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Analysis

Summary

Engine failure

Failure of the left engine from VH-EAQ occurred as a result of the fracture and liberation of approximately two-thirds of the length of a single first-stage low-pressure compressor (fan) blade. The loss of the blade section produced a significant imbalance in the fan rotor, which manifested as severe vibration of the airframe and produced heavy tip rub on the remaining blades against the fan case lining. The flight crew's subsequent observations of a high exhaust gas temperature indicated the development of anomalous combustion conditions within the engine because of the airflow interruptions produced by the fan failure.

Blade loss

Rotor kinematic laws predict that the partial loss of a fan blade will result in the fragment striking the fan case, before folding flat and moving forward from the plane of rotation with a helical motion. This motion will continue until the fragment either perforates the intake cowling forward of the fan case, exits the front of the intake cowling, or is drawn back into and re-ingested by the fan rotor. In this case, damage to the intake (nose) cowling indicated the loss of the blade section soon after liberation, with comparatively little other damage to the remaining blades. Measurements of the damage to the intake cowling indicated the blade exited the cowling with a forward and downward trajectory, sufficient to take it away from the aircraft without impacting any other part of the structure.

The radial forces transferred to the fan case by the initial impact of the blade segment were sufficient to break away several of the nose cowl bolts, one of which punctured the inboard fan case cowl. The fan speed (N1) sensor was also lost in a similar manner. Neither of these components had damaged the aircraft after exiting the engine nacelle.

Blade failure

Laboratory examination of the blade fracture surface confirmed the presence of a pre-existing trailing edge defect, from which high-cycle fatigue cracking initiated and propagated. The examination identified the defect was either produced by, or was present before the last major blade refurbishment operation incorporating an elevated temperature straightening operation. The size and nature of the defect was such that it should have been detectable by non-destructive means following the blade refurbishment operations. The engine manufacturer stated that the eddy-current method specified for this inspection had the capability to detect defects well below the size of the actual defect present. In this regard therefore, error by the inspecting technician was the most likely factor contributing to the failure to detect the defect.

Pratt & Whitney service bulletin SB 72-255 was the core document that specified the requirements for the periodic in-service inspection of the engine low-pressure compressor blades. The objective of that service bulletin was to provide opportunities for the early discovery and repair of foreign object damage, thereby reducing the potential for foreign object damage induced blade failures. The bulletin required the visual inspection of the fan blade assemblies and the blend repair of all leading edge damage found, with eddy current inspection included as an option at the operator's discretion. In this case however, visual inspection alone would have likely proved unsuccessful in detecting the defect at the origin of fatigue cracking, due to the small defect size and the absence of any associated foreign object damage.

In the current engine operator's case, an eddy current technique was used to complement the visual inspection, however this was a limited survey and did not extend to the examination of the blade trailing edges. It was not known whether any of the previous engine operators had used an eddy current inspection as part of their compliance with SB 72-255.

Continuing airworthiness

The US Federal Aviation Administration has published a draft advisory circular that provides a mechanism for the assessment of the continued airworthiness of powerplants and auxiliary power units on transport category aircraft (AC39-XX). The advisory circular describes the Continued Airworthiness Assessment Methodologies (CAAM) and uses them to identify unsafe conditions, before prescribing corrective actions in accordance with the Federal Aviation Regulations (FAR) part 39.

The CAAM recognise uncontained engine blade failures as an 'historically unsafe condition' and as such, require that an appropriate response be determined and carried out. In this case, the unsafe condition may be more specifically defined as the presence of undetected defects within the blade trailing edges, from which uncontained failure may result. Following from this, aircraft exposed to this unsafe condition are defined as those aircraft fitted with engines carrying defective fan blades. In the context of this occurrence, the assessment of the level of exposure (ie. number of aircraft) can only be determined by the inspection of each blade currently in service, to determine the presence or otherwise of the defect/s. Minimisation of the risk is achieved by the subsequent removal of all blades found to contain defects.

This action is proposed by the engine manufacturer and is detailed within section 4 of this report (Safety Action).

 
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