Helicopters will not fly over the Comms Bravo area any lower than 70m no matter what. Not even a flyinheight will get them to go to the correct altitude.

EDIT: Just discovered, any number above 10 used with flyInHeight results in the helo sitting at a fixed altitude, regardless of what number you set. If you set it to a number below 10 however, the helo will descend and sit at that altitude just fine.