In UFO literature  we can find hundreds of books, containing possibly thousands of reputable  eyewitness accounts of enormous rates of acceleration.

 So common and unmistakable is this feature that it would seem likely that the builders of these machines want us to notice.

This feature of UFOs is the one most vigorously denied by modern science. Because the scientists honestly believe that any machine or occupants could not survive such acceleration, they simply ignore or rubbish reliable eye wittiness reports .

The links above explore some sightings recorded on radar.
Enormous Rates Of Acceleration.