Incident: Virgin Blue B738 at Launceston on Aug 20th 2009, engine surge

27.07.2010 - 09:47

The Australian Transportation Safety Board (ATSB) released their final report concluding, that the contributing safety factors were: - Advanced wear of the Stage-3 variable stator vane (VSV) bushing and inner shroud promoted movement and mechanical interactions that led to the liberation of a segment of high pressure compressor (HPC) air seal while the engine was operating. - Liberation of the HPC honeycomb seal into the compressor gas path at the stage-3 blade trailing edges resulted in impact damage downstream, a loss of compressor efficiency and subsequent compressor surge during the takeoff from Launceston. - The CFM56-7B engine design was susceptible to VSV bushing and shroud wear that can lead to internal mechanical damage and potential in-flight performance difficulties. [Minor safety issue] - 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. [Minor safety issue] The ATSB reported, that shortly after takeoff multiple bangs consistent with a compressor surge were heard prompting the crew to reduce engine thrust to idle and call PAN. After ensuring a safe landing could be made at Launceston s wet runway the airplane landed safely back to Launceston about 30 minutes after departure. A walk around after the flight inbound to Launceston had revealed a small bird strike at the left hand wing s flaps near the trailing edge. No bird remains were found at Launceston however, maintenance did not detect any evidence of bird ingestion into one of the engines or bird remains in the engines. Following the return a boroscopic inspection of the engine revealed damage to the high pressure compressor (HPC) blades and vanes. The engine was removed from the aircraft and sent to the manufacturer for further analysis. The ATSB requested flight data and cockpit voice recorders to be removed from the aircraft and sent in for analysis. The flight data recorder (FDR) showed, that the exhaust gas temperature and the fuel flow of the left hand engine was slightly higher than for the right hand engine and was slightly higher during the last two flights than before, where the values of left and right hand engine had been almost identical. During the incident flight the FDR showed engine fluctuations for 7 minutes and a subsequent power reduction. The engine had not been subject to any known defects before the incident flight. It had accumulated 22978 hours since new. The engine was torn down at the manufacturer s facilities. The low pressure compressor showed no damage and no evidence of foreign object impact/ingestion. The high pressure compressor rotor however showed damage to the stage 3 and downstream blades, the damage consistent with hard object impact. None of the blades were fractured or missing however. All stator vanes exhibited some sort of damage. Further examination showed, that the inner shroud segments of the HPC stage 1,2 and 3 were loose and the bushings exhibited heavy wear. All straight pins were accounted for. Despite being loose the inner shroud segments of the stage 1 and 2 remained intact, the honeycomb seal did not move out of position. The stage 3 honeycomb seal located in the 9-12 o clock position however was missing, the honeycomb seat in the 12-3 o clock position had been displaced by about 4 inches. Two small pieces consistent with the missing honeycomb seal were found in the debris recovered from the engine. The anti-rotate pins at the 9 and 3 o clock position showed significant wear. The engine manufacturer advised that rotor to stator contact was a known issue at the time of the occurrence, several service bulletins had been issued requiring an inspection every 24000 hours and introducing new parts to the engine. The manufacturer concluded the sequence of events: - Wear of the variable stator vane (VSV) bushings and inner shroud had allowed movement between the seal retainer and inner shroud segments. - This subsequently produced heavy wear of the seal retainer anti-rotation pins. - A combination of the VSV bushing/shroud wear and loss of effectiveness of the seal anti-rotation feature allowed the seal retainer to make contact with the HPC rotor inter-stage seals. - The 3 - 9 o clock seal retainer further rotated on the inner shroud segments and was able to lift above the adjacent seal retainer. - The seal retainer was then able to further contact the rotor, which dislodged it completely from the inner shroud segments and into the compressor gas path. - The liberated seal segment then moved downstream, causing significant impact damage to the remaining stages and resulting in a loss of compressor efficiency and the subsequent compressor surge reported by the flight crew. The increase in EGT and fuel flow was consistent with the damage developing in the HPC. The incident engine did not have the new parts, but also had not reached 24000 hours yet and therefore was not required to undergo the additional inspection. The high pressure compressor, half of the stator being removed, note the rotated seal segment [arrow] (Photo: ATSB): Read the article at http://avherald.com/h?articleAe7e4a3/0000 Modify your subscription at http://avherald.com/h?login