ERA20LA237

The loss of directional control during landing due to the fracture of the steering horn bolts, which rendered the nosewheel steering ineffective and resulted from the airplane being towed over the left turn limits at an undetermined time before the accident.

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

Airframe Manufacturer Tow Test A simulated tow test was conducted to document the interference points in the NLG steering system, the witness marks created, the failure sequence of the bolts, and the forces required to fracture the bolts. The NLG strut was rotated left until the steering arm’s left flange aft face contacted the steering arm mount plate on the engine mount. The simulated left turning force of about 194 lbs was applied, and it produced a moment of about 8,560 in-lbs when the aft left bolt attaching the steering horn to the strut fractured. A slight separation occurred between the aft left portion of the steering horn base and the strut, but the NLG was still steerable from the steering arm through the steering horn. A left turning of about 144 lbs was then applied and produced a moment about 6,354 in-lbs when the aft right bolt fractured. A larger separation between the aft portion of the steering horn base and the strut occurred, and the steering horn base exhibited deformation. The steering horn remained attached to the strut; however, the inputs required to achieve rotation of the NLG strut increased substantially. The 35-lb force was then applied, which produced a moment of about 1,544 in-lbs when the forward bolt fractured, and the steering horn was completely separated from the strut. Examination of the steering arm left flange aft face after the test revealed a witness mark where the steering arm contacted the engine mount. Nose Landing Gear Steering The NLG is attached to the engine mount at the upper end of the trunnion assembly by two trunnion bolts. The NLG strut assembly is installed in the trunnion assembly where it rotates for nosewheel steering. The retract actuator is attached to the NLG trunnion assembly and engine mount to allow the NLG to retract aft into the wheel well. During retraction, the NLG strut and tire rotate 90° left through a mechanism that stows the NLG tire in a horizontal position. The NLG is steerable through a 50° arc, 20° left and 30° right, using the rudder pedals. Primary rudder mechanical stops are installed adjacent to the rudder sector in the empennage to limit the deflection of the rudder and thus the motion of the rudder pedals. Secondary rudder pedal mechanical stops are installed in a machined aluminum fitting aft of the steering bell-crank. The secondary stops allow for a small amount of additional rudder pedal movement as the rudder cables stretch before the stops engage the steering bellcrank. The NLG steering horn is attached to the top of the NLG strut with three bolts. When the landing gear are extended, the right and left forward faces of the steering arm engage the right and left pads and rollers on the steering horn to transmit the linear motion of the rudder pedals to a rotational motion of the NLG strut. Steering Horn Assembly During manufacture, a part number 85154-012 steering horn was installed. The steering horn is attached to the NLG strut with three bolts through the base. A rub block is attached to the forward leg of the top plate, a large roller is attached to the top plate’s left leg, and a small roller is attached to the top plate’s right leg. The two steering pads and two steering rollers interface with the steering arm to provide nosewheel steering with the NLG in the down position. The left roller should be up to 0.020 inch forward of the steering pad. Piper discovered alignment issues with certain steering horn assemblies installed on airplanes, including the accident airplane, where the location of the steering pads and rollers were not consistent due to variances in the tolerance stack up. The alignment could be adjusted by machining the faces of the steering pads to achieve the necessary dimensional relationship between the pads and rollers. Accident Airplane Maintenance History Review of maintenance records revealed that the airplane received its airworthiness certificate on December 7, 2019. On January 17, 2020, the airframe manufacturer reworked the steering horn to align the steering pad faces with the rollers, and it was then reinstalled in the airplane before delivery. The pilot purchased the airplane in June 2020, at which time it had accumulated about 23 hours of total flight time. After accruing 48.1 hours of flight time, the airplane underwent a 50-hour maintenance interval, which was completed on July 1, 2020. During the maintenance the left MLG strut and tire were replaced, and the MLG alignment and NLG squat switch were adjusted. Review of data from the airplane’s avionics unit revealed that, after the maintenance was completed, four flights were conducted in the airplane. The accident occurred upon landing after the fourth flight. The data also revealed that the airplane made multiple left and right turns during taxi operations before and after each flight. The pilot reported that there was no evidence of tow exceedance during his preflight inspection of the airplane on the day of the accident flight. Airplane Towing History and Guidance The airplane could be maneuvered on the ground by power equipment using a tow bar attached to the NLG axle. The airplane manufacturer had established turn limits for towing, and a placard installed on the lower end of the NLG trunnion assembly limits turning to about 20° right and left. Corresponding red lines on the NLG strut allow the operator to see the relationship between the turn angle of the NLG strut and the towing limits (see figure 1). Figure 1 - NLG turn limit placard. The Pilot’s Operating Handbook, Section 8, “Handling, Servicing and Maintenance,” contained a caution about towing, which stated, “When towing with power equipment, do not turn the nose gear beyond its steering limit in either directions, as this will result in damage to the nose gear and steering mechanism”. Similar cautions were contained in the Aircraft Maintenance Manual and a flight training manual. If the NLG is externally turned beyond the left or right placard limits, mechanical stops will engage where portions of the NLG and steering mechanism contact. If turned beyond the right limits, a machined stop on the lower portion of the rotating strut assembly will contact a machined stop on the lower end of the stationary trunnion assembly. The right stops will engage when the NLG tire is turned right about 36°. Further, turning right may damage the machined stops and will load the strut-trunnion interface; however, it will not affect the steering arm or steering horn. If turned beyond the left limit, the steering arm left flange aft face will contact the steering arm mount plate on the engine mount. The left stops will engage when the NLG tire is turned about 30.5° left. Further, turning left may damage the steering arm and the three bolts that attach the steering horn assembly to the top of the NLG strut. The rudder primary and secondary mechanical stops do not allow the mechanical turn limit stops to be engaged through pilot rudder inputs. The pilot stated that he was aware of the towing limitations on the airplane. The airplane was towed six times at his home airport, SPG, and he witnessed all the towing operations except when the airplane was in maintenance. It could not be determined if the airplane was towed during maintenance or, if so, how many times. Airplane Examination Examination of the airplane revealed the outboard 4 to 5 ft of the left wing was deformed upward, the upper wing skin was buckled, and both wing spars were bent. The left aileron was buckled upward about midspan. Four of the five propeller blades were fractured and separated. The fuselage’s left side was buckled above the rear spar attachment point. The lower portion of the strut was deformed about 15° right. The left MLG tire beads were separated from the tire, and the tire was deflated. Both MLG tires exhibited crosswise scuffing on the tread. The NLG tire was aligned with th