TL;DR - planes currently show very little signs of taking damage until they spontaneously explode, making ejecting a useless action (because you are never told when to). Jets should deteriorate over time, both visually and performance wise, and audio and/or in-cockpit warnings should advise the player when the aircraft's health has lowered to a critical level.
When aircraft reaches 100% damage, start a timer (for maybe, 3-5 seconds). The pilot has these seconds to eject before the plane explodes. If not, it blows up, taking him with it.
Another idea to add to that one, is to read the amount of damage past 100% is dealt. The more damage over 100, the less time to eject. That way, if you were killed by one too many bullets (perhaps the damage was at say, 103%), you would have a good 7 seconds before the systems melted down and the jet exploded.
However, if an AA missile exploded just a tad too close (112% damage), the time would be much shorter, maybe 2-3 seconds. A direct hit with a AAA missile from a tank might inflict much more damage (perhaps upwards of 125% damage), the jet would explode instantaneously, leaving no time for an ejection.
here is a simple linear equation that would calculate the time, where x represents damage level, and y represents seconds to eject:
y = 10-(x-100)
because we are calculating damage over 100%, we subtract 100 from x.
we subtract this overkill value from 10 (the maximum time to eject), giving us y, a time in seconds for the pilot to eject.
we could also give our pilot a slightly more forgiving curve, such as y = -1/2x + 10.
As I am not sure the exact damage values of the various weapons, it is difficult to come up with an exact equation on the spot, but with some data for damage output, it could be done quite easily.