on some of the aircraft the is a primer. what does it do?
The engine primer will inject a small amount of fuel directly into the cylinders to assist with engine starting. You should follow the POH's engine start checklist for details on when and how much to prime.
ok thanks but, i have just played with it, my planes never have need it. so does it really work? on FS 2004
Yes well, good point. You probably don't need to worry about it to get a plane to start in FS9 (since you can just autostart anyway). But if you're going for realism and want to follow a prestart checklist, that's what it does.
oh ok thanks!