[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.