What happened
On 4 November 2004, a Gazelle HT Mk 2, registration G-GAZL, was conducting a private flight from a landing site near Derby to another site near York. The flight included a planned refueling stop at Sheffield City Airport. While on a left-hand base leg for Runway 2 and approximately two miles south of the airfield, the pilot experienced a loud bang and a sudden jolt.
Suspecting a birdstrike, the pilot preemptively lowered the collective lever to prepare for a potential autorotation. However, because engine temperature, RPM, and pressure readings remained within normal parameters, the pilot continued the approach and landed the aircraft safely. Upon inspection after shutdown, the pilot discovered that the right-hand engine cowling had been torn away and found small marks on the main rotor blades. No injuries were reported during the incident.
The investigation
The AAIB examined the aircraft and analyzed fragments of the missing cowling recovered from the site. A metallurgical analysis of a fractured spigot fitting was performed using a Scanning Electron Microscope. This examination revealed that the fitting had suffered from fatigue cracking caused by repeated overloads over an extended period.
Investigators also examined the latching mechanism of the cowling. They found that the aft latch could appear to be in the locked position even when it had not actually engaged the roller fitting. Testing with a trial piece of cowling confirmed that it was possible to close the panels such that the latch remained unengaged while still appearing visually secure.
Findings
- The separation of the right-hand engine cowling was caused by a combination of fatigue cracking in the top hinge spigot fittings and an unsecured aft latch.
- The spigot fitting had developed cracks due to multiple overload cycles, likely occurring over several years.
- The aft latch mechanism was likely not fully engaged, a condition that is difficult to detect visually because the lever can appear locked when it is not.
- During the descent, aerodynamic forces likely pulled the unlatched cowling outward and upward, placing additional stress on the already weakened hinge fittings.
Safety action
- Safety Recommendation 2005-049: The UK Civil Aviation Authority should review periodic inspections of engine cowling spigot fittings for SA341-type Gazelle helicopters operated on CAA Permits-to-Fly.
- Safety Recommendation 2005-050: The European Aviation Safety Agency should review periodic inspections of these fittings for SA341-type Gazelle helicopters operated on Certificates of Airworthiness.