Aileron Mass Balance After Refinishing
During a pre-acceptance flight after hail-damage repair, the pilot of a retractable single-engine aircraft notices a light, high-frequency vibration through the control wheel at 140 KIAS in smooth air; it disappears when slowing to 120 KIAS. The maintenance release states that both ailerons and both elevators were stripped to bare metal, refinished with two coats of primer and three coats of polyurethane, and reinstalled. Asked about balance, the shop reports that each aileron gained 0.21 kg (0.46 lb) and that the manual allows a maximum change of 0.06 kg (0.13 lb) before rebalancing, but argues that "paint weighs almost nothing and the aircraft flew fine on the test hop". Control rigging, cable tension and hinge condition were checked and are within limits. The aircraft is due to depart into an area of forecast moderate turbulence. The pilot holds a CPL and must decide whether to accept the aircraft and what to require before flight.
1. Explain why a 0.21 kg paint gain matters on an aileron, and how it could produce the vibration reported.
Read the worked answer
Balance is about the moment of the surface's mass about the hinge line, not the mass itself. Primer, filler and topcoat applied aft of the hinge move the surface centre of gravity rearward, producing a trailing-edge-heavy condition. A trailing-edge-heavy control surface is more easily disturbed by the airflow and reduces the speed at which the surface can oscillate in torsion and couple with wing or tailplane bending - the mechanism of flutter. That is why the manual sets such a small permissible change (0.06 kg): the limit is on balance, not on weight.
The reported vibration is speed-dependent, appears only at the higher speed and vanishes when slower. That pattern fits a surface approaching an unstable condition, not an out-of-balance wheel or loose fairing. It should be treated as an incipient flutter warning: once flutter starts it can destroy a surface in seconds, and the forecast turbulence adds energy and reduces the margin further. The test hop proves nothing - flutter margins are speed- and stiffness-dependent, and a short flight in smooth air may simply not have reached the critical condition.
2. State what must be done before the aircraft flies again, and what you would require in the records.
Read the worked answer
The surfaces must be rebalanced on knife edges in accordance with the maintenance manual, using the specified pivot points, limits and tolerances after refinishing. Because the same process was applied to the elevators, they must be checked as well - an elevator imbalance affects pitch flutter and is at least as serious. If the surface cannot be brought within limits by removing finish, material must be removed or the surface replaced per the manual; adding weight forward of the hinge is only acceptable if the manual provides a procedure.
The records should show: measured balance before and after correction, the manual reference and limits used, the person certifying the work, and a functional check of control travel and cable tension after reassembly. A subsequent test flight should be flown in smooth air, at gradually increasing speed with a defined maximum, and any recurrence of vibration investigated. "It flew fine" is not an acceptable substitute for the balance check, and the aircraft should not be released into moderate turbulence until the surfaces are within limits.