Engine coupling failure leads to single-engine landing in AS355F1

Casualties unknown • Near Lasham Airfield, Hampshire, GB

A mechanical failure of the engine drive shaft coupling forced an Aerospatiale AS355F1 to perform an emergency single-engine landing near Lasham Airfield.

What happened

On 19 July 2004, an Aerospatiale AS3/355F1, registration G-FFRI, was returning to Lasham Airfield after transporting passengers to the Farnborough Airshow. While cruising at 1,500 ft amsl, the pilot heard an unidentified thump. Upon inspecting the instruments, the pilot observed that the No 2 engine had dropped to ground idle RPM. The pilot subsequently shut down the engine and completed a successful single-engine landing at Lasham. No injuries were reported, and the crew remained unharmed.

The investigation

An investigation of the aircraft in its hangar revealed that the flexible coupling, known as a 'Thomas' coupling, between the engine drive shaft and the combining gearbox had disintegrated. This failure caused the housing tube, which is a structural component of the engine accessory gearbox, to shatter. The coupling had broken into numerous pieces throughout the Main Rotor Gearbox (MRGB) compartment.

Investigators examined the engine mounting and the MRGB elastomeric suspension. While the engine mount showed some settling of the elastomer due to in-service weight, it was not considered a significant factor in the misalignment. Laboratory testing of the engine mount showed no significant material property differences compared to a new component. The investigation also noted that some coupling flange bolts appeared to have suffered severe frettage due to a loose-fitting split pin, though this was not deemed the primary cause of the failure.

Findings

  • The failure of the Thomas coupling was caused by the disintegration of the flexible coupling elements.
  • The destruction of the coupling led to the shattering of the coupling housing tube.
  • The engine was mounted on a single elastomeric point, and the housing tube provided the second point of location; without the coupling, the engine was able to move laterally and longitudinally around the single bolt.
  • There was no evidence that the failure was related to recent manufacturer alerts regarding freewheel rollers or incorrect surface coatings.

Safety action

  • Safety Recommendation 2005-081: It is recommended that Eurocopter review the design or maintenance procedures for installing ‘flector’ couplings to eliminate the potential for fretting within the split-pin, nut, and bolt assembly.

Probable cause

The failure was caused by the disintegration of the flexible Thomas coupling between the engine drive shaft and the combining gearbox, which subsequently destroyed the coupling housing tube.

Frequently asked questions

What happened in the 2004-07-19 Aerospatiale AS355F1 accident near Near Lasham Airfield, Hampshire, GB?

A mechanical failure of the engine drive shaft coupling forced an Aerospatiale AS355F1 to perform an emergency single-engine landing near Lasham Airfield.

What aircraft was involved and where did it happen?

The accident on 2004-07-19 involved a Aerospatiale AS355F1, registration G-FFRI, at Near Lasham Airfield, Hampshire, GB.

What was the probable cause of the accident?

The failure was caused by the disintegration of the flexible Thomas coupling between the engine drive shaft and the combining gearbox, which subsequently destroyed the coupling housing tube.

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