Boeing Rolls Royce and Boeing have received reports from several 757 operators with RB211-535E4 series engines with Phase 5 combustors installed. The reports describe a low frequency noise (rumble) in the flight deck that occurs at idle power on the ground, particularly for the short period when the engine stability bleeds are open immediately after engine start. The noise is more prevalent during low ambient temperature conditions. There are no unusual vibration indications on EICAS. The noise should cease shortly after engine start.
Investigation into the cause of the noise has shown that the phenomenon is a result of pressure oscillations within the combustor. The Phase 5 combustor was introduced in 1999 to meet the Committee on Aviation Environmental Protection (CAEP 2) emissions requirements. The features of the Phase 5 combustor that produce reduced emissions also tend to make the combustor more susceptible to the noise at low power. Low ambient temperatures tend to increase noise due to the effect on air/fuel ratio. Phase 5 combustor noise will have no impact on engine or airframe structural integrity, or airplane performance.
Boeing Flight Operations Technical Bulletin 757-70, 10th November 2002