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5th of September 2008, Blog #528

A Bell Helicopter Co 407 was being prepared for a touristic flight from the Cruise Ship True North, which was at anchor in Talbot Bay, Western Australia. Once all preparations were completed the six passengers boarded the aircraft and the engine was started. The weather was calm and ideal for a sightseeing flight.

Just moments after the aircraft hit the water, the rescue of the occupants is underway (© ATSB)

After the aircraft had lifted off the helideck the pilot moved the aircraft sideways, to the right of the vessel, with the intention to transition to forward flight. At that time a loud bang was heard and a total loss of engine power occurred and a loss of several cockpit indications. The helicopter rapidly descended into the water, impacting the water in a nose-down, right-side-down attitude. The cockpit and cabin quickly filled with water and the aircraft rolled onto its side, a short while later rolled full inverted. Only 3 seconds elapsed between the loss of power and the aircraft entering the water.

Several of the ship's staff entered the water to assist the occupants of the helicopter to exit the aircraft. Two of the occupants had been unable to free themselves from the helicopter cabin, however, they were freed by the crew. One of them lost consciousness before being freed.

The helicopter airborne from the helideck of the cruise ship, immediately prior to the engine failure (© ATSB)

The Australian Transport Safety Bureau (ATSB) was alerted and an investigation into the accident was launched. An on-site examination of the helicopter’s airframe and related systems showed damage consistent with the helicopter impacting the water with the main and tail rotors rotating. No defect in the airframe or its systems was identified, although the pilot’s perspex chin window had broken on impact with the water. The engine was removed from the wreckage and coated with a corrosion preventative compound to reduce the effect of the seawater. When examined, under ATSB supervision, the engine was found extensively corroded due to the seawater. However, a significant ‘rupture’ failure of the combustion case in the area of the horizontal weld in the left ‘armpit’ region of the casing was evident. No other anomalies were found. The damaged combustion case was removed from the engine and sent to the ATSB laboratory for analysis. There it was determined that the failure was the result of high cycle fatigue cracking (High cycle fatigue results in rapid, essentially unpredictable failures due to fatigue-crack propagation under ultrahigh frequency loading.) The cause of the accident, as stated in the ATSB accident report was;

"A ‘burst’ failure of the engine outer combustion case as a result of ongoing high-cycle fatigue cracking during normal engine operation."

The ruptured combustion case in situ on the engine (© ATSB)

As a result of this occurrence, the engine manufacturer conducted a computerised analysis of the design of the combustion case in an effort to more effectively address the relevant areas of high stress. In response to this and a similar failure in another helicopter 2 weeks earlier, the Civil Aviation Safety Authority released an Airworthiness Bulletin highlighting the circumstances of the occurrence to Australian helicopter operators.

Engine ECU (Electronic Controle Unit) data of the engine failure (© ATSB)

The ATSB report, on which this blog is based, is available for the blog readers' reference by clicking on the .pdf file below;

Ditching Bell 407 25-sep-2008
Download PDF • 3.17MB

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