In controls.con find the following:
ControlMap.create AirPlayerInputControlMap
Underneath that category look for:
ControlMap.addKeyToTriggerMapping c_PISprint IDFKeyboard IDKey_LeftShift 0 0
Beneath that line add:
ControlMap.addButtonToTriggerMapping c_PISprint IDFGameController_0 IDButton_1 0 1
Replace IDButton_1 with whichever joystick button you wish to use.
Then do the same beneath:
ControlMap.create HelicopterPlayerInputControlMap
Controls.con is generally found in something along the lines of:
C:\Documents and Settings\User\My Documents\Battlefield 2142\Profiles\0001