ERA20LA237 · Piper PA46 600TP

The pilot was conducting a cross-country flight. He reported that, although the flight had encountered turbulence, it was otherwise uneventful with no anomalies or malfunctions with the airplane noted. During landing, when the two main landing gear (MLG) touched down, the airplane started to veer left, so he relaxed the rudder pressure. As soon as the nose landing gear (NLG) touched down, the airplane immediately veered right. He attempted to correct the veer by applying left rudder and left and right brakes; however, the airplane did not respond. The airplane then exited the right side of the runway and skidded sideways through the grass, which resulted in the left wing and propeller striking the ground and resulted in substantial damage to the left wing and fuselage. The airplane came to rest in the grass about 100 ft from the runway edge.

Examination of the runway revealed distinct tire marks from the NLG and both MLG on the runway extending from the touchdown point to where the airplane exited the right side of the runway, which transitioned to ground scars in the grass. The NLG tire mark was a dark wide line with no evident tread pattern, consistent with the airplane skidding from the touchdown point to the runway departure point. The MLG tire marks had an evident tread pattern, consistent with the tires rolling while braking was being applied. A ground scar near where the airplane came to rest was consistent with a left-wing strike. Examination of the NLG and MLG revealed that dirt and grass were embedded between the NLG’s left tire bead, that the tire rim and left sidewall exhibited crosswise scuffing, and that the NLG fork was bent right about 25°. Both MLG tires also exhibited crosswise scuffing on the tread. Further, a review of the airplane’s avionics data revealed that, shortly after the left MLG touched down, the airplane’s heading and track began to diverge and remained apart throughout the ground roll and that its lateral acceleration increased. All of these factors are consistent with the NLG tire having been turned to the right at touchdown, and with the tire then skidding throughout the rollout.

Examination of the right NLG’s strut-to-trunnion mechanical stops revealed that the surface texture of the trunnion stop was embossed on the surface of the strut stop. This finding is also consistent with the NLG strut being forced to the right mechanical stop at touchdown and remaining there for the entire rollout.

Postaccident examination of the NLG steering system revealed that the aft face of the steering arm’s left flange exhibited a witness mark that matched the location and shape of the steering arm mount plate on the engine mount. The steering arm can only contact the mount plate if an external force is applied that turns the NLG left beyond the turn limits indicated on its placard. Therefore, at some point before the accident landing, the NLG had been turned left beyond the placard limits, which resulted in the steering arm contacting the mount plate. The three bolts that secured the NLG steering horn to the strut were fractured, and the steering horn was found separated from the strut. Examination of the bolt fracture faces, the steering horn bolt holes, and the damage to the top of the strut indicated the left aft bolt fractured predominantly in tension with some shear, the aft right bolt fractured in shear with some bending, the forward bolt fractured in shear with increased bending, and all the bolts fractured in overstress. The direction of the shear failures on the bolts and the strut damage indicated that the strut was rotated left with respect to the steering horn. It was determined that the aft left steering horn bolt fractured first, followed by the aft right bolt and then the forward bolt. The steering horn was reworked at the factory before delivery to better align the steering pad faces with the rollers. The steering horn was deformed during the accident sequence, which precluded taking all the dimensional measurements. The deformation of the left roller could be partially explained due to the exceedance of the left turn limits because the roller should be up to 0.020 inch forward of the steering pad. The remaining measurements did not indicate any gross discrepancies with the steering horn. Therefore, the steering horn was likely fully intact and manufactured in accordance with the drawing.

The NLG must rotate right from its stowed position to extension to align the tire with the airplane’s longitudinal axis. The extension mechanism will apply a right turning moment to the steering horn during extension. Given that the NLG was turned right at touchdown, the steering horn must have still been partially attached during gear extension before the accident landing. The NLG tire would only need be turned slightly to the right at touchdown for the frictional forces to result in a right hard-over of the NLG tire. The failure direction of the forward bolt is consistent with right turning moment on the steering horn during NLG extension. It is likely that the forward steering horn bolt fractured during the NLG extension before the accident landing.

If an external force is applied to turn the NLG wheel and strut beyond the left turn limit, the steering arm aft face will contact the steering arm plate on the engine mount. Further, turning left would induce a torsional moment between the strut and steering horn, which would result in the steering horn bolts fracturing in shear/bending in the direction observed. The right turning forces generated during the accident landing could not have produced the damage to the steering arm or the directional fracture of the steering horn bolts further indicating that they failed when the airplane was turned left beyond the limits.

The pilot stated that he was aware of the airplane’s towing limitations and witnessed all towing operations except when the airplane was in maintenance before the accident. It could not be determined if the airplane was towed during maintenance or if so, how many times. After the maintenance, four successful flights were conducted in the airplane, and the accident occurred on landing after the fourth flight. Review of the downloaded avionics data for the four flights revealed that the airplane turned left and right multiple times during taxi operations before and after each flight. At least one of the steering horn bolts must be attached for the airplane to be steerable; therefore, this evidence further supports that the airplane was towed beyond the left turn limits at some point before the accident flight, which fractured one or two of the steering horn attachment bolts. The failure direction of the forward bolt is consistent with right turning moment on the steering horn during NLG extension. The NLG rotates 90° left through a mechanism during retraction so that the tire is stowed in a horizontal position. If the tire is not horizontal, the NLG doors will not close, and an indication will be given in the cockpit. Given that the pilot reported no such cockpit indication, the steering horn must have still been partially attached during takeoff for the accident flight. As stated previously, the pilot noted that the nosewheel steering did not respond when he applied left rudder after landing. The pilot would have known that the rudder and the NLG were deflected right if the steering horn was in place at the time of landing. Further, the rudder pedals would not have been centered if the tire was deflected right, and the steering horn was in place. Based on all the evidence, all three steering horn bolts must have fractured before the accident landing as a result of being towed over the left turn limits at an undetermined time, which rendered the nosewheel steering ineffective and resulted in the subsequent loss of directional control.

Following the accident, the NLG tire pressure was within acceptable limits, and a review of maintenance records revealed that the NLG tire was properly serviced. Therefore, no issues with the NLG tire pressure contributed to the accident.

The left MLG torque link apex bolt torque links were separated, and the left wheel was not aligned after the accident. Although the apex bolt and cotter pin were not recovered, during examination of the bolt, deformation was found between the cotter pin hole and the bolt’s tip, and a remnant of a stripped thread was observed immediately adjacent to the bolt grip, which indicates that the left MLG torque link apex bolt nut and cotter pin were present upon landing.

Verbatim NTSB analysis and probable cause from the NTSB dataset