What happened
During the takeoff roll in a Piper PA-46-500TP, the pilot reported a smooth initial acceleration. However, the pilot noted that snow partially obscured the runway centerline, causing the aircraft to drift slightly to the right. As the aircraft reached a speed of approximately 50 to 60 knots, it suddenly turned hard to the left and began to skid. In response, the pilot reduced power to idle but did not utilize brakes or reverse thrust. The aircraft subsequently departed the left side of the runway.
The investigation
Investigators examined both the original engine mount and a supplemental type certificate (STC) engine mount installed on the aircraft. The aircraft had previously been fitted with an STC engine mount designed to eliminate the need for repetitive inspections required by manufacturer service bulletins following several instances of cracked mounts and nose gear failures.
During the post-accident examination, investigators found structural flexibility in the engine mount actuator attach feet in both the original and the STC versions. This compliance could lead to cracks at the actuator feet. A reexamination of the aircraft in 2008 revealed damage to the firewall that was consistent with the engine mount periodically contacting the bulkhead prior to the accident.
Findings
- The investigation determined that structural compliance in the engine mount could be exacerbated at certain speeds, creating a high shimmying effect.
- This shimmying resulted in an uncommanded left or right turn of the nosewheel during the takeoff phase.
- While the STC version reduced the likelihood of fatigue cracking, it did not eliminate the structural compliance that contributed to the shimmy.