What happened
NTSB summary, verbatimThe 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.
Photographs
1the aircraft, and what the investigators foundProbable cause
the Board's determinationThe flightcrew's failure to verify the engine instrument indications and powerplant controls while on autopilot with the autothrottles engaged, and their failure to recognize the drop in airspeed which led to an aerodynamic stall associated with the reduction in engine power. Factors were the presence of ice crystals at altitude, and the icing of the engine inlet probes resulting in a false engine pressure ratio indication.
Safety recommendations
2 issuedwhat changed because of this accidentTO THE FEDERAL AVIATION ADMINISTRATION: Issue a flight standards information bulletin to principal operations inspectors to alert all affected air carrier flight crews about the icing situation encountered by Spirit Airlines flight 970 and to emphasize the need to maintain vigilance for the signs of high altitude icing conditions, the effect these conditions can have on airplane and engine performance, and the need for the appropriate use of the engine anti-ice system.
TO THE FEDERAL AVIATION ADMINISTRATION: Actively pursue research with airplane and engine manufacturers and other industry personnel to develop an ice detector that would alert pilots of inlet pressure probe icing and require that it be installed on new production turbojet airplanes, as well as retrofitted to existing turbojet airplanes.
Sequence of events
3 timed eventsfrom the FDR factual report · claude-fable-5-1 (in-session)- SRN about 1,800takeoffp.2Takeoff from Denver (plot II-2)
- SRN about 4,725cruisep.2Start of the dual engine rollback in cruise at FL330, about 48 min 45 s after takeoff and 19 min 27 s before landing (plots II-3, II-5, II-7 cover the event: EPR both engines, flight controls, discretes and the autopilot FMA throttle mode)
- SRN about 5,892landingp.2Landing at Wichita Mid-Continent after power was restored
NTSB analysis
from the final reportThe 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.