About 25 years ago there was a TV documentary series on the development of the Boeing 777. In Episode 3 (link below) Pratt and Whitney had designed an engine for it and Boeing’s Director of Engineering, Ronald Ostrowski and others, didn’t want to test the engine on a Flying Engine Bed, i.e., a powered aeroplane with the test engine attached. They reasoned that the data from the ground engine testbed and computer models would be sufficient and they could skip the usual FEB testing, thereby saving Boeing ten million dollars and time. Boeing’s John Cashman, Chief Pilot, disagreed and managed to get Boeing to agree to a FEB and the test engine was attached to a 747. On the third flight the test engine surged on take-off, a problem that was not detected by the ground engine testbed. A surge is not a very serious problem on one engine, but if two engines had been tested on a 777 body and both engines had surged, as was quite likely, it would have led to some loss of power at completely the wrong time.
The whole series is an interesting insight into aeroplane development. Episode 4 covers the computer systems and should have been titled ‘Nobody knew how difficult it would be’. Of course, everybody should have known how difficult it would be.