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Acceptability of a flattened tube bend

Aviation Mechanic General (AMG) practice question 1 of 17 on Fluid Lines and Fittings. ACS element AM.I.D.K2 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONA mechanic bends a 1/2-inch outside diameter aluminum alloy tube with a hand bender and measures the small diameter of the flattened portion of the bend at 0.36 inch. What should the mechanic do with the tube?

  1. AInstall it; the flattened diameter is above 75 percent of the OD
  2. BInstall it; only kinked and wrinkled bends are cause for rejection
  3. CDo not install it; the flattening exceeds the allowable limit
Show the answer and explanation

Correct answer: C. Do not install it; the flattening exceeds the allowable limit

  • A — incorrect 0.36 inch is 72 percent of 0.5 inch, which is below the 75 percent minimum (0.375 inch).
  • B — incorrect The handbook says tubing with flattened, wrinkled or irregular bends should not be installed, and excessive flattening causes fatigue failure of the tube.
  • C — correct The small diameter of a flattened bend must not be less than 75 percent of the original OD. For a 0.5 inch tube that limit is 0.375 inch, and 0.36 inch is below it, so the tube should not be installed.

BACKGROUND

The objective in tube bending is a smooth bend without flattening the tube. Tubing under 1/4 inch in diameter can usually be bent by hand, while larger sizes need a hand bender or production bender. Tubing must be bent carefully to avoid excessive flattening, kinking or wrinkling. A small amount of flattening is acceptable, but the small diameter of the flattened portion must not be less than 75 percent of the original outside diameter. Tubing with flattened, wrinkled or irregular bends should not be installed; excessive flattening causes fatigue failure, and wrinkled bends usually result from bending thin-wall tubing without a bender.

STUDY TIP

Multiply the tube OD by 0.75 to get the minimum acceptable flattened diameter, then compare it to the measurement.

KEY POINTS

  • The flattened portion of a bend must measure at least 75 percent of the original OD.
  • For a 1/2 inch tube the minimum is 0.375 inch.
  • Flattened, wrinkled or irregular bends should not be installed; excessive flattening causes fatigue failure.

SOURCES

  • FAA-H-8083-30B ch09 "Tube Bending"
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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.