Plane A receives an "Oxygen out" critical hit. He is in the HI band, so he have 15 - 5 -5 = 5 turns before there is an effect.
By diving to the MH band, Plane A now uses the O2 Loss time limit for the new band which is 10 turns, thus he has 9 turns remaining before testing for unconsciousness.
If he reaches ML band, another 5 turns will be added back to his time limit. Assuming he continues to dive for richer air, he will be able to claim all 15 turns of non-effect before checking for LOC.
The trick is to get out of the HI band and into the MH band within the 5 turn limit, and then into the ML band within the 10 turn limit and below the ML band by end of the 15 turn limit. You don't reset the time upon going to a lower band, you just adopt its limits.
As an example, lets say he was in the VH band, where the time limit is 0. Let's say he takes three turns to get down to and end in the HI band (5 turn limit). He would test for unconsciousness on turns 1 and 2, but not turn 3 as he ended within the lower band's limits (5 turns). However, if he then could not descend to the MH band within the next 2 turns, he would have to start testing again and so on.
JD
(https://groups.yahoo.com/neo/groups/FW_TC_Darwin_RAAF/conversations/messages/517) 30/09/2014