Have the chopper as part of the tanks group, with lower rank than the tank and use column formation. When you want the chopper to move on, have it join grpnull and give it move commands. Choppers fly low by default, so setting it too low may make it crash into a tall tree. You could set the flyinheight at 25 and it should be ok since it'll be moving slow (speed of tank) and have time to react to terrain changes.