Ok, now I've understood you guys. And I've tried it myself.
1) The game does lock up when you try to shoot the aircraft with the zoomed (or at least when you try to zoom with it while pointing at the aircraft) Deck Gun, I do confirm it.
2) The game does lock up if you try to zoom with your Deck Gun in the area from about -90 degrees to +90 degrees while the aircraft is near the boat...
3) The game does not lock up if you turn your Deck Gun at the direct opposite direction of where the aircraft is coming from, zoom, turn the Deck Gun back 180 degrees and shoot (but you won't be able to hit the aircraft while zoomed).
4) It is possible to shoot the aircraft with Deck Gun without zooming
. In this case the aircraft "dies" in the normal way as if you shoot it with the AA gun (no extra explosions, no Hollywood style effects etc).
The conclusion:
Use Deck Gun for the ships and AA guns for the Aircraft and you'll be ok.