Advise ATC or the evaluator if unable to comply with a clearance.

Bottom line

After identifying an instrument or system failure, the pilot should usually advise ATC of the problem and, if necessary, declare an emergency before the situation deteriorates beyond the pilot's ability to recover. A pilot under radar control whose directional gyro or other stabilized compass is inoperative or inaccurate should advise ATC and request a no-gyro vector or approach.