It's very present. Not enough to distract from the 'adventure', but often referred to in passing as a response to sudden noises or stress. The C.O. even keeps tabs on who should be given leave and when, based on how worn-out they seem. He seems to know the process of breakdown, inside and out:

”If you did stay here, you'd be a sot in a month. Already you're drinking more than you used to; that's the beginning of the end. You'll be caught napping; you'll stall taking-off, or you'll hit a tree coming in. Cleverer pilots than you have gone out that way. You can't help it and you can't stop it. No one can stand the pace for ever. This game makes an old man of a young one without him knowing it. That's the truth, Biggles. You've got to have a rest. If you don't rest now you'll never be able to rest again.”