Roughly 600 yards for gv's when they are stopped. Roughly 1200 yards when gv's are moving. This is for all aircraft in AH, save for one: the Storch.
The Storch can see gv's while they are stopped at roughly 1200 yards. The Storch can see gv's while they are moving at roughly 2400 yards.
Also, the icon of the Storch is only visible at 3000 yards to other aircraft.
I applaud HTC's decision to allow the Storch better abilities to spot gv's, AND to be hidden a wee bit longer from the Vegas style icon that lights up the sky.