Federal Express Douglas MD-10 accident

According to the captain, the flight was uneventful until shortly before the airplane entered a holding pattern at FL330. The speed calculated by the flight management system (FMS) was 230 knots, 5 knots above the Vmin speed for the configuration and weight of the airplane. Both pilots expected the FMS to automatically default to the ICAO maximum holding speed of 265 knots, as described in the company flight manual. However, the captain decided to command the FMS to maintain 240 knots to increase the buffer between the holding speed and Vmin. Upon reaching the pattern fix, the airplane automatically banked left to enter holding and the captain noticed that the airspeed was lessening from 240 knots. He expected the airspeed to recover once the turn was completed. He also expected the bank to be limited to 15 degrees, but the autopilot actually commanded 23 degrees. Upon completing the turn, the airspeed was approximately 5 knots below Vmin. The captain expected the airspeed to recover now that the airplane was wings level. However, this did not occur, and when appropriate, the airplane started to automatically turn to the inbound holding heading. The crew then requested and was granted a clearance to descend to FL320 in the holding pattern. The captain then selected level change on the FMS to 320 and he also reduced the autobank controller to 15 degrees, from 23 degrees. During the descending turn, with the airspeed at about 220 knots, the first officer recommended extending the slats. The maximum slat extension speed at that juncture was 270 knots, according to the captain. When the slats extended, the maximum slat extension speed indicator on the airspeed indicator fell to below 220 knots. The captain then immediately ordered the first officer to retract the slats. He stated that at this point the airplane began to buffet, and an autoslat extension alert occurred. The first officer requested a further descent and was cleared to FL290. The captain then selected a level change on the FMS to 290. The buffeting ceased when the airplane was passing approximately FL300 during this descent. The flight landed uneventfully; however, the buffeting had caused substantial damage to both elevators and right horizontal stabilizer that was discovered during a post-flight inspection. According to the flight data recorder data and an analysis of that data by the airplane’s manufacturer, the slats were extended by the flight crew when the airspeed was 205 knots, well below the maximum slat extension speed of 260 knots. However the Mach number at that time was 0.59, well above the 0.51 Mach minimum slat extension speed. The target Mach then became 0.51, and the autothrottles reduced from 100% N1 rpm to about 50% N1 rpm. When the slats were retracted by the flight crew, the autothrottles advanced the thrust back to 100% N1 rpm. However, by the time the engine thrust had recovered, the airspeed had dropped to about 180 knots. The stick shakers activated about 5 seconds later and continued to operate for approximately one minute.

Verbatim NTSB analysis and probable cause from the NTSB dataset