The piece of aircraft debris thought to belong to the missing Malaysia Airlines Flight MH370 found Wednesday on an island beach in the Indian Ocean could not have been a spare part that was lost in transit or stripped from an old aircraft, according to an aviation expert. While others were pondering how the part got there, Boeing engineers have been quoted as suggesting that the piece, known as a flaperon, came from the wing of Boeing's 777 series of aircraft.

“If this is really a Boeing piece, and it seems it is, it narrows down the possibilities of where it comes from,” said Glenn Winn, an aviation security specialist and professor in the aviation security program at the University of Southern California’s Viterbi School of Engineering.

Winn added that the flaperon is never replaced. In other words, the piece of debris was likely to have only come from another 777 aircraft. “There have only been five total losses in the history of the triple seven series and those are all accounted for except MH370,” Winn said.

To add weight to Winn’s theory, a serial number (657-BB) was discovered on the inside of the debris is the same as noted in Boeing’s manufacturing manual, suggesting further that the part came from a 777 aircraft.

So where else could the debris have come from if not MH370?

A detailed analysis of Boeing’s entire 777 series history showed that the remains of two aircraft have not been accounted for. There are 30 Boeing 777 planes in storage around the world and six that were professionally scrapped by owners. 

Aside from a British Airways flight that crashed at London's Heathrow airport in 2008, Malaysia Airlines flight MH370, which was lost over the Indian Ocean in March last year, and MH17, which was allegedly shot down over Ukraine last summer, there remain two other planes that have uncertain whereabouts.

"Once an airplane accident investigation is complete, the wreckage is released back to the operator," explained Miles Kotay, a spokesman for Boeing Commercial Airplanes' Aviation Safety, Security & Compliance Communications. "Insurance companies typically are responsible for ultimate disposition."

There are no details about how Egypt Air’s 777 was disposed of after it suffered a cockpit fire in Cairo four years ago. Likewise, the remains of Asiana Airlines 777 which caught fire at San Francisco Airport in 2013 also remains unknown. Egypt Air and Asiana Airlines did not respond to requests for comment.

While Winn says that the possibility of any of the written-off aircraft’s flaperons turning up on a remote beach in the Indian Ocean was conceivable, he couldn't imagine how the logistics would make it possible.

“Those aircraft were probably broken up for scrap by the airline that owns them or returned to Boeing in Seattle for some sort of recycling, so I don’t see how they would have remained intact and then somehow got lost in the sea,” said Winn. "Saying that, it would be interesting to find out what happened to the Egypt Air 777."