What happened
On August 7, 2021, a Mooney M20M, registration N9156Z, was on an instrument flight rules (IFR) flight from Chandler Field Airport in Alexandria, Minnesota, to Flying Cloud Airport. While performing an instrument landing system (ILS) approach to runway 10R in instrument meteorological conditions (IMC), the aircraft began to deviate from its course.
During the final approach, the airplane tracked left of the ILS course and descended below the required altitude. The tower controller issued a low-altitude alert, which the pilot acknowledged. Following this, the aircraft made an abrupt left turn and entered a rapid descent. Radar contact was subsequently lost. The airplane impacted the ground upright in a nose-high attitude, followed by a postimpact fire. The accident resulted in 3 fatal injuries for the pilot and two passengers.
The investigation
An examination of the wreckage revealed that the aircraft experienced an in-flight breakup during the final seconds of flight. Debris, including the left horizontal stabilizer, left elevator, and a section of the main wing spar upper cap splice plate, were found away from the primary impact site. Investigators determined that both wings fractured near the outboard ends of the main landing gear wheel wells, and the horizontal stabilizer and elevator also suffered fractures due to overstress.
An ADS-B performance study showed that the aircraft's airspeed and altitude fluctuated erratically during the approach. The study found that the airplane's airspeed increased to over 180 knots during a steep descent, and the estimated load factor reached more than 8 Gs, exceeding the manufacturer's maximum positive load factor of 3.8 Gs. This high-G maneuver was consistent with an attempt to recover from a nose-low attitude after the ground became visible.
Postaccident examination of the engine and airframe found no preaccident mechanical failures or malfunctions that would have prevented normal operation. Toxicology testing of the pilot's tissue detected diphenragramine, a sedating antihistamine, though the investigation could not determine if its presence contributed to the accident.
Findings
- The pilot experienced spatial disorientation due to the lack of visual references while flying in IMC.
- The pilot's loss of control led to a spiral dive that subjected the airframe to loads exceeding its structural limits.
- The aircraft's structural failure, including the fracture of the wings and horizontal stabilizer, was caused by the extreme load factors encountered during the recovery attempt.
- The pilot's radio communications indicated he may have been task-saturated during the approach.