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Slack required in flexible hose installations

Aviation Mechanic General (AMG) practice question 2 of 17 on Fluid Lines and Fittings. ACS element AM.I.D.K3 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONWhen installing a flexible hose, how much slack or bend should be provided in the hose line to allow for the change in length that occurs when pressure is applied?

  1. A5 to 8 percent of its total length
  2. B10 to 12 percent of its total length
  3. C2 to 4 percent of its length
Show the answer and explanation

Correct answer: A. 5 to 8 percent of its total length

  • A — correct The handbook directs that flexible hose be installed with slack or bend of 5 to 8 percent of its total length, because the hose contracts in length and expands in diameter when pressurized.
  • B — incorrect No 10 to 12 percent figure is given; the stated allowance is 5 to 8 percent.
  • C — incorrect This is less than the 5 to 8 percent the handbook specifies and would not provide for the contraction of the hose under pressure.

BACKGROUND

Hose assemblies must not be installed in a manner that places a mechanical load on the hose. Flexible hose contracts in length and expands in diameter when pressurized, so the installer provides slack or bend of 5 to 8 percent of the hose's total length to allow for this change. Hose must never be stretched tightly between two fittings. Where hose is subject to vibration or flexing, enough slack must be left between rigid fittings so that flexure does not occur at the end fittings, and the hose must remain straight for at least two hose diameters from each end fitting. All flexible hoses should also be protected from excessive temperatures by routing or shrouds.

STUDY TIP

Remember the pair of hose installation numbers: 5 to 8 percent slack and straight for two hose diameters from the end fittings.

KEY POINTS

  • Provide 5 to 8 percent slack because hose shortens and swells under pressure.
  • Never stretch hose tightly between two fittings.
  • Keep hose straight for at least two hose diameters from the end fittings.

SOURCES

  • FAA-H-8083-30B ch09 "Installation of Flexible Hose Assemblies - Slack"
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MORE ON FLUID LINES AND FITTINGS

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.