Boeing Rolls-Royce and Boeing have received reports from 757 operators with RB211-535E4/E4-B/E4-C series engines with Phase 5 combustors describing a fuel odour in the flight deck and/or cabin following engine start. The odour occurs when the ECS packs begin using engine bleed air following engine start and may last for several minutes. The odour is predominantly experienced during tailwind starting conditions. This phenomenon has also been observed during production flight-testing at Boeing.
Investigation into the cause of the odour has shown that the phenomenon is a result of design changes associated with the combustor. The Phase 5 combustor was introduced in 1999 to meet CAEP 2 emissions requirements. As part of the change, the geometry of the high stage engine bleed duct was modified. This modification created a less restricted path from the combustor back to the high stage bleed duct. During tailwind starting conditions fuel vapour can be pushed back into the duct and enter the bleed system. Very small amounts of fuel vapour can create significant odours in the airplane and may take several minutes to clear. These short duration fuel odours following engine start are characteristic of this engine and are not an indication of a malfunction or condition requiring maintenance action.
The fuel odour will normally clear in less than 4 minutes, however may persist longer in situations involving low taxi power settings or short taxi times. If the odour persists for a significant period on the ground or longer than 2 minutes after takeoff power is applied, the Captain may consider at his discretion, accomplishing the SMOKE OR FUMES AIR CONDITIONING checklist.
Rolls-Royce has developed an engine modification (available via Service Bulletin) to help purge the high pressure bleed duct of fuel vapours during engine start and reduce the intensity of the odours.
Boeing Flight Operations Technical Bulletin 757-73, 17th June 2004