MvR and his boys didn't take off when the enemy was right overhead. They got advanced warning from the troops in the trenches so had enough time to take off, assemble, and climb to combat altitude before meeting the enemy.
I would assume that this probably happens to some extent in OFF. If you meet an enemy fighter flight, how do you know when it took off? You don't. I do know that airplanes on both sides take off while I'm in the air--I've seen them do it.
Only a complete fool tries to take off with the enemy overhead and attacking the airfield. In MMO flightsims, such idiots are sought out with glee because strafing them on the runway is such an easy way to pad your score. This is called "vulching" in MMOFS circles, and you'll often hear "the vulch is on at Airfield X" on the radio, amongst folks cackling as they call out their 6th or 8th kill of the same poor schmuck in as many passes down the runway

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I would hope, therefore, that the AI has enough smarts NOT to scramble when I'm overhead. Even if I'm not in the mood to vulch them, I'll certainly be long gone before they can climb up to my altitude.
On a similar subject, once I found out the 1st time that scramble missions are you getting to play vulcher fodder, I NEVER play those missions. In real life, the vast majority of pilots had more sense than to attempt to take off under such conditions, and I find it highly annoying that the game wants to treat me like I don't know any better

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