To inform flight crews of a checklist that addresses the loss of an engine pitot probe.
Revised to remove reference to EPR for accomplishing the "Engine Limit or Surge or Stall" procedure.
Rolls-Royce and Boeing have received numerous reports of air turn backs, diversions, and aborted takeoffs due to fluctuating engine parameters. During subsequent inspection, the affected engines are often found to be missing one or more "Pf rake" pitot heads. The Pf rake pitot heads provide input to the EEC. They are installed along the leading edge of the low pressure engine compressor fairings at the upper and lower bifurcation, and on the leading edge of the A-frame support structure.
Missing Pf rake pitot heads result in a fluctuating fan pressure measurement, which causes an unstable EPR calculation. At higher power settings (e.g. greater than approximately 50% N1), the EEC Supervisory control will react to the fluctuating EPR calculation, constantly adjusting the trim to the engine fuel flow to achieve commanded EPR. This causes the fuel flow to fluctuate, which in turn causes all other engine parameters (EPR, N1, EGT, N2) to fluctuate.
If the airplane has a Limiter Control only (no EEC Control switches), the missing Pf rake pitot heads will only cause fluctuating EPR (i.e. remaining engine parameters such as N1, EGT, N2, and fuel flow will not fluctuate) and this bulletin does not apply.
The new checklist includes instructions to select the EEC switches off. If engine parameters (N1, EGT, N2, and fuel flow) continue to fluctuate at high power with the affected EEC off, refer to the existing QRH procedure for "Engine Limit or Surge or Stall". If all engine parameters except EPR stabilise after selecting the affected EEC switch off, the new procedure will select the remaining EEC switch off and continue normal operations with the autothrottle turned off.
The autothrottle requires a reliable EPR indication from both engines to operate properly.
This Operations Manual Bulletin will be canceled after Boeing is notified that all affected airplanes in the operator’s fleet have been modified by Rolls Royce Service Bulletin RB.211-72-D718 or RB.211-72-E294.