What happened
While operating in cruise flight at flight level 330, the aircraft experienced a rollback of engine power on both engines. This loss of power led to a subsequent stall. During the event, the aircraft was operating with the autopilot and autothrendles engaged. The flight crew successfully restarted the engines, and the aircraft completed a precautionary landing without further incident. There were no injuries reported.
The investigation
An examination of the aircraft and its systems revealed no mechanical anomalies that could be attributed to the loss of power. However, weather conditions at the cruise altitude were consistent with the presence of ice crystals. Data from the digital flight data recorder (DFDR) indicated that the engine inlet probes became blocked by these ice crystals. This blockage caused a false engine pressure ratio (EPR) indication, which led the autothrottle system to retard the throttles.
Analysis of the DFDR showed that the power reduction, a decrease in airspeed, and an increase in pitch occurred over approximately five minutes prior to the engine rollback. During this period, airspeed decreased from 271.75 to 209.25 knots, while the pitch increased from 0.9 to 4.91 degrees. In the 24 seconds following the rollback, airspeed continued to drop to as low as 187 knots. The crew reported that their first indication of a problem occurred just before the stall and noted that engine anti-ice was not in use prior to the power loss.