What happened
On 28 January 1998, an Aerospatiale AS332L Super Puma, registration G-BWMG, was performing a public transport flight from the Tern Platform to Aberdeen. While cruising at 2,500 feet, the crew heard a loud bang, followed by a violent nose-down pitch and a rightward yaw. The autopilot disengaged, and the crew initially experienced significant pitch oscillations before the aircraft settled into a steady descent.
Following a MAYDAY call, the crew briefed passengers for a potential ditching. The commander eventually regained control and increased power to climb. While the crew noted some control difficulties regarding the yaw pedals, they observed that the engines were performing normally. To assist, an S61 helicopter was vectored to the scene to perform a visual inspection. During this inspection, the S6 and the crew of G-BWMG confirmed that the horizontal stabiliser had detached completely from the aircraft.
To mitigate the impact on the centre of gravity, the crew repositioned passengers toward the rear of the cabin. The flight was subsequently downgraded from an emergency to an urgency status. The helicopter landed safely at Sumburgh airport without further incident, and there were no fatalities or injuries among the 12 people on board.
The investigation
The AAIB examined the remains of the stabiliser spar stub, which had stayed within the pylon. The investigation revealed that the failure occurred within the glassfibre collar. Metallurgical analysis of the spar showed that the internal surface of the collar was stained with corrosion products and that the bond between the collar and the spar was poor, likely due to improper assembly during a previous maintenance period.
Testing of the spar material revealed that the steel tube had a significantly lower tensile strength compared to other examined components. This was attributed to surface decarburisation caused by incorrect heat treatment during manufacturing. While other spars had had this soft layer removed through grinding, the spar on G-BWMG had not.
Findings
- The primary cause of the failure was a fatigue crack in the stabiliser spar.
- The crack initiated due to a combination of surface decarburisation (which reduced the material's fatigue resistance) and corrosion micropitting.
- The presence of a soft, decarburised surface layer made the component highly vulnerable to crack initiation.
- A lack of adequate surface protection following a previous collar replacement likely contributed to the development of corrosion.
- The sudden transition from slow crack growth to rapid failure may have been exacerbated by increased cyclic loading, potentially linked to airframe icing encountered prior to the incident.