Flight 970 · McDonnell Douglas MD-82

The airplane experienced a roll back of engine power on both engines and subsequently stalled while in cruise flight at flight level 330. The airplane was operating on autopilot with the autothrottles engaged. The flight crew was able to restart the engines and a precautionary landing was made without further incident. Examination of the airplane and its systems revealed no anomalies that could be attributed to the loss of power. The weather conditions were consistent with the presence of ice crystals at the cruise altitude. The engine inlet probes became blocked due to the ice crystals resulting in a false engine pressure ratio (EPR) indication and subsequent retarding of the throttles by the autothrottle system. This is evidenced by the data from the digital flight data recorder (DFDR) which showed that the EPR indication increased while the airspeed decreased, the pitch attitude increased, the altitude remained the same, and the autothrottles had reached the EPR limit. The DFDR data showed that the power reduction, the corresponding reduction in airspeed, and the increase in pitch occurred over the course of about 5 minutes prior to the roll back of the engines. Over this time period, the airspeed dropped from 271.75 to 209.25 knots, and the pitch increased from 0.9 to 4.91 degrees. The DFDR data also shows that during the 24 seconds after the roll back of the engines, the airspeed continued to drop to as low as 187 knots. The only engine performance parameter recorded by the DFDR is EPR. Interviews of the pilots show that their first indication of a problem occurred just before the airplane stalled. The pilots also said in their interviews that engine anti-ice was not used prior to the loss of engine power.

Verbatim NTSB analysis and probable cause from the NTSB dataset