The crew was generally just one pilot/observer.
Their role? This will take some time. The mission of an FAC was to direct fighter bombers to provide ground support. Sounds like doubletalk, huh?
Occasionally, troops on the ground got into sticky situations and needed help. With this in mind, the Air Force would send flights of, say, F-4 Phantoms armed for air to ground to ‘stations’ above a sector but not with any assigned target. Cruise speed for a Phantom is say 430 knots. That’s going pretty fast to spot enemy troops in a jungle environment. Now, Army ground troops don’t have the capability to communicate directly with Air Force jets. The radios and frequencies are different. And even if it were possible, well, the directions a groundpounder (hey, I was one) could give a flyboy weren’t all that useful to the pilot. So, they came up with a FAC.
The FAC’s flew minorly modified civilian light aircraft. The only major modifications were 1. hard points on the wings for marker rockets (no, boom boom, but would make smoke) and 2. the right kinds of radios to allow them to talk to the troops on the ground AND the jets coming in. FAC’s were assigned to a sector for periods of time (months, maybe) so they did know the area. And they could fly slow enough to be able to spot troops on the ground and tell which side they were on. So, how would a mission evolve?
Ground troops in contact with the baddies would radio their net for air support. Their net would give them the call sign of the FAC up at the time and tell them they’d be contacted by the FAC. And the net would call in the AF. The FAC would fly to the area and contact via radio and ID the ground position of the friendlies. This is why I’d carry colored smoke grenades on patrol. The FAC then would call in the jets and scope out the area while they’re on the way.
When the jets arrive on station, they tell the FAC how they’re armed and how much fuel they have. The FAC tells the jets the situation, the winds, what direction they should head if they have to bail out and from what direction they should make their pass. Then the FAC goes in and marks the target with a marker rocket. It doesn’t have to be a direct hit. The FAC, after seeing where the rocket went might say “The target is 100 meters east of my smoke” or something like that. After the jets go in, the FAC sees if they need to make another pass, or clears the jets off. And if the jets get shot down, the FAC calls in the Search and Rescue folks.
Do they still have FACs? I don’t know. With the state of satellite and unmanned recon technology, they might not need them, or at least not as much.