From: Vienna, Austria
Air group fatigue is the average of the individual pilot fatigue of the squad.
This is important to know, since a fat. value of 20 could mean 2 pilots at 20, or 1 pilot with fat 0 and 1 pilot with fat 40.
Pilot fatigue can range from 0 (no fat) to 100, but usually the fatigue at turns´ end is not higher than 50-60 maximum per pilot. The reason is that for a pilot to fly a mission
he has to pass a fatigue check. If he fails the check he does not participate in the mission.
Highly fatigued squads perform worse due to two factors: high fatigue of the pilots flying the mission, and a lower percentage of pilots participating on the mission.
Fatigue negatively affects performance in mission specific dice rolls (e.g. air skill, navB skill,...), negatively affects coordination, and increases op losses.
The best way to keep fatigue at lower levels, even in a combat environment, is simply to finetune the rest settings.
As a rule of thumb, the higher the mission frequency (e.g. CAP has a high mission frequency as compared to bombing missions), the longer the mission range, the higher
# of airframes / # of pilots, the higher the mission altitude, the larger the percentage of actual engagements (e.g. CAP which constantly faces incoming strikes), the higher
your rest setting has to be to keep fatigue within bearable levels.
On the long run high fatigue levels also affect pilot morale. A constantly high fatigue gain with low rest settings will drop moral pretty fast (this one is measured the other way
around...100 is best, 0 is worst), which is harder to recover than fatigue.
Personally I consider an avg. fatigue level of 25 to be high, +-10 depending on mission/airframe type, and environment.