On 5 June 2001, a Fokker F27 Friendship took-off from Jersey Airport, Channel Islands for an empty positioning flight to Bournemouth with three crew members on board. Shortly after takeoff from Runway 27, an uncontained failure occurred to the left engine at an altitude of approximately 670 ft. This resulted in a sudden and complete loss of power from the left engine and a major fire external to the nacelle, but this was extinguished during the Engine Fire Drill as the fuel supply line was closed. The aircraft completed a left hand circuit and landed uneventfully back at the airport.
The investigation of the accident led by UK AAIB concluded that the engine failure was caused by high cycle fatigue (HCF) cracking of the High Pressure Turbine (HPT) disc. Five similar Dart HPT failures had occurred over the previous 29 years, the most recent at London Stansted Airport on 30 March 1998(G-OJEM). During the following necessarily protracted study, testing and analysis by the engine manufacturer, the evidence collected then indicated that a small gap, under running conditions, between the seal arm abutment faces of the HPT and Intermediate Pressure Turbine (IPT) discs could result in high cyclic stresses being present in the HPT seal arm radius at the disc diaphragm, and that these stresses could result in high cycle fatigue (HCF) cracking.
As a result of this, a manufacturer's Service Bulletin (SB), Modification No 1946, was issued in April 2001 with compliance date ‘not later than 31 December 2005’ and this was mandated by the CAA as an Airworthiness Directive. Jersey HPT disc failure let to the change of the SB to a cycle-based requirement with a compliance end date of 30 June 2004.
“The investigation identified the following causal factors:
- Minimal fatigue strength margin of the engine HPT disc resulted in it being susceptible to rapid cracking if subjected to vibratory excitation, such as resonance.
- The abutment between the HPT and IPT discs probably resulted in a small gap being present between the seal arms while the engine was operating. This allowed sufficient reduction in the natural frequency of the turbine disc vibratory mode for it to be excited while operating within the normal speed range of the engine.
- The protracted time taken following the G-OJEM event, due to the nature of the tests required to understand the cause of the failure, precluded the timely introduction of suitable preventative action aimed at avoiding recurrence prior to the HPT disc failure on G-CEXF.
- Fuel leakage from a severed low pressure pipe, part of the engine bay fuel system, led to a major fire, external to the nacelle.”
No new Safety Recommendations were made in direct connection with the accident. However, 19 recommendations were made in connection with the previous accident to G-OJEM. Two of them specifically address the HPT failure -these were included in the report:
- Safety Recommendation 2001-20 - “It is recommended that the engine manufacturer and the CAA reassess the susceptibility of the three-stage Dart turbine to HCF failure and ensure that effective action aimed at preventing recurrence has been taken.”
- Safety Recommendation 2001-21 - “It is recommended that the CAA and the engine manufacturer consider the need for further improvement to their systems to ensure effective action to prevent recurrence following potentially catastrophic in-service failures of UK type-certificated equipment used on public transport aircraft.”