Many years ago I had the pleasure of a ferry flight of an F27 from Manchester to Norwich. This aircraft had suffered a gear retraction whilst on roll-out (another story). It was certified for one flight, on a permit to fly only. After T/O on Manchester's westerly runway, I made a very gentle turn onto track for NWI, followed by a very gentle turn to land on the westerly at Norwich. The F.O commented that I seemed a bit reluctant to bank. Yes, the thing was dynamically unstable in roll. I don't know what they had to do to correct it, but among other things it involved re-skinning a section of the lower fuselage and replacement of all the lower aerials. I had a letter of thanks from the engineering director for the flight. I should have smelled a rat when all the more senior pilots suddenly found they were too busy.