I can't hit deep subs in RA using the FFG
 

The RA mod manual says you can manually set the depth of torps using the player helo control in the Oliver Hazard Perry, but all I can find is info on what kind of search parameters to give to the Seahawk.
Every time I drop a torp on a contact, it just circles around at about a depth of 50ft and the sub drives on by at 800 ft.

I even promoted my contact for the AI P-3, and the AI did fly directly for my contact. It dropped 3 torps, which all circled in shallow water.
Basically I couldn't engage the enemy sub at all. Is this a limitation of playing the OHP platform?

You can set manually depth of topredos only while you are Helo pilot. From Perry you can't set depth for torpedoes droped by your helos. Their depth is set by AI and they always makes circles.

Helo from Perry is your deadliest "weapon" - you just need to drop torpedo 600-800 yards form sub and no matter how deep your target is.