What happened
On 21 May 2005, an Enstrom 280FX, registration G-MHCK, was conducting a trial lesson at Barton Airfield, Manchester. Following a standard start-up, the pilot transitioned the helicopter into a hover when they detected a burning smell and smoke emanating from the engine compartment. The pilot subsequently experienced a loss of power and managed to land the aircraft back on the helipad.
Upon landing, air traffic control notified the pilot that the aircraft was on fire. The pilot immediately shut off the fuel and electrical systems, allowing the crew and passenger to evacuate the aircraft without injury. The fire, which had caused damage to the engine area and the aircraft skin, was extinguished by the airport fire services.
The investigation
Investigators examined the engine bay and discovered that the exhaust pipe leading from the turbocharger had become detached. A metallurgical analysis of the exhaust system was conducted to determine the cause of the failure. The examination of the fracture surface revealed significant corrosion on one half of the component, while the other half showed heat tinting, suggesting the metal had been subjected to high temperatures.
Scanning electron microscopy (SEM) identified intergranular cracking at the edges of the flange, with fatigue present in the center. The investigation also noted that the tube thickness had been reduced by corrosion, which eventually led to a secondary failure mode caused by cyclic loading.
Findings
- The turbocharger exhaust pipe failed at the weld connecting the flange to the main tube.
- Intergranular cracking was caused by the sensitisation of the 321 grade stainless steel during the welding process, which lowered the material's corrosion resistance.
- Severe corrosion on the tube led to a reduction in thickness, making the component susceptible to fatigue failure under cyclic loading.
- The detachment of the pipe allowed hot exhaust gases to escape through an aperture in the skin, heating the aircraft structure and causing the fire.