What happened
On 4 February 2008, a Schweizer 269D Configuration A (also known as a Schweizer 333), registration G-TAMA, was performing aerial work at Sheffield City Airport. The aircraft had recently completed an 800-hour maintenance check, which included the replacement of the main rotor gearbox (MRGB).
Following a post-maintenance flight to verify the tracking and balance of the main rotor, the pilot began the shutdown procedure. During this process, a loud bang occurred, followed by severe shaking, violent rocking, and a 45-degree nose-left yaw. The pilot managed to hold the collective lever down and successfully shut down the engine and fuel supply. The aircraft remained upright, and the pilot was able to exit without injury.
The investigation
Investigators examined the MRGB and found the pinion outer bearing had suffered extensive damage, including a fractured bearing cage and partially melted rollers, consistent with overheating and oil starvation.
An inspection of the bearing oil supply gallery revealed non-ferrous debris and a single piece of ferrous metal. Forensic analysis determined that this ferrous fragment was a piece of a broken stud extractor bit. Maintenance personnel reported that they had struggled to remove a blanking plug from the gearbox low oil pressure switch port because it had been overtightened. To remove the plug, they used a stud extractor, which broke during the process, and subsequently drilled the plug to loosen it. This process introduced the metal fragment into the oil supply.
Findings
- The primary cause of the gearbox failure was the seizure of the main rotor gearbox pinion outer bearing.
- The seizure was caused by oil starvation resulting from metallic debris entering the bearing oil supply gallery.
- Maintenance personnel introduced a fragment of a broken stud extractor into the oil supply while attempting to remove an overtightened blanking plug.
- The blanking plug had been installed too tightly, a situation exacerbated by the lack of specific torque instructions in the manufacturer's assembly procedure.
- The use of unconventional removal methods, such as drilling and using extractors, created the risk of internal contamination.