A CB, particularly a "BUS" one, needs some sort of overload to trip it - and I doubt it could have been tripped long term e.g. from start of sector without being apparent (e.g. Generator controls run off it).
Some sort of coincidence that a rarely required system is used, and it's supply CB "pops". Suspect that is not a coincidence. Whether it popped immediately (in which the crew's account does not tally) or later (with Altn Gear Extension being ON for some time?).
Fine for those flying in the FE days to say "unforgiveable" etc., but the modern aircraft are supposed to be designed around no FE. In practice, training and manuals are now rather "minimal", no doubt judged "cost effective." a few things immediately spring to mind to me:
- I'm a little surprised to find that circuit protected by both a Bus CB and it's own CB
- Again, surprised that there is no warning, or other indication of that Bus not being powered
- If you read the Tech Manual, you see reference to this Busbar. In your troubleshooting you might identify services from the same Busbar that remain powered... so a reasonable conclusion that is not the issue. Shame that in practice the busbar is split in 2, and half the "busbar" (only) protected by this CB. Thus with the CB popped, half the systems work and half don't... sounds to me like it's really 2 busbars
Accidents are rarely simple / one cause. As above, modern training also is to contact company for curious technical issues, and I am told they can get quick access to experts from the manufacturer. Failure for Gear Altn Extend to work, and nobody mentioning this CB, would indicate some sort of failure in that system.
Finally, looking at the items on that Bus, very different set of loads engines off sitting on the ground after the accident to airborne. I would not be too quick to hang the crew
Edit - really => rarely - thanks!