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Electronic Engine Control (EEC) Pressure Sensors Freezing
GOT-15
Flight Crew Operations Manual Bulletin
for
Gol Linhas Aereas S.A.
Number:
GOT
-
15
IssueDate:
December 21, 2018
Airplane Effectivity: 737-7/-8/-9
Subject:
Electronic Engine Control (EEC) Pressure Sensors Freezing
Reason:
To inform flight crews that if the pressure sensing system contains
moisture, freezing can occur which can cause N1 oscillations.
THE FOLLOWING PROCEDURE AND/OR INFORMATION IS EFFECTIVE UPON RECEIPT
Background Information
An operator recently experienced N1 oscillations on an engine during a level off
in descent. The root cause of the oscillations was found to be the freezing of
moisture which had accumulated in the engine control pressure sensing system
used to measure Compressor Discharge Pressure (PS3). The PS3 signal is used
by the EEC for engine control scheduling. The freezing causes erroneous PS3
values which can cause abnormal engine behavior. Changes in the PS3 values
being selected by the engine control system caused the N1 oscillations.
With the autothrottle engaged, the autothrottle attempted to recover the loss in
N1 by increasing thrust. The autothrottle reaction to the thrust oscillations
prolonged the event. During the N1 oscillations, the amber THRUST alert
flashed without steady illumination. The range of the N1 oscillations observed
were up to 12% N1. When the autothrottle was disengaged, the oscillations
stopped after a few more oscillation cycles.
The ENGINE CONTROL or the MAINT light can illuminate 30 seconds after
landing.
Information in this bulletin is recommended by The Boeing Company, but may not be FAA approved
at the time of writing. In the event of conflict with the FAA approved Airplane Flight Manual
(AFM), the AFM shall supersede. The Boeing Company regards the information or procedures
described herein as having a direct or indirect bearing on the safe operation of this model airplane.
March 1, 2021
