Pilot Debrief

SCHWEIZER 269C near Piney Flats, TN — 2012-05-24

Final reportERA12LA368
Sign in to save
Date
2012-05-24
Location
Piney Flats, TN, USA
Aircraft
SCHWEIZER 269C
Registration
N760DA
Category
Helicopter
Highest injury
None
Fatalities
0
Phase of flight
Approach

Probable cause

The mechanic’s improper maintenance of the main transmission aft pinion nut and belt drive system, which resulted in the uncoupling of the tail rotor driveshaft and the subsequent loss of helicopter control.

NTSB narrative

The pilot reported that, while on short final for landing, he heard a "high pitched whine" and that the helicopter's tail rotor authority subsequently degraded. He immediately initiated a full autorotation to landing. During the landing in a field with uneven terrain, the helicopter was substantially damaged. Postaccident examination revealed that, during the flight, the tail rotor driveshaft (TRDS) had become uncoupled from the drive assembly and main rotor belt transmission. The forward TRDS retention nut was found safety wired; however, it was less than hand tight. The main rotor transmission input aft pinion nut was found backed off the pinion; the nut and pieces of its fractured cotter pin were found captured in grease contained in the TRDS fitting grease cavity. The nut was intact, but it exhibited thread wear. The cotter pin was shorter and thinner than the cotter pin specified in the helicopter manufacturer's illustrated parts catalog. The grease in the aft pinion splines was discolored, caked, and dry, indicating that it had not been serviced in a long time. The main rotor transmission input pinion exhibited significant wear on the forward spline set and the aft spline set that engaged the driving spline, which was normally retained by the aft pinion nut. The aft upper H-frame bearing was displaced on the driving spline. The driving splines' interior spline teeth exhibited severe wear matching the wear on the pinion, and rotation damage was found on the spline ends. Other areas of the pinion exhibited fretting corrosion. According to the helicopter manufacturer's Handbook of Maintenance Instructions, during the 1,200-hour inspection, the mechanic was required to remove the upper pulley, inspect the pinion, and torque and safety wire the aft pinion nut during reassembly. The mechanic who performed the 1,200-hour inspection (and others) stated that, during the inspection, he had "never gotten into" the drive assembly, had not checked the torque on the aft pinion nut, and had not inspected the cotter pin.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Properly torque and safety the main transmission aft pinion nut during maintenance.

NTSB coding

Evidence available

  • Photos
  • 17 docket documents
View NTSB final reportView NTSB docket

Docket documents17

Related mishaps