Bonanza, N 3794N Accident Animation Audio Transcript

On February 3, 1959, at 12:55am, a chartered Beechcraft Bonanza took off from Mason City Municipal Airport on its way to Fargo, North Dakota. Prior to takeoff, the pilot had repeatedly checked the weather, beginning at 5:30pm, until just before takeoff. Initially, ceilings were reported at 5,000 feet or higher, with visibility of ten miles or more.

At takeoff, the local weather had deteriorated to a reported precipitation ceiling at 3,000 feet, sky obscured, and visibility of six miles. Light snow had begun to fall, with 20 knot winds, and gusts to 30 knots. The pilot was not aware of recent flash weather advisories issued prior to takeoff that called for generally deteriorating conditions along the route of flight. These conditions included visibilities below two miles, freezing drizzle, and moderate-to-heavy icing below 10,000 feet.

In weather conditions similar to those experienced during the accident fight, a pilot must rely on aircraft instruments to maintain a proper flight attitude and to navigate. The accident airplane was equipped with a Sperry F3 attitude gyroscope, which presents a different airplane pitch indication than a conventional artificial horizon.

A conventional artificial horizon provides a direct indication of the bank and pitch attitude of the aircraft, which is depicted by a miniature aircraft symbol displayed against a horizon bar, as if observed from the rear. The Sperry F3 gyro also provides a direct indication of the bank and pitch attitude of the aircraft, but is mechanized such that pitch information is presented in an opposite sense from a conventional artificial horizon.

In side by side comparisons, for the same pitch maneuver, the indicators would appear to move in opposite directions. Roll maneuvers would appear identical in both indicators. Investigators determined that the difference in pitch indication provided by the Sperry F3 gyroscope, compared to a conventional attitude indicator, could be confusing to an inexperienced instrument pilot.

Following takeoff, the aircraft made a left, 180-degree turn and climbed to approximately 800 feet above the ground. After passing the airport to the east, the aircraft headed in a north westerly direction.

With the loss of ground reference due to darkness and weather, a pilot may experience spatial disorientation. While not directly cited as a causal factor in this accident, spatial disorientation is common in this type of accident. Once an aircraft enters conditions under which the pilot cannot see a distinct visual horizon, disturbances in the vestibular system will remain uncorrected. Without reference to flight instruments, and in response to this sensory illusion, a pilot may misjudge the attitude of the aircraft and lose control.

Through most of the flight, the tail light of the aircraft was plainly visible to the owner, who was watching from a platform outside the airport control tower. When the aircraft was about five miles from the airport, the owner saw the tail light of the Bonanza gradually descend until it was out of sight.

The accident occurred on relatively level terrain, in a sparsely populated area. Investigators determined that the aircraft impacted the ground at a high airspeed and high rate of descent, in a steep right bank and a nose-low attitude.  

Investigators concluded that gusty winds and inflight turbulence may have affected the readability of other inflight instruments, forcing the pilot to focus his attention on the attitude gyro, an instrument with which he was not completely familiar. The pitch display of this instrument was the reverse of the instrument for which he was accustomed.  Investigators believed that he may have become confused, thinking that the airplane was in a climbing turn, while it was actually in a descending turn.

Investigators cited the pilot’s lack of instrument flying experience, the associated confusing nature of the primary attitude indicator, and inadequate weather briefings as the primary causes of this accident.

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