FAA Aviation Mechanic Powerplant (AMP)Engine InspectionEasy
During inspection of a radial engine crankshaft, a mechanic checks the dynamic counterweight (bifilar damper) assembly by hand-rocking each counterweight in its track. The rollers move noticeably more than the clearance limit specified in the overhaul manual. What action should the mechanic take?
- AShim the roller pins to reduce the measured clearance to within limits
- BReject the crankshaft assembly, since counterweight wear is not a field-repairable condition
- CLightly lubricate the rollers and re-check the movement after a short engine run
- DReface the roller tracks with emery cloth to restore a tighter fit
Show answer & explanationAnswer & explanation
Correct answer: B. Reject the crankshaft assembly, since counterweight wear is not a field-repairable condition
Dynamic counterweights are precision-tuned to a specific pendulum frequency to cancel torsional vibration at a particular engine RPM. Excessive wear in the roller-to-track fit destroys this tuning and cannot be shimmed, lubricated, or refaced back into tolerance in the field, so the crankshaft must be rejected/sent for overhaul.
Why the other options are wrong
- A. Shimming is not an approved repair for counterweight roller clearance.
- C. Lubrication does not restore lost tuning caused by worn rollers or tracks.
- D. Refacing tracks is not an approved shop practice and would further alter the tuned clearance.
Dynamic Counterweight (Bifilar) Damper
A pendulum-type damper mounted on the crankshaft cheeks that oscillates in its track to cancel torsional vibration at a specific engine speed.
- Tuned to a precise pendulum frequency matching engine firing order
- Roller-to-track clearance must be checked by hand-rocking during inspection
- Worn dampers are not field-repairable and require crankshaft rejection
Memory trick: A loose swing on the crank's pendulum means it's out of tune — send it away.