NYC04FA223 · Aero Commander 112

While proceeding eastbound at night along the shoreline of an island, the airplane began a rapid spiral descent, and impacted the ground. Despite an extensive search by multiple entities, the airplane remained missing for approximately 4 years; however, when the wreckage was located, there was sufficient evidence to show that the vacuum pump, which powered the attitude indicator and the directional gyro, had failed. Examination of Federal Aviation Administration (FAA) records revealed that the airplane had accrued in excess of 1,224.8 total hours of operation, but no evidence of the vacuum pump ever being overhauled or replaced was discovered. The pilot did not possess an instrument rating, and most likely had not flown at night for approximately 10 years. The airplane was equipped with an autopilot; however, the pilot was reported not to be experienced in its use. Taking into consideration the direction of flight, available weather data, and the lack of ground lights in the area, there would have been no visible horizon forcing the pilot to maintain control of the airplane solely by referencing the flight instruments. It has been well documented that such conditions can result in a myriad of vestibular illusions which can be extremely difficult to overcome. One illusion in particular can cause a pilot to believe the airplane is in level flight, when in reality, it is in a gradual turn. If the airspeed increases, the pilot may experience a postural sensation of a level dive and pull back on the yoke, which tightens the turn. If recovery is not initiated, a steep spiral will develop and continue until the airplane impacts the ground or water. The FAA's Airplane Flying Handbook states that "unless a pilot has many hours of training in instrument flight, flight in reduced visibility or at night when the horizon is not visible should be avoided." FAA Advisory Circular AC 91-75, highlights that pilots tend to rely heavily on the attitude indicator in instrument meteorological conditions, and that vacuum system failures can be a significant cause or contributor to fatal accidents because in most cases the corresponding instruments slowly becomes inaccurate, making the failure difficult to recognize.

Verbatim NTSB analysis and probable cause from the NTSB dataset