Accident Overview
On 21 May 2005 at 1015 hrs, an Enstrom 280FX helicopter, registration G-MHCK, was conducting a trial lesson at Barton Airfield, Manchester. The aircraft was powered by one Lycoming HIO-360-F1AD piston engine and was manufactured in 1985. On board were the pilot, aged 67 with an Airline Transport Pilot's Licence and 20,000 flying hours (12,000 on type), and one passenger. No injuries were sustained.
Sequence of Events
Following a normal start-up, the pilot lifted the helicopter into a hover and noticed a burning smell and smoke from the engine area. He reported a loss of power and landed back on the helipad. Air Traffic Control informed the pilot that the aircraft was on fire. He shut off the fuel and electrics, and both occupants exited without injury. The Airport Fire Services extinguished the fire.
Post-Fire Examination
After the fire, the pilot looked inside the engine bay and found the turbocharger exhaust pipe detached. The exhaust from the turbocharger outlet passed via a short pipe through an aperture in the skin to overboard. The pipe had failed circumferentially around the weld attaching the flange to the main tube. The material of the main tube and flange was specified as 321 grade stainless steel; the filler weld material was AMS5680 (347 grade stainless steel).
Metallurgical Findings
The exhaust pipe was sent to the AAIB for metallurgical examination. Visual examination of the fracture surface revealed one half severely corroded and the other half exhibiting blue/purple colours indicative of heat tinting. Scanning Electron Microscopy (SEM) showed intergranular cracking on the outer edges of the coloured area, with fatigue cracks in the centre. Approximately 60% of the fracture surface was severely corroded, indicating the crack had been present for some time.
Conclusion
The Enstrom 280FX turbocharger exhaust pipe failed due to intergranular cracking around the weld connecting the flange to the main tube. It is probable that the cause of the intergranular cracking was sensitisation of the austenitic 321 stainless steel during welding; sensitisation occurs in the temperature range 500–850°C, lowering corrosion resistance. Fatigue was a secondary failure mode initiated by cyclic loading on the reduced thickness from the intergranular cracking.
Previous Events
A previous incident involving an Enstrom 280C (G-BRPO) reported a fatigued clamp securing the exhaust pipe, but the manufacturer's agent advised that fractures of that clamp type are rare and there have been no reported incidents of exhaust pipe failure.