Elevator trailing-edge heavy after refinishing
A Piper PA-28-161 Warrior III has its elevator stripped to bare metal and refinished during a 100-hour inspection because of surface corrosion. The paint shop applies epoxy primer plus two polyurethane topcoats, adding approximately 0.11 kg, most of it concentrated near the trailing edge and around the trim tab hinges. The aircraft maintenance manual requires that after any refinishing the elevator be statically balanced: it must be neutral or leading-edge heavy by no more than 0.25 in-lb; if it is trailing-edge heavy at all, or leading-edge heavy beyond 0.25 in-lb, it must be rebalanced by adding weight to the leading-edge horn, up to a maximum of 0.9 kg. On the balance stand, the trailing edge settles firmly and you measure 0.9 in-lb trailing-edge heavy. The shop foreman says the paint is only a couple of coats, the aircraft is a trainer that never exceeds 110 KIAS, and the customer needs it back for a lesson block tomorrow morning.
1. Explain why a trailing-edge-heavy condition matters, and why the balance limit is not a cosmetic or paperwork issue.
Read the worked answer
Static balance controls where the control surface's centre of gravity sits relative to the hinge line, and that position governs whether the surface is stable or unstable when it is disturbed.
- Leading-edge heavy is the safe side. The surface's centre of gravity sits ahead of the hinge, so an angular acceleration produces an inertia moment that tends to reduce the deflection. This is positive aerodynamic damping.
- Trailing-edge heavy reverses that. With the centre of gravity behind the hinge, the inertia moment adds to the deflection instead of opposing it. A small oscillation can grow rather than decay, and the surface can become a flutter driver.
- Flutter is not a low-speed problem. It is a dynamic instability that depends on stiffness, mass distribution and aerodynamic damping, and it can initiate from a gust, a turbulence input or a control input. Once it starts, the amplitude builds rapidly and the surface, or the tail, can fail in seconds. The 110 KIAS limit is no defence: the manual limit comes from the type's flutter analysis and flight test and applies at all speeds within the envelope.
- The added mass is in the worst place. Paint near the trailing edge and around the trim tab hinges has a long moment arm from the hinge, so a small weight produces a disproportionate moment. The trim tab itself is part of the same balance problem.
So the reading is not a tolerance issue to be discussed with the customer. It is the condition the flutter clearance was built to exclude.
2. What do you do about it, and how would you rebalance the surface? What would make you reject it?
Read the worked answer
The aircraft does not leave the hangar in this condition. The manual is unambiguous: trailing-edge heavy at all means rebalance.
Actions:
- Do not release. Record the balance reading on the work order with the equipment identification and date, so the state of the aircraft is documented.
- Rebalance by the approved method. Add weight to the leading-edge horn, attached per the manual (normally bolted with a specified fastener and locking method, never just adhesively bonded in an unspecified way). The added weight is limited to 0.9 kg total; if the required weight exceeds that, the surface is rejected.
- Re-check after any change. Balance, then check free play at the hinges and the trim tab, and check that the trim tab was balanced separately if the manual requires it. Hinge bearing condition affects the reading as well as the mass distribution, so do not chase a balance problem caused by a stiff or worn bearing.
- Sign only against the final measured value, not against the intent to rebalance.
What would make me reject the surface: required balance weight above the manual maximum; any doubt about the paint thickness or the adhesion; distortion or a bent horn; elongated hinge bolt holes; or a trim tab that cannot be brought into limits. Then the surface is replaced or sent for overhaul, and the aircraft is grounded until it is fitted and rechecked.
A useful calculation for the discussion: at a moment arm of about 150 mm from the hinge to the added weight, 0.9 in-lb corresponds to roughly 70 grams. That is a trivially small weight, which is exactly why the foreman's 'only a couple of coats' argument fails.
3. You later discover the aircraft flew 6 hours between the refinishing and your balance check. What does that change?
Read the worked answer
It changes the problem from an unfinished job to an airworthiness event.
- The flights were made in an unairworthy condition. The release after the paint work was invalid with respect to the missing balance check. This is a reportable matter to the CAA, and it needs an internal review of how the job was signed off without the required check.
- The surface has been loaded in flight, possibly with flutter inputs. Inspect the hinges, hinge brackets, attachment bolts and their holes for elongation, wear and cracking; inspect the trim tab and its hinges; check for skin cracks around the horn and trailing edge. Elongated holes or fretting at the hinges would suggest vibration.
- Talk to the pilots. Ask specifically about vibration, a buzzing or humming through the controls, control heaviness, or trim changes. Any such report upgrades the inspection to an OEM-supported one and may require the surface's removal.
- Rebalance regardless. The condition that existed before the flights still exists and still has to be corrected before further flight.
- Record and follow up. The occurrence report, the corrective action, and the process change (a balance check as a mandatory hold point after any refinishing) all have to be documented. 'It flew fine' is not evidence of airworthiness; a control surface can be stable in the conditions flown and still be outside the flutter margin for the envelope.