I may have your answer!
it's either a bug, or sabotage!
Just happened to me twice on patrol in the Celebes Sea.
No damage of any kind I'd dove to 100ft to have a listen,
Ahead 1\3 I set TC to 64 to speed things up a bit when I notice a message "compress air down to 50%
I click on the comp tab and it's dropping like a rock, I exit TC, BLOW TANKS! AHEAD FULL! and all most didn't make it.
So I started watching it every time I'd dive, and it happened again:hmm: Seems random, but if your not watching it once you run out of compress air there's no were to go but down.
I did finish the patrol...taking my ol S-18 in for a refit
Just ran another mission, I think it's some how related to high sea's, and the S boats.
I don't know but I would lose compressed air even faster than batteries, kept having to surface til finally got nailed by a Bete. Half the crew injured, and moral broken, had to return to base!...they offered me a teaching position at Annapolis....I said NO WAY! dismissed all the weaklings and shanghaied a new crew!