FedEx MD-10 Freighter Suffers Elevator Damage During Holding Pattern (N554FE)

Incident Overview and Flight Details On June 14, 2008, at 1015 eastern daylight time, FedEx flight 764, a McDonnell Douglas/Boeing MD-10-10 freighter registered as N554FE, encountered aerodynamic buffet and stickshaker activation while descending from flight level 330 in a holding pattern near Raymond, Pennsylvania. The aircraft was en route from Memphis, Tennessee, to John F. Kennedy International Airport in Jamaica, New York. Following the incident, maintenance personnel discovered substantial damage to the right and left elevators and the right horizontal stabilizer tip structure. None of the three flightcrew members on board were injured, and the flight landed uneventfully. The aircraft, serial number 46708, had 86,958 total airframe hours and was last inspected under a continuous airworthiness program on May 8, 2007. Both the captain and first officer held air transport pilot ratings and were type-rated in the McDonnell Douglas MD-11, a rating that also applies to the MD-10-10. Both held current first-class medical certificates and had successfully completed FAR 121 flight checks in May 2008. The captain had 6,814 total flying hours with 2,740 in the MD-10, while the first officer had 2,334 total hours with 820 in the MD-10. ## Sequence of Events According to the captain, the flight was uneventful until shortly before the aircraft entered the holding pattern at HOXIE with an inbound course of 115 degrees at flight level 330. The holding pattern legs were 20 nautical miles long. The speed calculated by the flight management system (FMS) was 230 knots, five knots above the Vmin speed for the aircraft's configuration and weight. Both pilots expected the FMS to automatically default to the ICAO maximum holding speed of 265 knots, as described in the FedEx company flight manual. However, the captain decided to command the FMS to maintain an airspeed of 240 knots to increase the buffer between the holding speed and Vmin. Upon reaching HOXIE, the aircraft automatically banked left to enter holding, and the captain noticed the airspeed decreasing from 240 knots. He expected the airspeed to recover once the turn was completed and anticipated a bank limited to 15 degrees, but the autopilot actually commanded 23 degrees of bank. Upon completing the turn, the airspeed was approximately five knots below Vmin. The captain expected the airspeed to recover once the aircraft was wings level, but this did not occur. The captain then asked the first officer to request a lower altitude, and Cleveland ARTCC cleared the flight to descend to flight level 320. The captain selected level change on the FMS to 320 and 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 immediately ordered the first officer to retract the slats. At this point, the aircraft began to buffet, and an autoslat extension alert occurred. The first officer requested a further descent, and ARTCC cleared the flight to descend to flight level 290. The captain selected a level change on the FMS to 290. The buffeting ceased when the aircraft was passing approximately flight level 300 during this descent. Visual meteorological conditions existed at the time of the buffeting event, and no turbulence was encountered. ## Flight Data Recorder Analysis According to flight data recorder data and analysis by Boeing engineers, 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 five seconds later and continued to operate for approximately one minute. About 10 seconds after initial stick shaker activation, the aircraft pitch reached 12 degrees nose up. Ten seconds later, the pitch was recorded to be 3 degrees nose up, and 17 seconds after that, the pitch was recorded to be about 12 degrees nose up again. Almost immediately after that, the pitch was recorded to be 2 degrees nose up. Pitch then increased to about 7 degrees nose up, stabilized at about 5 degrees nose up, rapidly decreased to 0 degrees nose up, and then recovered to a stable 5 degrees nose up over the next 40 seconds. During the period of stick shaker activity, the altitude decreased from flight level 340 to flight level 306. The aircraft continued to descend to flight level 290 as the airspeed recovered to 230 knots. ## Additional Information On January 21, 1988, McDonnell Douglas Aircraft Company (subsequently acquired by Boeing) issued All Operator Letter C1-E60-HHK-99-L033, titled High Altitude Stall Buffet. Although addressed to all DC-10 operators, it also applies to the MD-10, which is aerodynamically identical to the DC-10. The letter's subheading on buffet characteristics states that for Mach numbers below 0.84 in 1.0g flight, buffet is related to maximum lift capability or stall. The buffet onset or stick shaker speed, whichever is greater, should be used as the minimum speed below which aircraft structural damage, in the form of wrinkled elevators, could potentially occur. The letter's stall recovery procedure section advises that should a crew encounter buffet in the clean configuration below 0.84 Mach, prompt stall recovery corrective action should be taken. Maximum continuous thrust should be applied and pitch attitude reduced as required to minimize altitude loss. If below the slat limit speed, slats should be extended. At heavy weights or high altitudes, if the airspeed/Mach has become low, this technique alone may not be adequate for recovery, and altitude should be traded for airspeed to accelerate out of the buffet. Incomplete recovery may result in a secondary stall or inability to accelerate to cruise Mach with thrust available. If recovery action is not promptly initiated, a substantial loss of altitude may result. The Boeing MD-10 Flight Crew Operations Manual section titled Recovery from Approach to Stall states that at the first indication of approach to stall, crews should simultaneously apply maximum available thrust, level wings, and adjust pitch as required to minimize altitude loss. The first indication may include a rapid decrease below selected airspeed, airspeed decay below the Vmin indicator, pitch attitudes approaching the PLI, or stick shaker or initial stall buffet. At the first indication of a stall with auto flight engaged, crews should immediately disconnect auto flight and initiate stall recovery, remaining alert to counteract excessive nose-up trim conditions. Above 25,000 feet, stall can occur prior to stick shaker, with buffet serving as the only stall warning. The FCOM section on Automatic Flight – Description and Operation states that the commanded speed will be the FMS speed if the FMS SPD is engaged. If the bank angle limit selector is set to the AUTO position, the bank angle limit value is limited to 25 degrees between Mach 0.17 and Mach 0.55.