Mooney M20M N9156Z accident near Victoria, Minnesota

Probable cause

The pilot’s loss of airplane control due to spatial disorientation during final approach, which led to a spiral dive that overstressed the airplane and resulted in an in-flight breakup.

NTSB analysis

The pilot and two passengers (one of whom was a student pilot) departed on a personal flight. The pilot was cleared by air traffic control (ATC) to fly an instrument landing system (ILS) approach in instrument meteorological conditions (IMC). About 10 miles from the runway while on final approach, the airplane slowed to 80 knots, tracked left of the approach course, accelerated to about 140 knots, and descended about 300 ft. The airplane subsequently turned right and descended below the designated altitude for the approach, which triggered a low-altitude alert that the controller transmitted, and the pilot acknowledged.

The airplane then abruptly turned left and entered a steep descent. The airplane continued in a left-turning spiral and descended below an overcast ceiling. The airplane subsequently impacted the ground upright about 8 miles west of the destination airport. Both wings and the right stabilizer were deflected upward in a vertical position. No preaccident mechanical failures or malfunctions were found with the airframe and engine that would have precluded normal operation. The airplane debris on the ground—the left horizontal stabilizer, left elevator, and part of the main wing spar upper cap splice plate--showed that an inflight breakup occurred during the final seconds of flight.

The performance study for this accident revealed that the airplane exceeded its maximum positive load factor during the spiral descent. As the airplane descended below the overcast ceiling, a rapid groundspeed increase and heading change occurred, which were consistent with the pilot (or possibly the student pilot) attempting to recover the airplane from a nose-low attitude after seeing the ground. The spiral descent and attempted recovery overstressed the airplane, which caused the in-flight breakup.

The left horizontal stabilizer, left elevator, and spar cap were found southwest of the accident site. Postaccident examinations of the airplane revealed that both wing main spars and both sides of the horizontal stabilizer had fractured due to overstress. The wings fractured first, and the horizontal stabilizer, elevator, and spar cap fractured immediately afterward.

While the pilot was flying the final approach, several of his radio transmissions to ATC were either delayed or disjointed, indicating that the pilot was task-saturated. The performance study showed that, when the airplane made the series of turns while on final approach, erratic altitude and airspeed fluctuations occurred. These airspeed and altitude fluctuations and the tight spiraling turn that began afterward were consistent with the pilot becoming spatially disoriented due to the lack of visual references while the airplane was operating in IMC. The pilot’s spatial disorientation led to his loss of airplane control.

A friend of the accident pilot stated that the pilot had adopted an instrument flying habit in the Mooney airplane that involved making turns on approach primarily with the rudder and adjusting pitch attitude with the pitch trim. If the pilot controlled the airplane in such a manner during the accident flight, especially in response to the controller’s lowaltitude alert, the application of rudder could have exacerbated the pilot’s erratic airplane control inputs while on approach.

The pilot’s electronic logbook did not show any logged instrument approach procedures in 2021, and the accident pilot did not fly with his usual safety pilot during 2021. The pilot’s last flight review, in October 2020, did not include any instrument approach procedures. Neither the safety pilot nor the accident pilot’s flight instructor knew whether the accident pilot had flown with another safety pilot to log instrument time. As a result, the investigation was unable to determine if the accident pilot met the Federal Aviation Administration’s regulatory requirements for instrument experience.

Diphenhydramine (commonly marketed as Benadryl) was detected in the pilot’s liver and heart tissue; no blood specimen was available to assess therapeutic levels. Diphenhydramine causes sedation and can slow psychomotor responses and reaction times, which can contribute to susceptibility to spatial disorientation. However, without a diphenhydramine blood level, the investigation was unable to determine whether the effects of the pilot’s use of diphenhydramine contributed to this accident.

Text from the NTSB. Open the NTSB docket for CEN21FA360 for the full record.