made of the appropriate material.

STUDY TIP

Pair each fluid with its seal family in memory, and confirm compound identification from the basic part number on the sealed envelope rather than relying on color coding alone.

KEY POINTS

  • Neoprene and Buna-N seals go with MIL-H-5606.
  • Butyl rubber and ethylene-propylene elastomer seals go with phosphate ester-based fluids.
  • Incompatible seals lead to leaks and system malfunctions, so verify material on every replacement.

SOURCES

  • sources/ama-airframe/ch12.txt "Seal Materials"
  • sources/ama-airframe/ch12.txt "Compatibility with Aircraft Materials"
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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.