US Air Force investigators have determined that a Boeing B-52's incorrectly-positioned horizontal stabiliser trim led to the fatal crash of the bomber, northwest of the Pacific island of Guam last year.
But the inquiry by the USAF's Air Combat Command could not establish the reason why the trim was improperly set, owing to the lack of wreckage retrieved from the ocean and an absence of emergency radio calls from the crew.
None of the six airmen on board survived the 21 July accident, which occurred while the jet was on a training flight.
Salvage work recovered enough parts of the aircraft to demonstrate that the stabiliser trim had been set between 4.5° and 5° nose-down pitch.
"This indicates an improper stab trim setting of an aircraft in a nose-low descent at low altitude," says the accident investigation board.
It states that the aircraft had been performing a descending left turn at the time. Radar data and modelling enabled the board to simulate the turn, and rule out other possible accident scenarios.
While the difficulties of obtaining direct evidence have prevented the inquiry from reaching full conclusions on the reason behind the trim positioning, the board says the "late recognition of the serious nature of the situation" by the crew contributed to the accident.
"Any experienced crew could have found it difficult to recognise, assess and recover from the rapidly-developing situation involving the stabiliser trim setting," it adds.
While the B-52 was assigned to Louisiana's Barksdale air force base, 20th Bomb Squadron, it had been temporarily transferred to Guam's Andersen base as part of the 20th Expeditionary Bomb Squadron. The jet had been due to participate in a flypast for Guam's liberation anniversary, the day of the accident.
Source: Flight International
