Frontier flight 7 was a regular service between Denver, CO and Phoenix, AZ with several intermediate stops. The flight departed Prescott, Arizona at 13:21 for the final leg. Cruising altitude was 6500 feet. En route the airplane descended due to decreasing visibility. At an altitude of about 4600 feet the left wing impacted the side of a mountain ridge. The portion of the left wing and the aileron torn off was from station 286.4 outboard to the tip. A safe landing was made at Phoenix at 14:05. PROBABLE CAUSE: “The Board determines that the probable cause of this accident was the attempt by the pilot to fly over mountainous terrain by visual reference to the ground in weather conditions which severely restricted forward visibility and necessitated a descent to a dangerously low altitude.”