Piper PA-18 Tailwheel Shimmy Leads to Runway Excursion and Upside-Down Crash at Peyresourde Altiport

Incident Overview and Aircraft Details On 4 June 2012 at 15:20 local time, a Piper Aircraft PA-18-150 with registration F-BKBE was involved in an accident at the Peyresourde altiport in the Hautes-Pyrénées department (65). The aircraft was operated by a flying club and was conducting an instructional flight. The accident resulted in severe damage to the aircraft, while both occupants escaped without injury. ## Flight Circumstances The instructor and student pilot departed from Luchon aerodrome (31) for the Peyresourde altiport for an instructional flight aimed at obtaining the mountain qualification (wheels). The student was at the controls and performed a high and low reconnaissance for runway 09. During the reconnaissance, the instructor observed that the wind was from the northwest sector with an estimated strength between 5 and 10 knots. The instructor described the landing as normal. ## Accident Sequence After touchdown, the instructor noticed significant tailwheel shimmy. He took control and attempted to relieve the tailwheel to minimize the effects of the shimmy, but his action on the elevator had no effect. The aircraft became uncontrollable. The student suggested a go-around, but the instructor discouraged this and reduced power while the trajectory abruptly turned 90 degrees to the left. The aircraft left the runway and overturned onto its back. The occupants, uninjured, managed to exit the cabin by breaking the side windows. ## Runway and Meteorological Conditions The runway at Peyresourde altiport has a length of 335 meters and a gradient of 15%. The elevation at threshold 09 is 5,039 feet. The meteorological conditions were: wind 320° to 010° at 7 to 10 knots, maximum 15 to 20 knots, visibility 9,999 meters, broken cumulus at 2,000 feet, isolated towering cumulus, light turbulence, temperature 12°C, QNH 1019 hPa. ## Personnel and Maintenance Findings The instructor had a total of 22,387 flight hours, including approximately 200 hours on type and 3,500 hours on tailwheel aircraft. The student had 160 flight hours, including approximately 45 hours on type, and held a site qualification to access the Peyresourde altiport. The tailwheel shock absorber's blade retaining bracket is attached to the fuselage using two bolt-nut assemblies. The left bolt-nut assembly was damaged, resulting in abnormal play (lateral and vertical) in this assembly. The examination revealed that the diameter and length of the left bolt differed from those recommended by the manufacturer. The left bolt was a metric standard bolt, while the nut used on this bolt was American standard and had a different thread pitch than the bolt. Its hardness was also lower than that of the bolt. The observed damage was therefore a consequence of the use of incompatible parts. The bracket was re-welded, indicating a previous repair before the accident but not contributory. The replacement of these two bolt-nut assemblies with assemblies not meeting the manufacturer's specifications was carried out in March 2012 within the club. The club did not hold the approval or the necessary equipment for this type of operation. Approximately 15 hours after the nut replacement and 5 hours before the accident, the aircraft had undergone a 100-hour annual inspection at its usual maintenance unit. During this inspection, no defect was detected. ## Technical Analysis The tailwheel shimmy phenomenon is an oscillation of the wheel around the pivot axis. It is related to the landing gear geometry and wheel damping. The landing gear is less sensitive to shimmy the more the caster angle is positive or zero. An excessively positive caster angle reduces turning efficiency. When the aircraft is loaded, the spring blades are compressed and the caster angle decreases. The landing gear becomes sensitive to shimmy when the caster angle is negative. During the event, the play present in the bolt-nut assembly acted on the damping constant and allowed a lateral displacement of the blade stack, favorable to shimmy. This play, sufficiently important, combined with the amplitude of the shimmy, led to the misalignment of the tailwheel and its locking at 90 degrees. Normally, action on the rudder pedals allows, thanks to the coupling springs between the rudder and the tailwheel, to orient the wheel a few degrees. However, it does not allow the wheel to be returned to the centerline when the wheel is locked at 90 degrees.