Strugglin' with last Stalingrad mission on Sim

[Axe99]

Just wondering if anyone else was having trouble with this - I've had a crack at it three times, and on all three occasions when I think I've still got my last target in my crosshairs, still in the middle third of the map, it says the mission has failed as the transports have reached the airfield (Petomkin or similar). Am I just being too slow destroying the transports? I've had four cracks at it now, and it always gives me the mission failed at pretty much the same point - my guess is that there's an invisible line on the map that's the 'transports go over this and you lose' trigger in the game, but just want to make sure before my fifth attempt .