Debris is scattered across at field after a B-1B bomber crashed Aug. 19 in a remote area near Broadus, Mont. The four crew members survived after ejecting from the aircraft. (Bill Stuver / Power River Examiner via The AP)
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A B-1B Lancer, from the 37th Expeditionary Bomb Squadron, returns to mission after receiving fuel from a KC-135 Stratotanker, assigned to the 340th Expeditionary Air Refueling Squadron, while flying over Afghanistan in support of Operation Enduring Freedom, March 29. (Master Sgt. William Greer / Air Force)
The explosions that caused a B-1B Lancer to crash in August occurred when a mechanical malfunction led to a fuel leak, investigators found.
“There was no evidence that human factors contributed to the mishap,” the Accident Investigation Board report determined.
The $318 million aircraft was destroyed when it crashed on Aug. 19 near Broadus, Mont., but all four crew members ejected safely with non-life threatening injuries, according to the report, which was released on Monday.
The plane and crew both came from the 34th Bomb Squadron, 28th Bomb Wing at Ellsworth Air Force Base, S.D., the report says. On the day of the crash, the crew was conducting a post-deployment training flight. The plane was scheduled to begin 60 months of depot maintenance in just four weeks.
The wings of B-1B bombers sweep back so that the plane can perform better at certain speeds, but when the pilot moved the wings from the forward to aft position during the training flight, a piece of the aircraft that was supposed to fold down became detached, the report says.
The aircraft part that malfunctioned is called the fold down baffle. It is supposed to fold flat when the wing moves to allow the wing to sweep over the top, the report says. It is not known why it became detached.
“The preflight inspection does not require a specific check of the fold down baffles to ensure proper attachment or function,” the report found. “The preflight inspection was valid at the time the MA [bomber] departed Ellsworth AFB.”
As the wing moved, the detached fold down baffle cut a hole in the main fuel line, causing 7,000 pounds of fuel to leak, the report says. The fuel indicator, which is inside the fuel tank, did not detect the leak.
“Because fuel is pumped from the tank, a downstream fuel leak cannot be detected from inside the tank,” the report found.
When the fuel ignited, it set off a series of explosions that led to a “permanent loss of power to the crew compartment,” the report says. The four crew members punched out as the bomber plummeted toward the ground at about three-quarters the speed of sound.
Three of the crew members suffered minor injuries and the fourth crew member injured his leg during the ejection, possibly because the ejection seat’s leg restraints did not function as they were supposed to, the report found.
The last B-1 crash was at the start of the war in Afghanistan in December 2001 when a bomber went down in the Indian Ocean near Diego Garcia. In April 2008, a B1B exploded on the ground at al-Udeid Air Base, Qatar, when its brakes failed and the plane rolled into a concrete barrier.