Monday, September 21, 2009

NTSB issues probable cause report for 2008 safety incident aboard Obama's plane

by B. N. Sullivan

On July 7, 2008, a chartered Midwest Airlines MD-81 (registration N804ME), which was carrying then-Senator Barack Obama and his presidential election campaign entourage, diverted to St. Louis due to a mechanical problem. The aircraft was en route from Chicago to Charlotte, but had developed a "flight control anomaly" during initial climb. The aircraft made an uneventful landing at Lambert-St. Louis International Airport, after which it was discovered that the escape slide in the aircraft's tailcone had inflated in flight, apparently interfering with the aircraft's elevator cables. Today the U.S. National transportation Safety Board (NTSB) issued its report on the incident, including a statement of probable cause.

The NTSB determined the probable cause(s) of this incident as follows:
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.
There were no injuries among the two pilots, four flight attendants, two airline representatives, and 43 passengers. The passengers included Mr. Obama, members of his staff, news reporters, and United States Secret Service (USSS) personnel.

Flight Details

Deployed escape slide inside aircraft tailconeAccording to the NTSB report, the flight crew told investigators that the problem arose shortly after departure from Chicago Midway International Airport. During initial climb, the aircraft's pitch increased without a corresponding flight control input and exceeded normal limits before the captain was able to regain control.

Quoting from the NTSB report:
The captain reported that after liftoff the airplane's pitch continued to increase without a corresponding flight control input. The airplane's pitch reached 20-25 degrees nose up before he regained control using control column and stabilizer pitch trim inputs. The flight crew noted that the pitch control pressure required to level the airplane was "higher than normal."
Although the flight crew was able to regain control of the aircraft, a significant restriction in pitch control still remained. Consequently, the crew elected to divert to St. Louis instead of continuing to Charlotte-Douglas International Airport, the intended destination.

The captain told investigators that as the airplane passed through 15,000 feet mean sea level (msl), normal pitch control pressures returned. No further flight control restrictions or anomalies were encountered during the remainder descent and landing.

Inspection of the aircraft on the ground in St. Louis revealed that the tailcone evacuation slide had inflated inside the tail area of the aircraft. Investigators "found the deflated slide lying in and around its cover." The NTSB report describes it this way:
The slide cover was overturned immediately aft of its normal location at the end of the walkway. The slide's inflation cylinder was empty and lying inside the slide cover. The slide cover and base, including the hinges, forward tie-down straps, deployment lanyard assembly, and floor mounting hardware were undamaged. A bracket that secured one of the walkway railings to an overhead structural support had fractured.
The investigation found that the slide cover had not been secured to the floor fittings on the walkway before the flight, although "it could not be determined why the slide's cover was not secured." Normally, the cover is secured by the mechanic who installs it and should remain secured until it is removed from the airplane.

The NTSB concluded that the pitch control restriction experienced during the flight "was caused by the inflated slide and a subsequently damaged walkway railing that impinged on a set of elevator cables in the tailcone." The set of elevator control cables ran vertically in close proximity to the railing.

More About the Slide

Undeployed escape slideThe tailcone of the MD-81 is attached to the aft end of the fuselage and can be jettisoned to provide an opening for an emergency exit. This exit can be accessed from inside the passenger compartment through the aft bulkhead pressure door and aft accessory compartment.

Here is the NTSB's description of how the aft slide normally works:
The tailcone can be released either from inside or outside the aircraft. A mechanism is integrated into the aft bulkhead door, that when armed will jettison the tailcone and initiate 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 which inflates the slide.
The incident slide had been installed on December 10, 2007. The last visual inspection of the tailcone evacuation slide area (service check) was on June 5, 2008, with no anomalies noted. The inflation cylinder pressure was last checked on June 20, 2008, without any significant findings.

Flight crew statements to NTSB investigators indicated that "they did not hear the slide inflate in flight, nor did an airline mechanic who was seated in the rear of the airplane." The results of an internal investigation carried out by the U.S. Secret Service (USSS) revealed that "no USSS personnel or USSS support personnel interfered with or altered the aircraft's hardware or systems relating to the tailcone evacuation slide" during their security sweeps of the incident aircraft.

Inertial calculations were performed using data from the incident aircraft's flight data recorder (FDR). The NTSB determined that "during 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." The NTSB noted further that the incident aircraft had flown 15 flight legs since the last service check of the tailcone evacuation slide, and had experienced inertial loads of similar magnitude, but concluded that "they were of insufficient duration to result in slide inflation."

Maybe, but at the same time, the NTSB's 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."

So then, how was it that the slide container was not properly secured in the first place? The NTSB report offers no conclusion.

Meanwhile, though, a Maintenance Alert Bulletin issued last October added an additional step to the MD-80 Service Check "to ensure the security of the slide cover tie-down straps." Midwest Airlines also released a revision to the work card for the general visual inspection (service check), adding specific language calling for an examination of the tie-down straps to ensure their proper installation and security.

About the photos: The first photo, near the top of this page, shows the deployed tailcone evacuation slide in the tailcone of the incident MD-81 aircraft. The second photo shows an undeployed tail cone evacuation slide as installed on another aircraft. Both photos were supplied by the NTSB. [Click on the photos for a larger view.]

If you would like to read the NTSB report for this incident, here are the links: NTSB Identification: CHI08IA182 Summary; and Full Narrative.

[Photo Source]