CEN11IA156

During the initial approach, as the airspeed decreased through about 70 knots, the helicopter began an uncommanded right yaw of more than 100 degrees, and the yaw could not be stopped by the application of left pedal. The crew diverted to a nearby airport and performed a successful roll-on landing, touching down at 72 knots using differential braking to maintain the runway heading. An after-landing inspection showed that the tail rotor pitch change beam retaining nut, pitch beam retaining washer, and associated screws and washers were missing. The remainder of the tail rotor assembly remained attached and did not depart the helicopter. Recent maintenance had been performed to replace the pitch change beam retaining nut and pitch beam retaining washer. According to maintenance personnel, the maintenance had been performed per the existing maintenance procedures, and the mechanics did not notice anything odd or out of place. Evidence of witness marks along with plastic deformation on the end faces of the pitch change shaft splines is consistent with the retaining washer installed with its indexing spline slightly off, resulting in a cocking condition and incomplete engagement of the spline teeth. Accordingly, there was likely an improper alignment of the components during reassembly, which would have prevented the nut from locking in place. Subsequent tests on an exemplar assembly showed that it was possible for the pitch beam retaining washer to be misaligned with the teeth on the shaft splines, which then showed barely visible evidence of the misalignment.

On January 19, 2011, at 1134 central daylight time, a Sikorsky 92A helicopter, N920AL, experienced a flight control malfunction and sustained minor damage during initial descent for landing to Air Logistics Galliano Heliport (2LS0), Galliano, Louisiana. Visual meteorological conditions prevailed and a company flight plan was filed for the 14 Code of Federal Regulations Part 135 non-scheduled passenger flight. The three crewmembers and 15 passengers were not injured. The flight originated from an off-shore helipad at Green Canyon 858 in the Gulf of Mexico, with 2LS0 as its intended destination. During the initial approach into 2LS0, as airspeed reduced through 70 knots, the helicopter began an uncommanded right yaw of more than 100 degrees that could not be stopped by application of left anti-torque pedal. The crew immediately lowered the nose to regain airspeed and streamline the aircraft. They diverted to South Lafourche Airport (GAO), Galliano, Louisiana, and performed a successful roll on landing, touching down at 72 knots, utilizing differential braking to maintain runway heading. An after landing inspection showed that the tail rotor pitch change beam retaining nut, pitch beam retaining washer, and associated screws and washers were missing. The tail rotor hub and paddles remained attached to the pitch change links and the pitch beam assembly and did not depart the helicopter. Recent maintenance had been performed to remove and replace the pitch change shaft. Following that maintenance the tail rotor pitch change beam retaining nut and pitch beam retaining washer had been reinstalled and the retaining nut had been torqued to the required value. All of the operator’s mechanics said they thought that all the maintenance items had been performed correctly and they did not notice anything odd or out of place. Following this incident the manufacturer conducted tests on an exemplar assembly that showed it was possible for the pitch beam retaining washer to be misaligned on the spline and have it partially engage the shaft teeth as the retention nut was rotated and tightened. With the locking washer misaligned, it turned approximately 90 degrees clockwise as the retention nut was being preloaded. At that point the locking washer partially engaged the spline teeth and remained in the cocked condition until the full amount of torque was achieved. The gaps created by the cocked condition and the thread protrusion of the pitch beam retaining nut were visible and measureable. The post-test condition of the teeth on the locking washer included barely visible evidence of the misalignment against the shaft splines. The manufacturer subsequently issued a change to the aircraft maintenance manual which included additional confirmation of proper alignment of the pitch beam retaining washer and a check for thread protrusion of the pitch change shaft above the tail rotor pitch beam retaining nut.