Boeing continues to receive reports of flight crews performing engine in-flight shutdowns based solely on reasons such as loss of engine parameter indications, high engine Airborne Vibration Monitoring System (AVM) indications or appearance of EICAS Status messages. These inappropriate shutdowns can have an adverse and unnecessary impact on flight safety margins.
Boeing's philosophy regarding the need to shut down an engine in flight is described in the current Operations Manual Non-Normal Checklist Introduction:
Checklists prescribing an engine shutdown must be evaluated by the captain to ascertain whether an actual shutdown or operation at reduced thrust is the safest course of action. Consideration must be given to probable effects if the engine is left running at minimum required thrust.
The following guidelines are provided for flight crews to consider when anomalous engine behaviour is encountered.
Boeing has received reports regarding loss of an engine parameter indication while in-flight on 747-400, 757, 767 and 777 airplanes. Examples of loss of engine parameters are: no EPR indication, RPM indicator zero, blank or zero EGT indication, etc. In each case there were no other abnormal engine indications. In some cases, the affected engines were shutdown when it was not necessary to do so.
Flight crews should be advised that the loss of an engine parameter indication while in flight does not necessarily constitute a situation that would require the engine to be shut down. As long as there are no other accompanying indications of an engine malfunction, the engine can be operated normally for the remainder of the flight.
Current FAR's require that AVM systems be installed in all large transport airplanes and, except for short periods allowed by the Master Minimum Equipment List (MMEL), they must be operative.
AVM limits, if any, are provided by the engine manufacturer in their engine Operating Instructions. However, except in a few cases, the engine manufacturers have not defined "hard" AVM limits (ie. amber bands, red lines, lights or EICAS messages) where crew procedures or an engine shutdown is required solely on the basis of high AVM indications and without other abnormal engine parameter indications. Instead, the engine manufacturers have provided AVM operating "guidelines" which are used to provide AVM procedures on three crew and 737 series airplanes.
Currently, flight crews on 707, 727, 737 and three crew 747 airplanes monitor engine and system indications and take actions based on observed trends to prevent exceeding engine operating limits. This sometimes includes precautionary engine shutdowns. AVM indications are included as one of the engine parameters to be used in judging the condition of the engine. Relative changes in AVM readings, as well as absolute values, are used as a cue to cross-check other engine parameters to determine the procedure to be used. This requires a continuous monitoring or awareness of the AVM indications.
With the introduction of EICAS equipped 757, 767, 747-400, and 777 airplanes, procedures were re-evaluated and all monitoring functions were placed in EICAS. Only parameters with published operating limits were programmed to generate crew Alert messages or indications. Procedures were written so that flight crews are not required to monitor, analyse or troubleshoot engine indications to diagnose problems or initiate procedures. The EICAS monitors designated engine parameters and alerts the crew when published limits are exceeded. Flight crew procedures are accomplished in response to these alerts.
On some of the EICAS equipped airplanes, an AVM "pop-up" is provided at a predetermined value. However, with exception of the Rolls Royce powered 757, there are no associated high AVM procedures. The reason for the "pop-up" is for crew awareness and for entry in the airplane log book for maintenance trend monitoring. No crew action is required for an AVM "pop-up" unless there are other abnormal engine indications in these airplanes.
Since there are no published or defined AVM limits on EICAS equipped airplanes (except on Rolls Royce powered 757's), EICAS only indicates existing AVM values but does not generate crew Alert messages based on these values. Because of this and because there are no defined "high" AVM limits, flight crews are not expected to respond to AVM indications without other abnormal engine indications.
On airplanes with AVM procedures, flight crews should also be made aware that AVM indications are not valid while at takeoff power settings, during power changes or until after engine thermal stabilisation. High AVM indications can also be observed during operations in icing conditions.
For the above reasons, Boeing does not publish AVM procedures on EICAS equipped airplanes unless the engine manufacturers provide "hard" (not guideline) AVM limits. Boeing recommends that the AVM system be used for long-term engine condition trend monitoring. This can be accomplished by the flight crew recording the AVM values in the airplane log book during stabilised cruise or by automatic recording as appropriate.
In line with the above, flight crews should be instructed, in the absence of published AVM procedures, not to shut down engines based solely on high AVM indications. However, high AVM indications when combined with other abnormal engine indications, extreme vibrations, severe damage, and/or engine limit exceedances should be addressed in accordance with the appropriate Non-Normal Checklist.
Oil pressure is considered as the most significant of several Oil System indicators. In addition, oil temperature and oil quantity indications, in concert with oil pressure indications, enable the flight crew to recognise a deteriorating oil system. While engine operation is governed by both oil pressure and oil temperature limits, there is no minimum oil quantity limit.
When abnormal oil quantity Indications are observed, check oil pressure and temperature to confirm the abnormal quantity indication. A sudden or complete loss of oil quantity without abnormal indications of pressure or temperature is most likely a quantity indicator malfunction. In either case, if oil pressure and oil temperature indications are normal, operate the engine normally. If any operating limit is exceeded, take appropriate action as specified in the Operations Manual.
On some engine models, oil quantity variations can also occur due to characteristics of the lubrication system. When the engine is running, the oil quantity will decrease due to additional distribution of oil within the lubrication system (i.e., sumps, gearboxes and supply and scavenge fines). The magnitude of the quantity decrease, commonly referred to as "gulping", varies with engine RPM. Oil temperature (thermal expansion), aeration and airplane pitch attitude will also influence "gulping".
When a steady decrease in engine oil quantity is observed over a period of time, check oil pressure and oil temperature and anticipate an engine shutdown. When an operating limit is reached, take the appropriate action.
EICAS Status messages annunciate system failures that affect dispatch. A Status message in absence of an Alert message indicates a loss of system redundancy, not function, therefore does not require flight crew action. Status messages should never be considered as sole justification for an engine in-flight shutdown. System failures that require flight crew attention and/or action are annunciated by Alert level (Advisory, Caution, Warning) messages.
Passengers or cabin crew may occasionally report flames, sparks, or fluid emanating from an engine. Flight crew judgement must be applied in these cases, but it is prudent to corroborate these reports by qualified personnel before taking action.
More than one engine shutdown has resulted from a cabin report of fluid "leakage" in the vicinity of the engine nacelle, which was later identified as de-icing fluid. A suspected fuel leak should be addressed in accordance with the appropriate non-normal checklist, taking other available information into account.
Many engine surge or stall events are recoverable and permit continued engine operation. Stabilised engine indications and decreasing EGT indicate the surge/stall has cleared. In these cases, the engine may be operated normally or at a reduced thrust level which is surge and stall free.
Following an engine flameout, in-flight restart may be attempted if no damage is apparent. Restart should be accomplished in accordance with the appropriate non-normal checklist. Flight crews should recognise that engine acceleration to idle may be slow. The start should be allowed to proceed as long as N2 (or N3 as appropriate) is steadily increasing and EGT remains within limits.
In summary, consider an engine shutdown only as directed by a non-normal checklist. If available engine parameters remain within limits, continued engine operation is normally preferable to shutdown, even if the thrust must be reduced to idle.
Boeing Flight Operations Review 43, 28th February 2003