This bulletin provides information explaining event history and conditions that can lead to inadvertent engine failure. Engineering changes and crew procedures are specified.
Boeing and Rolls-Royce have received seven reports of rundown of RB211- 535E4 series engines during descent. Each of the events were reported to have occurred during potential icing conditions or adverse weather, and between 25,000 and 30,000 feet altitude. Six of the events occurred during acceleration to approach idle which is automatically commanded upon selection of engine antiice. The seventh event occurred during an acceleration from minimum idle which was initiated by the autothrottle. Boeing and Rolls-Royce have reviewed flight data recorder data from each of the rundowns. In four of the events, the data indicated that the other engine, which did not run down, exhibited parameter fluctuations which are consistent with a compressor stall.
Because all of the events occurred in adverse weather, we have concluded that weather caused a deterioration in the engines' handling characteristics. Analysis of all available information suggests that icing of the compressor inlet or ingestion of ice shed from the compressor inlet caused a reduction in engine stall margin. To address this condition, Boeing and Rolls-Royce have considered several modifications to improve the engine compressor stall margin. We have determined that the most appropriate means of accomplishing this improvement is to modify the Fuel Flow Governor (FFG) to increase the engine's minimum idle speed.
Flight crews shall turn on engine anti-ice prior to thrust lever retard when beginning a descent. Below 20,000 ft. engine anti-ice may be selected off if atmospheric conditions permit. Follow normal engine anti-icing procedures for the remainder of the descent.
To correct the top of descent point (TOD) for these conditions, flight crews should forecast engine anti-ice on by entering 25,000 on the DESCENT FORECASTS page. Insert this information at least 200 NM prior to the unadjusted top of descent point.
Due to the longer descent, additional trip fuel of approximately 400 lbs/180 kgs will be required.
After engine modification, with Engine Anti-ice off:
As a result of the slight increase in minimum idle thrust, the TOD points on airplanes with one /two modified FFG's will experience an actual TOD point 10/20 NM before the "normal" TOD point currently being computed by the FMC. We feel that this error is small enough to be accepted until the FMC can be updated during a normal update at which time the correct thrust data will be incorporated.
If one or both FFG's are modified, the following is offered as an interim procedure to allow flight crews to more accurately program the TOD point until the FMC is updated. If this procedure is not used, speed brakes may be required to maintain the VNAV path. To program the FMC with the correct top of descent point for the new minimum idle descent, flight crews should forecast engine anti-ice on by entering 10,000 FT if one engine is modified and 20,000 FT if two engines are modified on the DESCENT FORECASTS page. Insert this information at least 200 NM prior to the unadjusted top of descent point.
Due to the slightly longer descent, additional trip fuel of approximately 100 lbs/45 kgs will be required.
If only one engine’s FFG is modified, the following effects will be observed/experienced when the thrust levers are at the minimum idle position, with engine anti-ice selected off.
Due to the increased minimum idle setting which will be effective both in flight and on the ground, landing ground roll at high altitude airfields has increased slightly. Corrected performance data will be provided by Airplane Flight Manual and Operations Manual revisions. In the interim, operators are advised to determine the allowable landing weight by reducing the Available Landing Field Length, with operational anti-skid, by 80 ft. With anti skid inoperative reduce the Available Landing Field Length by 300 ft. In addition, the Maximum Quick Turn around Weight should be reduced by 1500 lbs/680 kgs. The effect on all other performance, both in flight and on the ground, is considered negligible.
Upon compliance with the corrective action specified in the soon to be released Airworthiness Directive the requirement to activate engine anti-ice prior to all descents is removed.
This anomaly is corrected by Rolls-Royce Vendor Bulletin RB211-73-B869.