What happens when a vehicle is "disabled" in Battlefield 3?
I just investigated your question on my server (standard - not hardcore in case it matters) with a couple of M1 Abrams tanks. I haven't confirmed these numbers for other vehicles, so results might be different for other vehicles.
The magic number
The number for disabling looked like 50%. I would have guessed it was that, but I'd never tested to be sure. A 52% tank was fine, and a 50% tank was on fire.
Bleed effects
Once the tank was disabled, I had a grace period of a few seconds before health started bleeding. It seemed like about a 10 second period.
But 50% was the point where it started bleeding. Once it starts bleeding you have to either get it repaired or, failing that, bail out before it explodes.
Audio cues
- Down to about 60% damage I heard a slow klaxon-like sound (like an angry little alarm clock).
- From 50-60% the klaxon got very fast.
- At 50% and below, once the vehicle was on fire, the "woop-woop" alarm sound was added.
Heal effects
When a you repair a vehicle's health to the point that it's no longer disabled, all adverse effects from disabling are repaired too.
When a vehicle is anywhere above a disabled state, after avoiding additional damage for a few seconds it'll slowly regenerate its health back up to 100%. In my tank testing it took about 12 seconds of non-damage before regeneration started.
Other
Though a disabled vehicle's movement is impaired, its weapons function normally.
When a vehicle has been disabled it will have its moving severely impaired. You can control some parts, but it will move slowly and you're basically a sitting duck (or in the case of airborne vehicles, a falling one :)
The only way to get a vehicle back from a disabled state is to have a friendly engineer repair it, or - for airborne vehicles - by using the "Extinguisher" vehicle upgrade that will put out the fire on disabled crafts. This is probably what you have been experienced when you saw an air vehicle fly away after being disabled.
I could not find any reliable sources on what exact percentage that causes the vehicle to enter the disabled state (or the "on fire" state), but the battlefield wikia suggests it's movement start to degrade around 50% and catch on fire around 20% Typically, for airborne vehicles, one hit with a stinger will disable it, leaving it plummeting to the ground unless it's fire is extinguished or an engineer inside it repairs it.
Also, as you've noticed, a vehicle on fire will "bleed" in that it will slowly do damage over time until it explodes.