Possibly could account for some of the sightings, but like i said there are a number of restrictions of where secret aircraft are allowed fly. And i'm sure that military pilots would recognize their own aircraft; there have been some cases where pilots flying the stealth bombers have spotted these objects. The craft that are spotted are not just saucer shaped either, some are triangular, some are cigar shaped. And there are a few cases where some of these objects are over a mile in diameter, with radar confirmation. Now i expect at least some of the sightings are in fact militray, and there have been a number of officials who have told that the US in particular have tried to figure out the Flying saucer technology. Flying saucers was the old term for UFOs in the 1950 and 60s. Ben Rich former head of lockhead Martin warned on his death bed that, and i'm quoting him now. "We already have the means to travel among the stars, but these technologies are locked up in black projects and it would take an act of God to ever get them out to benefit humanity….. anything you can imagine we already know how to do." Also President Eisenhower's National security chief, COL Philip Corso, explained that technology such as Fiber optics and lasers have come from back engineered extraterrestrial technology As for UFO crashes, they are very rare and have only been a couple known cases; Roswell was one and has been confirmed by hundreds of witnesses, including militray officials who were involved with the incident. But unfortunately the US has been developing some very advanced weaponry which some claim have been trying to bring down UFOs, whether they have been successful i haven't got a clue; and nobody has been able to clarify these claims.