Pneumatic System Duct Pressure Fluctuation with Thrust Lever Movement

Boeing Service experience indicates that some flights operate at thrusts close to the bleed air system low-to-high pressure switch-over engine pressure ratios (EPR). Such operations may cause either or both of the following effects:

Duct Pressure Split

  1. Pneumatic duct pressure fluctuations due to the bleed system switching between low-pressure (LP) and high-pressure (HP) engine compressor sections (stages).

  2. Large (approximately 20 psi) differences between the right and left duct pressure indications. The probability of the above effects occurring increases with high altitude, low gross weights, and low engine thrust settings.

If either of the effects is observed, a slight EPR increase (approximately 0.05 EPR) of the engine displaying high or fluctuating duct pressure is recommended to allow the bleed air system to resume normal operation.

Boeing is currently flight-testing possible modifications to correct the above effects.

Note: If a large difference (20 psi or more) between the right and left duct pressure indications remains, make an entry in the Tech Log as it may be that the high stage system is not controlling properly.

Boeing Flight Operations Technical Bulletin 757-31, 1st September 1989