The CFM56-7B engine had sustained bushing and shroud wear sufficient to cause rotor-to-stator contact, after a time in service that was less than the minimum threshold period specified by the manufacturer, for an initial inspection targeted at identifying this problem
While the failure occurred at less than the time specified on the service bulletin, the manufacturer believed that the probabilty of a failure below this level was extremely low, and the ATSB issued an accompanying SAN to advise operators of the possibility of failure.
The TSN of the subject engine was lower than the 24,000 hours specified in the service bulletin. As such, this engine was not required to have undergone borescope inspection in accordance with the service bulletin. The manufacturer responded to the ATSB that the recommended inspection period is not necessarily set to prevent every event; but is based on the consequence of that event. The manufacturer used Weibull analysis to meet a combination of safety goals and operator needs (scheduling, costs etc). The manufacturer also stressed that only one other of the IFSD events (March 2009) occurred at less than the 24,000 threshold, at 23,700 and went on to state that; “While it may be desirable to prevent every event, a single-engine event is not in itself a threat to continued safe flight, and the aircraft are certified to fly with a single IFSD. Thus, thresholds are set to ensure that dual-engine events are extremely improbable, while not imposing an undue burden on the operators. In this case, the events are being monitored and Weibull’s updated and it is not considered necessary to reduce the inspection threshold at the moment.” ATSB assessment of response/action The ATSB is satisfied that the action taken by the engine manufacturer adequately addresses the safety issue at this stage. The ATSB will continue to monitor the issue in regard to future related occurrences and the outcome of ongoing updated Weibull analyses by the engine manufacturer. At the same time, in the interests of ensuring awareness of the safety issues identified in this investigation, the ATSB issues the following Safety Advisory Notice to all operators of CFM56-7 and CFM-56-5 engines and their variants.
The Australian Transport Safety Bureau draws the attention of all operators of CFM56-7 and CFM56-5 engines and their variants to the safety issues identified by this investigation. In particular operators should be aware of the potential for premature wear within the compressor variable stator vane bushings and shroud to develop to levels where it may precipitate the failure of the engine while in-service, and within a timeframe that is less than the minimum threshold for the initial inspection for the problem (24,000 hours TSN, per S/B 72-0515). Operators are encouraged to review their procedures to ensure an appropriate awareness of the issues among maintenance personnel.
