[a-priori]

That will never happen. It would take the simultaneous failure of two independent systems, each of which are highly reliable. If the MTBF (mean time between failures) of one component were, let's say, ten years of continuous use then they each have a 1/87600 chance of failing in each usage-hour.

The odds of a simultaneous (within one hour) double failure is the square of that, or 1/763760000 per hour. This corresponds to a MTBF of roughly 3836880000 hours or 438000 years.

[noir_lord]

1000 of same model flying 12 hours a day (utilisation is nearer 11[1], Some airliners where made in numbers a lot more than 1000, 737's where over 8000[2]) for 20 years is 10000 flight years (very crudely).

438000/10000 is 43.8, so 2.3% chance over 20 years.

It's a longshot but it's not never.

[1] http://web.mit.edu/airlinedata/www/2014%2012%20Month%20Docum...

[2] https://en.wikipedia.org/wiki/List_of_most-produced_aircraft

So yes the odds of any one plane experiencing that problem are absolutely tiny but across the fleet not so much.

[TheCoelacanth]

But compared to the other failures modes, still extremely unlikely. It's not really worth trying to prevent an error that happens to the whole fleet once every 400 years when you could work on fixing other problems that cause plane crashes much more frequently.

[noir_lord]

Agreed but I never said anything about engineering priorities, my observation was that unlikely events happen at scale.

[robbiep]

I understand 'wear and tear' failure is extremely unlikely to strike simultaneously within the same flight, but what about intentional disruption? Is that possible or has it been explored?

[PhasmaFelis]

If the bad guys can disrupt one clock, they can probably disrupt all the clocks you have, whether that's three or twenty.