As you know, Pilot need to use O2 in high alt.
so
1) Pilot will be black out if continue to fly over 18000ft.(I don't know exact alt)
2)Add O2 supply switch
3) Set time usage until running out of Oxygen cylinder like wep
4) Pilot have to go low alt if O2 bin empty.
5) Need damage model O2 cylinder
if Hit O2 cylinder critical damage of plane or Pilot can't grab high alt if any trouble of it.
Is it more real isn't it?

It will make more story isn't it !!
Case 1)
You fight in high alt and no advantage.
however if his O2 cylinder empty, He have to go low alt otherwise black out.
Case 2)
You chase Buff in High alt.
You have to give up if running out of O2.
Case 3)
Your plane has clitical damage if bullet hit your O2 cylinder unfortunately.
The big effect is
Fighter can't stay long time in high alt like before. right
