Ive got call of duty 4 at the moment and im stuck at a level,i need help?

Ive had call of duty 4 for about 2 months and i found it easy before i got stuck at this level,i don't know the name of the level,but it's where you have to shoot down zhakaev in the helicopter,every time i try and do it,it always says zhakaev escaped unharmed,how do i p this part