Engine Spool-ups

From:         gordona@wimsey.com (Gordon R. Andrews)
Date:         08 Mar 95 02:59:49 
Organization: Wimsey Information Services
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In response to the discussion on engine spool ups (unintended), I would like 
to describe an event that happened to me.  

We were climbing out of Maui in a RR powered 757 (180 min ETOPS) bound for 
Vancouver, B.C.  Through 25000 feet we noticed that the right engine EGT was 
boxed yellow at 795 degrees.  The right EPR guage was sitting at about 1.83.  
I pulled back the right thrust lever and nothing happened.  Repeated movement 
of the thrust lever did not affect the engine at all.  We elected to shut the 
engine down with the fuel control switch.  We returned to Honolulu, where a 
series of checks in the E and E bay found no faults.  We did a dry motoring,  
an idle run at the gate, a full power run on the runway and then took the 
airplane to 39000 feet on a test flight.  No problems.  The aircraft was 
subsequently dispatched and flew 7 more sectors before the right engine Fuel 
Flow Governor was pulled and sent to Woodward for bench testing.

These are excerpts from their report.

Findings from the investigation were reproted by Woodward in an Engineering 
Alalytical Report, EAR 1944763 - 940107.  Briefly, it was discoveded that 
accel and decel scheduling was extremely enriched because the CDP Servo Piston 
seized in a high CDP position.  The piston seized due to interference of a 
press fit pin inside the piston with the ID sidewall of the servo sleeve.

Based on the engine parameters from the DFDR data during th eevent, the 
calculated CDP was approximately 355 psia which was representative of takeoff 
CDP.  Therefore, it was deduced that the piston became stuck during takeoff 
which held the decel fuel limit artificially high.  During takeoff at high 
power, this did not affect performance because the fuel flow requirement was 
high.  However during climb when the fuel flow requirement was reduced, the 
enriched decel limit exceeded the engine's required-to-run line and prevented 
the pilot from reducing engine power with the throttle.

Conclusions

The unit could not have been calibrated if the pin the CDP Servo Piston had 
been installed improperly.  Also since the FFG had operated normally for 7600 
hours ans since the wear on the ID sidewall of the CDP Servo Sleeve from the 
pin was minimal, it was concluded that the pin moved in service.

Based on one problem in several hundred million operating hours of experience 
in similar applications, plus the lack of a definite cause for the problem, 
the probability of another occurrence should be considered extremely unlikely. 
 No further corrective action is planned at this time.


I am wondering if the problem stated in a previous posting of unscheduled 
spool up, is related to this event.

If anyone has any comments let me know

Gordon Andrews
Vancouver
gordona@wimsey.com