Olive Branch, MS, USA
N627PR
PIPER AIRCRAFT INC PA-46-600TP
The pilot was landing the airplane with a right quartering headwind, resulting in a right crosswind component of about 10 to 14 kts, which was less than but near the airplane’s maximum demonstrated crosswind component of 17kts. The airplane touched down left of runway centerline. After touchdown, the pilot attempted to correct to the right toward the centerline with rudder inputs. As the airplane decelerated through about 68 kts and approached the runway centerline, the airplane entered a hard right turn about a 45° angle, and the pilot reported that he was unable to control the turn with left rudder input. The airplane departed the right side of the runway, about 1,330 ft from the threshold and about 570 ft after touchdown, and the nose landing gear (NLG) collapsed. The airplane sustained substantial damage to the engine mount. Postaccident examination of the runway surface revealed rubber transfer markings consistent with the NLG, right main landing gear (RMLG), and left main landing gear (LMLG). The NLG transfer mark began about 900 ft from the threshold and about 10 ft left of the centerline, with light intensity and its tread pattern evident, consistent with a rolling, non-skidding tire. The rolling tire marking continued until the right veering began, at which point it was of moderate intensity with tread pattern evident and transitioned to heavy intensity and no tread pattern, consistent with a skidding tire. A review of the airplane’s avionics data revealed that the RMLG touched down first, followed by the LMLG about 1 second later, consistent with a right crosswind landing. The NLG touched down about 1 to 2 seconds after the RMLG. Starting about 2 seconds after LMLG touchdown, the heading and track diverged, with the nose to the right of the track, until near the runway departure location, indicating that the airplane was in a skid. The maximum divergence occurred about 1 second before the runway excursion, when the heading was about 223° and the track was about 195°. At the same time, the lateral acceleration increased, also consistent with the airplane being in a left skid. The airplane departed the runway with the heading and track mostly aligned. Postaccident examination of the NLG assembly did not reveal anomalies that would have affected its operation or steering. The rake angle of the NLG is designed to be 90° from the airplane longitudinal axis with a very tight tolerance, allowing 0.5° forward rake and zero aft rake. The zero-rake angle is good for static stability and taxiing at slower speeds but can be problematic for dynamic stability at higher speeds. A zero or aft rake can tend to cause the NLG tire to turn more sharply if deflected in some conditions. On NLG touchdown, the NLG strut will deflect aft due to spin-up loads, increasing the negative rake that could cause instability, and will undergo a damped oscillation. The design of the NLG with zero rake and no trail requires that the NLG tire be aligned with the runway upon touchdown, which it was during the accident landing as evidence by the runway markings. Any misalignment or steering input may result in a steering hardover due to the dynamic instability inherent in the NLG design. The circumstances of this accident indicated that the NLG turned hard right soon after touchdown, when the NLG dynamic instability manifested itself as the pilot was correcting back to the centerline of the runway with rudder input. The manufacturer redesigned the lower NLG fork area to move the axle 0.5 inches aft of the steering axis. This is equivalent to raking the NLG forward 5°, making it inherently dynamically stable. The new NLG should be dynamically stable even during aft deflection since the steering axis will remain forward of the vertical axis (center of the tire contact patch) and provide a restoring force to the NLG tire if deflected. The new NLG improves the dynamic stability of the NLG allowing the nose wheel to self-center.
HISTORY OF FLIGHTOn March 23, 2022, at 1038 central daylight time, a Piper PA-46-600TP (M600) airplane, N627PR, was substantially damaged when it was involved in an accident near Olive Branch, Mississippi. The pilot and passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that he conducted the instrument landing system approach in visual meteorological conditions to runway 18 with the wind reported as 220° at 16 knots (kts) gusting to 22 kts. As the airplane approached the final approach fix, the pilot reduced engine power and extended the landing gear. About 550 ft above ground level, he disconnected the autopilot. The airplane landed “softly”, about 85 kts, to the left of the runway centerline. The airplane rolled about 1,200 ft as the speed decreased, and the pilot reported that he was in control moving toward the centerline. About 68 kts as the airplane approached the runway centerline, he had both hands on the yoke, holding it back slightly, and was making light rudder inputs to get to the airplane to the runway centerline. At that time, the airplane entered a hard right turn about a 45° angle. The pilot stated the left directional input was not working and that he applied more pressure with the left rudder. He reported, “It felt as if the nose quit rolling for a nano second, and the tail of the plane was skidding to the left.” The pilot applied neutral rudder pedals and the skid reduced. The airplane exited the right side of the runway and continued across wet and muddy grass when the nose gear collapsed, and the airplane came to rest nose down. (See figure 1.) The airplane departed the runway about 1,330 ft from the threshold and about 570 ft after touchdown. The engine mount sustained substantial damage. Figure 1. Accident airplane (Source: OLV airport operations) PERSONNEL INFORMATIONThe pilot obtained his private pilot certificate in September 2015. Between 2015 and 2021, his flight experience included about 185 total hours in a Cessna 182 airplane, and 737 total hours in Cessna T240 airplanes. In November and December 2021, after purchasing the accident airplane, the pilot received 20 hours of M600 training from a Piper Aircraft dealer pilot, and 10 hours from Legacy Flight Training, Vero Beach, Florida, an authorized Piper Aircraft training provider. The 10 hours provided by Legacy Flight Training included 4 hours in a simulator and 6 hours in the accident airplane. According to the pilot, during his initial flights with the accident airplane, he commented to the sales staff that the nose wheel steering and responsiveness was quite different from other airplanes he had experience operating (the T240 airplane’s nose wheel is free-castering). The sales staff responded that his experience was normal for this airplane, and he should get used to it. AIRCRAFT INFORMATIONAccording to the Piper M600 Pilot’s Operating Handbook (POH), the maximum demonstrated crosswind component for the airplane is 17 kts. Based on the reported wind conditions at the time of the landing, the crosswind component was about 10 to 14 kts. According to the POH, the width between the left main landing gear (LMLG) and right main landing gear (RMLG) is 163.44 inches, and the width between the nose landing gear (NLG) and each main landing gear (MLG) is 81.72 inches. The longitudinal distance between the NLG and main landing gear is 99 inches. The NLG rake angle on the M600 is the angle between the lower fuselage skin and the centerline of the NLG strut. The rake angle is set at the factory to be 90° with a tolerance of +0.5°/0.0°. Piper Service Letter 1286, issued on August 18, 2020, provides instructions for inspection and adjustment of the NLG steering components including rake angle. The service letter has been revised four times since original issuance, but the rake angle has not changed. At a rake angle of 90° for the M600, the steering axis passes through the NLG axle and intersects the ground coincident with the center of the tire contact patch with the airplane stationary (zero trail). At a rake angle of 90.5° for the M600, the steering axis still passes through the NLG axle, but intersects the ground slightly forward of the center of the tire contact patch with the airplane stationary (positive trail). Examination of the structural response of a landing gear at wheel touchdown showed that the gear will deflect aft initially due to wheel spin-up loads and then undergo a damped oscillation about its equilibrium point. Landing Procedures The POH contains the airplane normal procedures in section 4.5p. (see figure 2.) The procedures caution the pilot about centering the rudder prior to nosewheel touchdown. Figure 2. POH normal landing procedures Maintenance History A review of the maintenance records revealed that the accident airplane underwent its manufacturer’s warrantable first 50-hour inspection at a Piper authorized service center on January 12, 2022, at a total airplane time of 46.6 hours. During the maintenance visit, both MLG and the NLG were removed, painted black, and reinstalled. The maintenance was completed in accordance with the airplane’s maintenance manual. Piper Service Letter 1286D, NLG Steering Components Inspection and Adjustment, was also complied with during the maintenance. The most recent maintenance on the airplane was performed on March 11, 2022, at a total time of 64.5 hours, when a new auto-throttle and autopilot controller were installed. At the time of the accident, the airplane had accrued 74.8 hours total time. AIRPORT INFORMATIONAccording to the Piper M600 Pilot’s Operating Handbook (POH), the maximum demonstrated crosswind component for the airplane is 17 kts. Based on the reported wind conditions at the time of the landing, the crosswind component was about 10 to 14 kts. According to the POH, the width between the left main landing gear (LMLG) and right main landing gear (RMLG) is 163.44 inches, and the width between the nose landing gear (NLG) and each main landing gear (MLG) is 81.72 inches. The longitudinal distance between the NLG and main landing gear is 99 inches. The NLG rake angle on the M600 is the angle between the lower fuselage skin and the centerline of the NLG strut. The rake angle is set at the factory to be 90° with a tolerance of +0.5°/0.0°. Piper Service Letter 1286, issued on August 18, 2020, provides instructions for inspection and adjustment of the NLG steering components including rake angle. The service letter has been revised four times since original issuance, but the rake angle has not changed. At a rake angle of 90° for the M600, the steering axis passes through the NLG axle and intersects the ground coincident with the center of the tire contact patch with the airplane stationary (zero trail). At a rake angle of 90.5° for the M600, the steering axis still passes through the NLG axle, but intersects the ground slightly forward of the center of the tire contact patch with the airplane stationary (positive trail). Examination of the structural response of a landing gear at wheel touchdown showed that the gear will deflect aft initially due to wheel spin-up loads and then undergo a damped oscillation about its equilibrium point. Landing Procedures The POH contains the airplane normal procedures in section 4.5p. (see figure 2.) The procedures caution the pilot about centering the rudder prior to nosewheel touchdown. Figure 2. POH normal landing procedures Maintenance History A review of the maintenance records revealed that the accident airplane underwent its manufacturer’s warrantable first 50-hour inspection at a Piper authorized service center on January 12, 2022, at a total airplane time of 46.6 hours. During the maintenance visit, both MLG and the NLG were removed, painted black, and reinstalled. The maintenance was completed in accordance with the airplane’s maintenance manual. Piper Service Letter 1286D, NLG Steering Components Inspection and Adjustment, was also complied with during the maintenance. The most recent maintenance on the airplane was performed on March 11, 2022, at a total time of 64.5 hours, when a new auto-throttle and autopilot controller were installed. At the time of the accident, the airplane had accrued 74.8 hours total time. WRECKAGE AND IMPACT INFORMATIONRunway and Ground Scars Examination Examination of the runway surface revealed a light rubber transfer mark consistent with the LMLG that began about 760 ft from the runway 18 threshold and about 17 ft left of the centerline. The transfer mark was continuous until it departed the right side of the runway and varied in intensity. A light rubber transfer mark with the tread pattern evident, consistent with the NLG, began about 900 ft from the threshold and about 10 ft left of the centerline. The mark was mostly continuous until it departed the right side of the runway and varied in intensity. The LMLG and NLG rubber transfer marks were aligned, separated by about 80 inches, and drifted to the left until about 1,140 ft from the threshold when they veered sharply to the right. (see figures 3 and 4.) Figure 3. Initial touchdown runway markings (Source: Piper Aircraft, Inc., annotated by NTSB) Figure 4. Runway markings (Source: Piper Aircraft, Inc., annotated by NTSB) A rubber transfer mark from the RMLG was not discernable until about 1,170 ft from the runway threshold and was not continuous to the departure point. At the point where the RMLG mark became evident, the perpendicular distance between the RMLG and NLG marks measured 51 inches and between the LMLG and NLG measured 106 inches. Throughout the right veer, the LMLG rubber transfer mark varied from moderate to heavy intensity with the tread pattern evident consistent with moderate to heavy braking. The NLG rubber transfer mark during the veer was initially of moderate intensity with the tread pattern evident but transitioned to heavy intensity with no tread pattern evident, consistent with a skidding tire. Between the centerline and the departure point, the NLG skipped once and transitioned to light intensity with a tread pattern just before the departure. The RMLG rubber transfer mark was sporadic from its beginning to departure with moderate intensity. At the runway centerline, the perpendicular distance from the RMLG to NLG measured 33 inches and from the LMLG to the NLG measured 107 inches. At the departure point, the perpendicular distance between each MLG and the NLG measured 82 inches. The airplane departed the right side of the runway, about 1,330 ft from the threshold. There was no disturbance of the grass for about 10 ft beyond the runway edge, after which there were three deep ruts in the grass; each rut displayed a tire tread pattern in the mud. The NLG rut stopped about 68 ft from the initial point, and the airplane came to rest about 80 ft further along the path. Airplane Examination Postaccident examination of the airplane revealed that the NLG, retract actuator, engine mount, and emergency extension spring sustained damage. The NLG wheel and tire were installed and rotated freely. The NLG tire pressure measured 84 lbs per square inch (psi). There was some crosswise scuffing on the NLG tire tread, but no evidence of scuffing on the sidewalls. The damage precluded examination of the rake angle, NLG steering adjustments, and MLG alignment. There was no damage to the LMLG or RMLG. The tire pressure on each MLG measured 90 psi. There was some crosswise scuffing on both MLG tires, but no sidewall scuffing or flat spots were noted. The NLG assembly was removed for further examination. The NLG assembly was further examined at Piper Aircraft Inc., Vero Beach, Florida, under the supervision of the National Transportation Safety Board. The NLG tire spun freely and there was minimal perceptible play between the wheel and axle. The axle nut was installed finger-tight. The wheel, torque links, and wire harness were removed, and the NLG was disassembled. No anomalies were noted during the dimensional check of the components that would have precluded normal operation. Recorded Flight Data The Garmin G3000 Integrated Avionics System on the airplane records certain flight parameters at a rate of 1 hertz for each flight. A maintenance-oriented data set (CMC data) is recorded and stored internally on the unit, and flight-oriented data (log data) is recorded directly to an SD card. Both the CMC and log data for the accident flight were downloaded from the airplane. According to the data (see figure 5), the right main landing gear (RMLG) touched down first at a recorded indicated airspeed about 76 kts, followed by the LMLG about 1 second later at a recorded indicated airspeed about 74 kts. The NLG touched down about 1 to 2 seconds after the RMLG. Starting about 2 seconds after LMLG touchdown, the heading and track diverged, with the nose to the right of the track that continued until near the runway departure location, indicating that the airplane was in a skid. The maximum divergence occurred about 1 second before runway departure when the heading was about 223° and the track was about 195°. At the same time, the lateral acceleration increased to a maximum of about 0.88 g about 2 seconds before runway departure, also consistent with the airplane in a left skid. The airplane departed the runway about 6 seconds after the RMLG touchdown at a ground speed about 47 kts with the heading and track mostly aligned. The pitch parameter showed a sharp decrease about 2 seconds after runway departure, consistent with a collapse of the NLG. Figure 5. Plot of Garmin data from accident landing (Source: Piper Aircraft Inc.) ADDITIONAL INFORMATIONThe Federal Aviation Administration Airplane Flying Handbook (FAA-H-8083-3C), Chapter 9, page 18, states in part, “In those airplanes having nose-wheel steering interconnected with the rudder, the nose-wheel is not aligned with the runway as the main wheels touch down because opposite rudder is being held for crosswind correction. To prevent swerving in the direction the nose-wheel is offset, the corrective rudder pressure needs to be relaxed as the nose-wheel touches down.” In preparation for an unpaved field operational environment, Piper began installing a new NLG on the PA-46-600TP airplanes starting at S/N 4698198 (accident airplane was S/N 4698190). The new NLG incorporates an updated lower NLG strut assembly with a different fork and redesigned torque links. The new fork is a machined aluminum structure, sized by analysis and testing to accommodate the loads generated by the increase in geometric trail. The new fork offsets the NLG axle aft 0.5 inch from its previous position, which results in a trail value of 0.5 inch, in which the steering axis is 0.5 inch forward of the center of the tire contact patch. The new NLG with offset is equivalent to 5° of forward rake angle, resulting in positive trail and increased positive dynamic stability, which Piper desired for unpaved field operations. A kit for the new NLG is offered on airplanes S/N 4698001 – 4698197. TESTS AND RESEARCHNLG Design Considerations NLG trail refers to the location of the steering axis in relation to the vertical axis through the axle and can be positive, neutral, or negative. Neutral trail (see figure 6. Item A) occurs when the steering axis is vertical and coincident with the vertical axis and axle location resulting in minimum steering forces required and no appreciable torque about the steering axis when the wheel is rolling (neutral dynamic stability). This configuration has neutral static stability. Positive trail (see figure 6. Item B) occurs when the steering axis intersects the ground forward of the vertical axis and results in a self-aligning torque about the steering axis when the wheel is rolling (positive dynamic stability). This configuration requires increased steering forces and is statically unstable if not restrained. Negative trail (see figure 6. Item B except angle is aft) occurs when the steering axis intersects the ground aft of the vertical axis and results in a torque about the steering axis that acts to increase the turn angle (negative dynamic stabili
The airplane’s uncommanded hard right turn and loss of directional control during landing in crosswind conditions. Contributing to the accident were the dynamic instability of the nose landing gear and the crosswind conditions.
Source: NTSB Aviation Accident Database
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