What happened
On 20 July 1998, an AS3/32L Super Puma, registration G-PUMB, was conducting a scheduled air test at Aberdeen Airport following recent engineering work. The flight was intended to verify the engine's maximum contingency power rating. During the procedure, the crew was operating in a visual circuit at approximately 800 feet.
While maintaining level flight at 100 knots, the pilot reduced the fuel flow to the number 1 engine to perform the required power check. As the crew attempted to achieve the target rotor speed, a loud bang and significant vibration occurred, followed by a descent. The pilot immediately responded by reducing collective pitch and restoring power to the number 1 engine. The crew noted that the number 2 engine was running down, and the aircraft was subsequently landed safely on Runway 32.
The investigation
An investigation by the AAIB revealed that the power turbine output shaft of the number 2 engine had been destroyed. Subsequent metallurgical analysis showed that the surface temperature in the area where the splined sleeve meets the Bendix shaft had exceeded 700°C, leading to friction welding between the components.
The investigation established that six bolts, which were required to secure the splined sleeve to the aft end of the power turbine output shaft, had not been installed. The aircraft had recently undergone a 500-hour engine inspection alongside a 1200-hour inspection of the Bendix shafts.
Investigators found that the maintenance process was compromised by several factors related to shift changes and personnel transitions. An unlicensed engineer had been working on the engine and, due to a lack of formal handover information during a shift changeover, mistakenly believed the engine was in a different state of assembly than it actually was. During the reassembly, the absence of the attachment bolts went unnoticed by both the unlicensed engineer and the licensed engineer who assisted in the installation.
Findings
- The primary cause of the engine failure was the omission of six critical bolts intended to secure the splended sleeve to the Bendix shaft.
- A breakdown in the handover process occurred because the shift supervisor deviated from standard practice, leaving the engineer without direct supervision or written instructions during the early part of the shift.
- The engineer involved was working without direct supervision for several hours and had deviated from his usual assembly sequence due to the difficulty in locating necessary tools.
- Maintenance worksheets lacked clear identification of critical inspection stages, making it difficult to identify points where components could be missed.
- The lack of a face-to-face handover between the departing and arriving engineers contributed to the misunderstanding of the work progress.