Revealing mistake: When String uses Airwolf's rotor wash to blow over the pickup, when the camera is on the truck, you can see a cable pulling the truck up from the right side.
Continuity mistake: Just as Santini lands his helicopter after his rear rotor was destroyed, Hawke gets into his jeep and drives off. The number plate (licence) is reversed and Hawke's steering wheel is on the right side like British vehicles are. The next shot you see the shot has been flipped the correct way with both plate and steering wheel on the left side. (00:24:14)
Continuity mistake: When Hawke is first seen running on the ranch, his shirt is intact. A moment later, it's torn on the back, and the sleeve is ripped off.
Continuity mistake: During the final battle Airwolf shoots a Bell UH-1. In the following explosion shot a Hughes OH-6 is blown up.
Other mistake: During the helicopter chase at the end a pilot can be seen in Airwolf's left seat although Hawke is sitting on the right side and Santini in the back.
Continuity mistake: For most of the flight shots, Mace is shown in the right seat. Part way through the chase, he's shown in the left seat, and everything is reversed.
Continuity mistake: When String tells Dominic about his plans to act on the phone call, Dominic says that things had been quiet for two months. When they're proceeding from liftoff in Airwolf, he said that three months hadn't harmed her, referring to Airwolf.
Continuity mistake: When String and Dominic first look at Airwolf, the cockpit is dark. When they get out of the jeep, the cockpit is lit up with a greenish light.
Answer: I don't think it's ever explained why Mac refused to tell Hawke where St. John is.