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Calculation method of control surface rebalancing

Aviation Mechanic Airframe (AMA) practice question 15 of 16 on Flight Controls. ACS element AM.II.C.K8 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONA repair to an aileron removes 0.20 pound of material and adds 0.45 pound of new material. The center of the repair area is 12.00 inches from the hinge center line. Using the calculation method, what is the result?

  1. A5.40 inch-pounds
  2. B3.00 inch-pounds
  3. C7.80 inch-pounds, using total weight added
Show the answer and explanation

Correct answer: B. 3.00 inch-pounds

  • A — incorrect That result uses only the 0.45 pound added and ignores the 0.20 pound of material removed.
  • B — correct Net gain is 0.45 − 0.20 = 0.25 pound, and 0.25 × 12.00 inches = 3.00 inch-pounds.
  • C — incorrect That result adds the two weights instead of subtracting the weight removed from the weight added.

BACKGROUND

Any time repairs on a control surface add weight fore or aft of the hinge center line, the surface must be rebalanced, because an out-of-balance surface is unstable and in extreme cases can flutter badly enough to cause loss of the aircraft. The calculation method has one advantage over the scale and balance beam methods: it can be performed without removing the surface from the aircraft. In the calculation method, the weight of the material removed from the repair area and the weight of the materials used to make the repair must both be known; the weight removed is subtracted from the weight added to get the net gain. The distance from the hinge center line to the center of the repair area is then measured in inches, to the nearest one-hundredth of an inch. Multiplying that distance by the net weight gives an answer in inch-pounds, and if the result is within specified tolerances the control surface is considered balanced.

STUDY TIP

Always subtract the weight removed from the weight added before multiplying by the arm; using the gross weight added is the classic error.

KEY POINTS

  • Net weight = weight added − weight removed.
  • Net weight × distance from the hinge center line = inch-pounds.
  • If the inch-pound result is within tolerance, the surface is considered balanced.

SOURCES

  • sources/ama-airframe/ch02.txt "Rebalancing Methods" (calculation method)
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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.