Z depth with parrents

[Jan]

I have a helicopter flying around. The helicopter is sitting in the middle and all the other actors are moving. Now I got this controle tower and if the chopper is in front of it the tower has to slide behind the chopper. If the chopper is behind the tower, the tower should slide in front of the chopper. To move all the actors (excluding the chopper) I parented everything to the background So everything has the same (parent) Z depth. I have made a check to see if the chopper is in front or behind the tower. If the chopper is behind the tower I have it change the Z depth of the tower so it appears in front of the chopper but also the background and the other actors change their Z depth placing my chopper behind the background. Is there a way that only the Z depth of the tower can be change if it's parrented to the background? Anybody?