Page:CAB Accident Report, Frontier Airlines Flight 32.pdf/7

- 7 - It is impossible to reconstruct the precise flightpath or exact altitudes of the aircraft during its final few minutes of flight. It must be borne in mind that strong winds with blowing wet snow and low ceilings could and probably did significantly distort both sound and sightings as described by witnesses. The evidence indicates that the aircraft flew over the VOR at approximately 2048 about 1,000 feet above the surface. The aircraft at this position, according to the witness, appeared and sounded normal an all respects. Yet, the point of impact was located only 1.7 miles from the VOR where the minimum descent altitude should have been approximately 400 feet above the ground.

Because there is no evidence of any failure or malfunction of the aircraft or navigational aids, nor any message received from the crew pertaining to any difficulties, the Board cannot state, with any degree of certainty, the reason for the unexplained departure from the minimum descent altitude. However, based on the evidence available the following possibilites appear to be the most plausible:

1. An attempt to Conduct the Final Portion of the Approach by Visual Reference to the Ground

The witness located at the VOR site stated that visual contact with both the aircraft and the airport beacon was possible from his position. It as, therefore, possible that when the aircraft was an the vicinity of the VOR, visual contact with the ground and/or airport beacon was established by the crew. If this were the case, it is conceivable that with both pilots attempting to maintain visual contact with the ground and/or keep the airport beacon in sight, a continued unmonitored descent below the existing ceilings could have resulted.

The final approach speed for the DC-3 1s approximately 105 knots and the aircraft's ground speed at impact was computed to be approximately 137 knots. Under the existing wind conditions the indicated airspeed (TAS) at impact would have been approximately 134 knots. To obtain the speed, with landing gear down and flaps 1/4 extended, either high engine power settings or a decrease in the angle of attack or a combination of the two would be required. The evidence indicates that normal engine power was being utilized by Flight 32 until just prior to impact.

Therefore, it can be postulated that an order to continue the flight by visual means during the final portion of the approach a rapid descent became necessary to remain below the clouds and to keep the ground and/or airport beacon in sight. If both pilots were concentrating on conditions outside the cockpit, and the altimeters were unmonitored, it is possible that the descent was continued until contact with the ground was unavoidable. Power was added by the crew when it was realized they were at a dangerously low altitude. However, the power application was too late to prevent ground impact.

2. Airframe Icing

It was determined that weather conditions an the vicinity of Miles City were conducive to moderate to heavy airframe icing in clouds and precipitation. Strong gusty winds over the rough terrain would likely have produced moderate to severe turbulence in the area.

Under these conditions large ice accretions on the wing surfaces, if undetected throughout the descent and initial approach, would have become a