A B767-300's EEC malfunctioned at cruise altitude causing a precipitous airspeed loss with no warning; annunciations or throttle movements.
Synopsis
A B767-300's EEC malfunctioned at cruise altitude causing a precipitous airspeed loss with no warning; annunciations or throttle movements.
Narrative
It appears our situation isn't the first of it's kind in a flat panel 767. Approximately 3 hours into our flight we were at FL 340; operating with LNAV/VNAV and autothrottles on. I had just scanned the flight instruments. I looked over as the Captain plotted the 2 degree/10 minute mark. Approximately 2 minutes later we realized our airspeed had decayed because we got an initial buffet. I disconnected the autothrottles and pushed them forward. I also disconnected the autopilot and began descending slightly to gain airspeed. The Pitch Limit Indicator (PLI) was in view; but well above the command bars. We never had a stick shaker. I don't recall the altitude loss; but know it wasn't more than a few hundred feet because it didn't take much to climb back to FL 340. After recovering the airspeed; we reengaged the autothrottles and autopilot. The commanded airspeed was .82 Mach and 285 knots. The speed increased to 292 knots. We disconnected the autothrottles and flew the rest of the trip without autothrottles. What I believe is significant to note is that we had no EICAS message; MC; MW or aural warnings. The throttles had not physically moved. We just lost airspeed for no apparent reason. We were dispatched with an inoperative EEC under MEL 73-01. After reviewing the MEL; we chose to use the autothrottles. Departing; we were cleared to 6000 feet. As we were leveling off; the throttles attempted to retard to idle. I turned them off and pushed the power forward to maintain 250 knots. We reengaged the autothrottles. This happened again as we were climbing though 9000 feet. I continued the climb to cruise with manual throttles. We reengaged them and everything seemed stable for 3 hours.
Second reporter narrative
At that time we disengaged and flew the remainder of the flight auto throttles off; manually setting all power settings. This proved to take constant attention as throttles were quite sensitive as was airspeed although had a mushy feel. This aircraft was on a 5th day of repeated EEC Pireps (5 in 5 days) with two EEC changes. We dispatched MEL 73-01 which did not prohibit autothrottle operations; nor were they placarded. On initial climbout leveling at 6000' commanding 250 kts as per the SID the autothrottles keep trying to retard to idle on speed; either in VNAV or Alt Hold/Speed Mode. On climbout we had one additional anomaly passing through FL90 for FL120 they retarded again. After that point the system operated normally and as usual autothrottles were used for speed protection. We had 3 hours normal Operations. No indication including throttles physically retarding was ever noted. As a note there is a paper trail since of 5 write ups regarding EEC. QRH requires disengaging the autothrottles if the EEC abnormal occurs in flight but as we dispatched MEL 73-01a and they worked normal through takeoff roll to first level off and after that point wonder if perhaps an unknown associated failure in automation occurred. It is again important to note no visual; annunciated; throttle movement or audio warnings existed. Several hundred feet was used in Oceanic Airspace to regain airspeed promptly; no exceedences on power plants occurred as we were aware of loss of over boost protection with EEC's INOP. When the decaying airspeed was noted it was insidious and startling fast; without any cockpit indications as mentioned; no change for hours in auto throttle mode or operation other than normal Operations. Airspeed does take an extra second or two to 'breakout' and quickly ascertain with the flat panel; PLI was WELL above our attitude. When the First Officer recovered the airspeed (doing a GREAT JOB) I recall getting a momentary shaker. The First Officer felt just a little bit of burble and does not recall shaker activation at any time. Checking the performance chart afterward; the charted stall speed at 338;000 LBS as I recall this aircraft configuration was well below our speed. I don't recall the initial shaker my side however the First Officer does on the First Officer side.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.