Originally Posted by
Uplinker
So a combination of particular magnetic headings, latitude, longitude and mag variation might cause the Nav display to output nonsense, but all six displays blanked??
Why the PFD?. Why the Engine/system display?
Please tell me this is untrue or that at least the PFDs are given a higher level of coding security.
I expect it failed in some section that would ordinarily be labeled "THIS CANNOT FAIL," not because it is impervious to failure but because there is no sensible response if it does. AT&T managed to crash almost all of its communications network because of a that kind of bug and, unlike airport information, they were in control of every detail of their hardware operation. See
All Circuits are Busy Now: The 1990 AT&T Long Distance Network Collapse
The 737 is rumored to be running an 80286 processor which is a well understood design. However I don't see the architecture for the software. It should have an independent watchdog timer processor to automatically reboot if there is a fundamental software failure, but maybe only the thread/task/subsystem to update the displays failed and the watchdog reset and all other controls is still running. I've seen software fail because it was awaiting feedback that never happened. It may be that one display got a message it could not handle and never responded. That would quickly stop the task to update the displays. If the displays are programmed right they should have their own watchdog timers to stop displaying when too long a time has passed without an update rather than freezing and giving the impression they are still functioning.
There was a question of why not every runway was tried on the software. While hindsight is great on this sort of problem, it also suggests trying every combination of: altitude, airspeed, pitch, AoA, lat, long, N1, fuel load, weight, true airspeed, relative airspeed, and on and on.
The reason I wondered about the magnetic heading earlier is because this last year cannot be the first time a 737NG flew into Barrow, Alaska. Can it?
The last two airports on the list are nearly 2 degrees from a true heading of 270 degrees. It's not likely for that to result in a divide by zero problem.
It's a shame, but I doubt the actual source of the problem will be revealed.