Apache Helicopterscan sometimes fly to your location and aid you for a limited amount of time. These helicopters will hoverin front and above your own vehicle and match your speed and direction with a slight delay.
They come armed with heavy machinegun turrets and rockets and will shoot up a path for you.
In all the challenge modes except Rescue Ops and F.U.B.A.R., they will be available every time a certain distance is reached and can be called in when you desire. Upgrading the Chopper Support will make it available more frequently and will help you for a longer time.