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Holding the connector while tightening a flareless tube nut

Aviation Mechanic General (AMG) practice question 15 of 17 on Fluid Lines and Fittings. ACS element AM.I.D.R6 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONWhile using a wrench to make the final tightening of a flareless tube fitting nut, what should the mechanic do with the connector?

  1. ALet it turn freely so the sleeve seats evenly on the tube
  2. BCoat its faces with sealing compound before tightening
  3. CHold it with a wrench so it cannot turn
Show the answer and explanation

Correct answer: C. Hold it with a wrench so it cannot turn

  • A — incorrect Allowing the connector to turn is the opposite of the instruction; the connector is held stationary while the nut is tightened.
  • B — incorrect Compound must never be applied to the faces of a fitting or flare, because it destroys the metal-to-metal contact needed for the seal.
  • C — correct The handbook directs the mechanic to use a wrench on the connector to prevent it from turning while tightening the nut one-sixth turn (one flat) from the point where the nut begins to bottom.

BACKGROUND

In flareless tube installation the nut is tightened by hand until an increase in resistance is felt; if it cannot be run down with the fingers, a wrench is used with care to note the first signs of bottoming. Final tightening starts at the point where the nut just begins to bottom: the mechanic uses a wrench to turn the nut one-sixth turn (one flat on a hex nut) while using a second wrench on the connector to prevent it from turning. The system is then pressure tested. If a connection leaks, the nut may be tightened an additional one-sixth turn for a total of one-third turn, but never beyond one-third turn (two flats), because that can permanently damage the sleeve and nut. If leakage continues after one-third turn, the assembly is removed and inspected for scores, cracks, foreign material or overtightening damage.

STUDY TIP

Picture two wrenches: one on the nut counting flats, one holding the connector still; then remember the one-sixth, one-third limits.

KEY POINTS

  • Hold the connector with a wrench so it cannot turn while tightening the nut.
  • Final tightening is one-sixth turn (one flat) from where the nut begins to bottom.
  • An extra one-sixth turn is allowed for a leak, but never exceed one-third turn total.

SOURCES

  • FAA-H-8083-30B ch09 "Flareless Tube Installation"
  • FAA-H-8083-30B ch09 "Connection & Torque"
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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.