WASHINGTON (CNN) - Federal accident investigators Monday closed the books on the mysterious and dangerous deployment of an evacuation chute aboard a Barack Obama campaign plane in the summer of 2008.
The chute deployed because it wasn't properly fastened to the floor and it shifted when the plane made a steeper-than-normal takeoff, the National Transportation Safety Board said.
But investigators weren't able to determine why the chute container wasn't properly secured to the floor in the first place.
The board's report gives two possible explanations, but indicates neither could be established as the cause.
One potentially embarrassing explanation is that Secret Service agents conducting a pre-flight security check could have loosened the container fasteners to peek inside. But, the report says, "an internal (Secret Service) investigation revealed that no (Secret Service) personnel ... interfered with or altered" the evacuation slide systems.
As to whether transportation safety board investigators independently interviewed the Secret Service agent or agents who inspected the plane, the board said only that they "corroborated with the Secret Service regarding their preflight security survey activities." It declined to be more specific.
Nor does the report implicate maintenance personnel. It says an airline inspector conducted a visual inspection one month before the incident and did not note any anomalies. That inspection would have included a check of the tie-down straps that secure the evacuation slide cover to the floor, but "there would be no reason for the mechanic to touch the strap during this inspection," the report says.
"It could not be determined why the slide's cover was not secured," the report says. "In normal circumstances, the cover is secured by the mechanic who installs it and should remain secured until it is removed from the airplane."
Then-candidate Obama was on the MD-81 charter aircraft at the time of the July 7, 2008, incident, accompanied by his staff, Secret Service personnel, reporters and the plane's crew. An airline mechanic was also on board and was seated in the rear of the aircraft, not far from the evacuation slide in what is referred to as the plane's tailcone.
Neither the flight crew nor the mechanic heard the chute deploy shortly after the plane took off from Chicago Midway Airport.
The pilot first noticed that something was amiss when the plane's nose-up pitch continued to increase even without his input, the transportation safety board said. The pilot regained control using the control column and stabilizer pitch trim inputs.
The plane diverted to Lambert-St. Louis International Airport and made an uneventful landing.
The evacuation chute mechanism on the McDonnell Douglas plane is unlike those found on many commercial planes. The chute is deployed when the plane's tailcone is jettisoned. As the tailcone falls away from the plane, an attached lanyard pulls open the cover over the evacuation chute, the board's report said. This in turn rotates the slide pack and a second lanyard then triggers an inflation cylinder that inflates the slide.
Investigators said flight recorder data shows the plane took off at a steeper-than-usual angle, and that there was sufficient inertia to allow the
unsecured slide cover to rotate and inflate the slide.
After the mishap, Midwest Airlines, operator of the charter, released a maintenance bulletin adding an additional check to ensure the security of the slide cover tie-down straps.