As was said before:

Your Planesman helps to recover your air. Without him you will start every battle without
air (and start to refill very slowly cca 270s to 100%). With Planesman you will start the
battle with air and time to refill is 112s (and geting better as Planesman is geting better).

Keep your Planesman with full crew otherwise he will drop his ability again to 270s
bellow 80% of crew .

on your SS1, 2 & 3, there is only one gunner slot.

You need to cycle your two gunners on the slot to get them up to L74 for the SS4.

You need the following:

One Sonarman, One Planesman, One Engineer, One Bridge Operator, Two
Torpedomen, two Gunners (eventually).

Planesman helps recharge your air supply. Sonarman gives you underwater vision.
Both should be 14+ potential, with good repair/restore numbers too.