The last few minutes in the cockpit of two Boeing 757s were spent in fatal confusion.
The computer-driven displays were warning the pilots they were flying too fast and too slow. Too fast and the airframe could break up. Too slow and the aircraft could stall.
What could the pilots do?
Air crash investigations found that the external pitot tubes which feed air speed information to the onboard systems were blocked.
In one case a maintenance worker had put tape, similar to DIY tape, on the sensors while he cleaned the outside of the aircraft. He'd forgotten to take it off.
Before boarding the plane, the pilot had not noticed the tape, probably because it was night and the tape was a similar colour to the aircraft.
In the second accident, mud dauber wasps which choose tubes for nests, are thought to have made a home in an uncovered pitot sensor.
In both cases the auto-pilot had taken its information from the blocked pitot tube. When the auto-pilot reached the limits of its authority it disconnected.
The auto-pilot was also disconnected in the Air France Airbus, Flight 447, which crashed into the sea off the coats of Brazil on 1 June 2009, with the loss of 228 lives. Before that crash, too, the pitot tubes might have been blocked, say French investigating authorities.
In the three crashes a total of 487 lives were lost.
What happened in the last moments of flight on the Birgenair Flight 301 and Aeroperú Flight 603 is reported on ComputerWeekly.com's homepage. They're extraordinary stories of how pilots reacted when told told they were flying too fast and too slow. Should they slow down or speed up?
They're extraordinary stories because, apart from the blocked pitot tubes and the inconsistent data they presented to the pilots via the onboard systems, both the 757s were technically and mechanically sound.
Yet they both went into the sea.
Were the last moments of the pilots the Air France Airbus spent such in fatal confusion?
Crash One - ComputerWeekly.com
Crash Two - ComputerWeekly.com
Airbus replaces pitot tubes after crash - Guardian