The missing FIR prevented the board from determining the cause of the crash and fuelled a rare public dispute between accident investigators and the head of Air Force Special Operations Command (AFSOC) over opposing theories.
The president of the accident investigation board, recently retired Brig Gen Donald Harvel, believes chances are greater than 50% that the CV-22 crashed as a result of dual engine power loss.
AFSOC issued a press release on 16 December dismissing Harvel's theory as unsupported by a "preponderance of credible evidence", and the command's internal - and unreleased -- investigation report already blamed the crash on pilot error.
The truth about what happened on the night of 9 April in a wadi roughly 5,000ft above sea level near Qalat, Afghanistan, remains a mystery because the rescue crew failed to recover the FIR from the aircraft.
Asked about the puzzling omission, Harvel, currently a Delta Air Lines Boeing 777 captain, replied in an interview: "That's kind of a funny story."
AFSOC inherited instructional manuals - called "Dash-1s" - for the CV-22 from the Marine Corps' MV-22B units. It was necessary to translate the manuals from Marine piloting and maintenance jargon to USAF terminology, but the translators made a few mistakes, Harvel says.
"Somehow in that translation there was nothing in [the AFSOC manual] that showed this aircraft had a FIR," he says. "They had absolutely no idea."
As a result, he adds, the FIR "was never on the list to get that off the airplane" after a crash.
After recovering the 16 survivors and four bodies, plus other sensitive equipment, from the crash site, the remains of the CV-22 were destroyed by a bomb, the board's report says. Still, the FIR was designed to survive such an explosion, so it was possible the device was retrievable.
An army unit arrived at the site one day after the crash and recovered major pieces of the aircraft still intact, including the left engine nacelle, Harvel says. The army unit also took photos of items that would be retrieved the next day. Among the items photographed, Harvel says, one of the objects resembled the structure that contains the FIR. However, several of the items left behind on the first day were missing by the time the army unit returned, Harvel says.
The absence of the FIR means several theories about the cause of the CV-22 crash are possible, he says.
"I could understand how there could be people looking at the
same evidence and come to different conclusions," Harvel says.