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: 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: 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.