History of the Flight
The helicopter, registration G-SUEZ, was conducting a pipeline inspection approximately 2 nm north-west of Perth, Scotland, at 1500 hrs on 20 February 2012. At about 600 ft agl, the crew heard a loud bang, the helicopter yawed left, main rotor rpm decreased, and the engine wound down. The pilot executed a successful autorotation into a field and subsequently reported the incident to air traffic control. There were no injuries.
Investigation Findings
Initial inspection revealed the engine had suffered a failure of the axial compressor, with the compressor case breached. No damage was found to other helicopter components. The engine was removed and examined at the operator's maintenance organisation, where fragments of compressor blades and stator vanes were recovered. Further examination at an approved overhaul facility, under AAIB and engine manufacturer supervision, showed multiple ruptures of the compressor case in the plane of stage-two and three compressor discs. All blades from stages two through six had separated from their discs; stage-one blades remained attached but were damaged. Stage-two and three stator vanes were heavily damaged; subsequent stage vanes had separated. The centrifugal impeller showed impact damage but remained intact. Impact damage was also noted on diffuser tubes and turbine. The extent of damage precluded optical identification of the failure cause, so the engine was sent to the manufacturer for detailed investigation.
Metallurgical Examination
Metallurgical tests confirmed no material abnormalities and no Foreign Object Damage (FOD). Scanning Electron Microscope (SEM) examination of fracture surfaces indicated most failures were due to tensile overload. However, a section of a fracture face on one stage-two compressor blade showed evidence of crack progression in fatigue. Further examination suggested the crack propagated in High Cycle Fatigue (HCF) from the suction side. Secondary damage prevented identification of the crack initiation point. SEM also revealed pits in the leading edge blade root area of several stage-two and three blades, with the largest pit measuring 0.0053 inches deep and 0.0055 inches wide.
Maintenance Activities
G-SUEZ was subject to routine compressor washes as part of daily maintenance when operating from its base. At the time of the incident, the helicopter had been away from its main base for two days and had not received a compressor wash during that period. The compressor had been installed on 19 August 2011 and operated 341 flying hours prior to failure. A 300-hour inspection was completed in January 2012. The maintenance organisation's 300-hour inspection programme included a task to inspect the compressor case, referencing the manufacturer's manual, but the reference was generic and ambiguous, lacking specific references to the detailed inspection requirements for compressor rotor damage and pitting.
Safety Action
Following the investigation, the maintenance organisation revised its maintenance programme to include a specific task for inspection of the compressor rotor during the 300-hour inspection, with explicit references to the applicable sections of the manufacturer's manual regarding inspection criteria and limitations.
Probable Cause
The probable cause of the event was the fracture of a stage-two blade due to crack progression in fatigue.