In mission 7 "Midnight Departure" eliminate Davidov as quick as 
you can. Pick his stuff up and if you're on secret or oo agent 
get the ID badge from the shack in the woods. Get back on the main 
path and don't let the alarm get pulled. The first truck you see 
after you cross the bridge(kill the three guys on the right side 
of the truck.) Then climb in the back of the truck(wait a few 
seconds for the truck to start) and then the truck will take you 
to the air field.(This works on all difficulties and is great for 
the cheat.)