The time slowdown when your radar man spots an air contact at 10 miles away depends on the mod you play, plus wether you have air radar installed or not.
To see wether a radar contact on your map is a plane, click on the contact and if you read 'speed very fast' then it's a plane and not a ship.
What I do with air radar contacts is: I determine the course, draw the course line on the map then I go flank at perpendicular route and I put a man at the flack gun if I think the plane will not spot me visually (else I crash-dive and I'm always in time), . In most cases I don't have to even dive (in FOTRSU). However losing patience and trying to slug it out with the plane can go terribly wrong if you are hit directly by a bomb
XS