Air Force officials say it was the most expensive mishap since the next-generation fighters began being deployed to operational squadrons last year.

The accident occurred Oct. 20, when the Air Force's first Raptor squadron, based in Langley, Va., was at Hill for an inaugural training deployment.

A 22-page investigative summary, released this week, concludes the engine was damaged when a mechanic failed to remove a safety pin from the plane's forward landing gear.

Just after the pilot, Maj. Evan Dertien, had started the Raptor's twin 35,000-pound-thrust engines, Senior Airman Arthur Blosser noticed the pin was still installed. Blosser signaled for Dertien to shut down the left engine so that he could approach and remove it.

As Blosser removed the pin, the streamer attached to it was caught in the jet intake of the Raptor's right engine, ripping the pin from his hand and sucking it into the engine.

Dertien, according to the report, "heard a crunch and a winding down sound" as witnesses outside the aircraft "saw sparks coming from the engine."

"For this particular accident, the dollar amount of the damage to the right engine is approximately $6,754,275," said Air Force spokesman Lt. Daniel Goldberg.

The cost of a landing gear pin, Goldberg said, "is minimal."

Investigators concluded that, while Air Force guides correctly instructed Raptor mechanics to install the landing gear pins before performing maintenance on the airplanes, there were no similar step-by-step instructions to ensure mechanics remember to take the pins out prior to clearing the aircraft for use.

Critics have said the Raptor program - at one time planned to cost $35 million per aircraft but at times since has approached $200 million per unit - is too expensive and largely irrelevant to modern war-fighting needs.

That, however, did not diminish the excitement at Hill in October when the Langley squadron arrived for two weeks of flying. On Oct. 18, the squadron's commander, Lt. Col. Jim Hecker, dropped the first bomb from a Raptor over the Utah Test and Training Range.

Hecker later called the time spent at Hill, where dozens of Raptors will be maintained at the Ogden Air Logistics Center, "invaluable."

mlaplante@sltrib.com