As a pilot in the RAH-99 Blackfoot or the Mi-48 Kajman, you have the "Manual fire" option, which allows you to control the weapon system on the helicopter. (Rotary cannon, guided missiles and unguided missiles)
However, you can not lock onto targets using manual fire(whether by pressing the tab key; clicking the right mouse button or pressing and holding the right mouse button)
You can still shoot your missiles, but they will be shot straight like dumb rockets.
However, if you have an AI gunner and are using manual fire, you are able to lock onto targets and fire on them.