What happened
On 20 February 2012, an Agusta Bell 206B Jet Ranger II, registration G-SUEZ, was performing a pipeline inspection approximately 3.4 miles northwest of Perth, Scotland. While flying at 600 feet agl, the crew heard a loud bang, followed by a left yaw and a visible wind-down of the engine. The pilot successfully executed an autorotation into a nearby field. There were no injuries to the pilot or the passenger.
The investigation
Following the incident, the AAIB examined the engine's condition. An initial inspection showed that the axial compressor had failed, leading to a breach in the compressor case. Upon removal and inspection at the operator's maintenance facility, investigators found numerous fragments of compressor blades and stator vanes. The engine was subsequently sent to an approved overhaul facility for detailed examination under the supervision of the AAIB and the engine manufacturer.
Detailed analysis revealed that the compressor case had suffered several ruptures in the plane of the stage-two and three compressor discs. The investigation found that blades from stages two through six had separated from their discs. While the stage-one blades remained attached, they sustained significant damage. The extent of the internal damage was so severe that the initial point of failure could not be identified via optical examination.
Metallurgical testing of the remaining components showed no evidence of Foreign Object Damage (FOD). However, scanning electron microscope (SEM) examination of certain blades revealed pits at the leading edge blade root area. While the primary failure was identified as tensile overload, the presence of these pits suggested they could have acted as a potential initiator for the crack.
Findings
- The engine failure was caused by the fracture of a stage-two compressor blade due to fatigue crack progression.
- The failure of the stage-two blade led to significant downstream damage, including the separation of multiple compressor blades and vanes and the rupture of the compressor case.
- The maintenance programme used by the operator contained ambiguous wording regarding the 300-hour inspection task, which failed to specifically reference the detailed inspection criteria and limitations found in the manufacturer's manual.
- At the time of the incident, the aircraft had been operating away from its main base for two days and had not received a compressor wash during that period.
Safety action
Following the investigation, the maintenance organisation revised its 300-hour inspection programme. The updated programme now includes a specific task for inspecting the compressor rotor and provides explicit references to the relevant sections of the manufacturer's manual to ensure inspection criteria and limitations are clearly understood.