Incident Summary
An airplane encountered a sudden loss of power on both engines while in cruise flight at flight level 330 (FL330). The aircraft was operating on autopilot with the autothrottles engaged at the time. Following the power rollback, the airplane stalled. The flight crew successfully restarted the engines and performed a precautionary landing without any further incidents. Examination of the aircraft and its systems revealed no anomalies that could be attributed to the loss of power.
Weather Conditions and Engine Anti-Ice
Weather conditions at the cruise altitude were consistent with the presence of ice crystals. According to pilot interviews, engine anti-ice was not used prior to the loss of engine power.
Investigation Findings
The investigation determined that the engine inlet probes became blocked due to ice crystals. This blockage resulted in a false engine pressure ratio (EPR) indication, which caused the autothrottle system to retard the throttles. Digital flight data recorder (DFDR) data confirmed that the EPR indication increased while airspeed decreased, pitch attitude increased, altitude remained constant, and the autothrottles had reached the EPR limit.
Analysis of Flight Data
DFDR data showed that the power reduction, corresponding decrease in airspeed, and increase in pitch occurred over approximately five minutes before the engine power rollback. During this period, airspeed dropped from 271.75 knots to 209.25 knots, and pitch increased from 0.9 degrees to 4.91 degrees. In the 24 seconds immediately following the rollback, airspeed continued to decline to a low of 187 knots. The only engine performance parameter recorded by the DFDR was EPR.
Pilot Awareness
Interviews with the pilots indicated that their first indication of a problem occurred just before the airplane stalled.