If I remember from law school, a "frolic of his own" is not just not following instructions. If an employee runs a red light in a company car and causes an accident, the company is liable, no matter how many times it reminded or ordered the employee not to do so. This, by the way, does not mean that the employee is not liable, only that the company can be sued as well.
If the company sends the employee across town to make a delivery, and he runs for a quickie with his girl friend 50 miles away, that's a frolic of his own, and the company is not liable if he gets in an accident on the way.
A mechanic's job is to fix the company's airplanes, and it would be pretty hard for the company to escape liability for anything he did while doing so. Well, maybe if he snuck into the cabin for a smoke and managed to start a delayed fire, or something like that.