MCDONNELL DOUGLAS MD-81 — St. Louis, MO, USA, 2008-07-07

NTSB summary

The incident occurred during a charter flight for a political candidate, his staff, news reporters, and United States Secret Service (USSS) personnel. According to postincident flight crew statements, during the initial climb, the MD-81 airplane’s pitch increased without a corresponding flight control input and exceeded normal limits before the captain was able to regain control. Although the flight crew was able to regain airplane control, a significant restriction in pitch control still remained. Consequently, the flight crew elected to land instead of continuing to the intended destination. Normal pitch control pressures returned during descent, and no further flight control restrictions or anomalies were encountered during the remainder of the flight. Postincident inspection of the airplane indicated that the tailcone evacuation slide had inflated inside the tail area of the airplane as the airplane lifted off. The pitch control restriction was caused by the inflated slide and a subsequently damaged walkway railing that impinged on a set of elevator cables in the tailcone. The investigation further revealed that the slide cover had not been secured to the floor fittings on the walkway before the flight. It could not be determined why the slide’s cover was not secured. In normal circumstances, the cover is secured by the mechanic who installs it and should remain secured until it is removed from the airplane. The airplane’s tailcone is attached to the aft end of the fuselage and can be jettisoned, either from inside or outside the airplane, to provide an opening for an emergency exit. In an evacuation, when the aft bulkhead door is opened, an integrated door mechanism jettisons the tailcone, thus initiating the evacuation slide deployment. The tailcone falls away from the aft fuselage, and an attached lanyard pulls open the evacuation slide cover. This, in turn, rotates the slide pack aft, and a second lanyard triggers the inflation cylinder to inflate the slide. Results of calculations performed using flight data recorder data indicated that, during the incident flight’s takeoff rotation and initial climb, there were inertial loads of sufficient magnitude and duration to allow an unsecured slide cover to rotate open and initiate slide inflation. Postincident testing showed that the slide pack could not have rotated enough to activate its inflation cylinder if the slide container had been properly secured. Further, a properly secured slide cover would have contained the slide if the inflation cylinder had improperly discharged. Postincident testing of the inflation cylinder did not reveal any anomalies that would have resulted in an unintentional discharge of the cylinder for other reasons. The tailcone evacuation slide’s last service check before the incident flight was performed by the airline on June 5, 2008, and no anomalies were noted. That check was a general visual examination of numerous items throughout the cabin, which included inspection of the forward tie-down straps that secure the slide cover to the floor fittings. There would be no reason for the mechanic to touch the straps during this inspection. In addition, on June 20, 2008, the tailcone slide’s inflation bottle pressure was inspected; this inspection did not specifically call for looking at the straps. A review of flights since that service check indicated that the airplane had experienced load magnitudes that were similar to the incident flight; however, the duration of the inertial loads experienced was not long enough to result in slide inflation. Since the last service check, the incident airplane had flown three flight legs with the presidential candidate aboard. Because a presidential candidate was aboard the airplane, the USSS performed security sweeps of the aircraft before flight. Following the incident, an internal USSS investigation revealed that no USSS personnel or USSS support personnel interfered with or altered the aircraft’s hardware or systems related to the tailcone evacuation slide. Shortly after the July 2008 incident, the airline released a maintenance alert bulletin describing the incident and initial findings from the investigation. The airline also released a revision to the service check work card which added specific language calling for airline maintenance personnel to examine the tie-down straps to verify their proper installation and security during reoccurring service checks. Only two previous incidents of inadvertent tailcone evacuation slide inflation in MD-80 series airplanes have been reported (one to the Federal Aviation Administration’s Service Difficulty Reporting system and one to Boeing, which holds the MD-80 type certificate); the causes of each of these inflations could not be definitively determined. No actions were taken.

Probable cause

The inadvertent partial inflation of the evacuation slide within the tailcone during takeoff and subsequent binding of the elevator control cables. The partial inflation resulted from the tailcone evacuation slide cover failing to be secured to the floor fittings on the walkway for undetermined reasons.

Flight data recorder

— 112,700 samples, 17 parameters, SRN 361,180 to SRN 364,400.

Sequence of events (FDR factual report)

Recorder documents in the docket

Docket: data.ntsb.gov/Docket/?NTSBNumber=CHI08IA182. This brief is an analytical aid assembled from public records; it is not an investigative finding.