Mooney M20F N9524M accident near Yosemite Valley, California

Probable cause

The pilot’s decision to continue flight into instrument meteorological conditions, which resulted in his failure to maintain situational awareness and sufficient altitude to clear mountainous terrain.

NTSB analysis

The instrument-rated pilot departed on a visual flight rules cross-county flight. Review of radar data showed the airplane departing and then proceeding on a northeasterly heading toward the intended destination. For the first 35 minutes of flight, the airplane remained below 10,000 ft mean sea level (msl). The airplane then ascended to a maximum altitude of 17,000 ft msl, initiated a descent, and turned north. As the airplane continued to descend through 14,300 ft msl, it turned west. (The airplane operated above 10,000 ft msl for 1 hour 8 minutes.) The data then showed the airplane perform a series of 360-degree descending turns. During the series of turns, the airplane descended to 6,200 ft msl and then ascended while turning east. The last recorded radar target was located about 12 miles west of the accident site at an altitude of 9,800 ft msl. Examination of the accident site revealed that the airplane impacted mountainous terrain just below the top of a ridgeline at an altitude of about 10,679 ft msl.

Airmen's meteorological information for instrument meteorological conditions (IMC), mountain obscuration, and moderate turbulence conditions below 18,000 ft were in effect throughout the area about the time of the accident. Satellite imagery depicted a layer of low clouds over the area with a thick, overcast layer of clouds over the accident site. Weather radar imagery depicted scattered echoes within the immediate vicinity of the accident site, which indicates that precipitation was falling over the area. However, due to the possibility of beam blockage from the higher terrain, the full extent and intensity of the precipitation over the accident site could not be determined. It could not be determined if the pilot obtained a weather briefing before the flight.

A postaccident examination of the airframe and engine revealed no evidence of preimpact mechanical malfunctions or failures that would have precluded normal operation. Wreckage impact signatures and radar data were consistent with continued flight toward the intended destination before the accident. It is likely that the pilot was attempting to maneuver around the IMC but did not maintain situational awareness and sufficient altitude above the mountainous terrain and entered IMC at the time of the accident.

As noted, the airplane operated 1 hour 8 minutes over 10,000 ft msl; this included 42 minutes over 14,000 ft msl. The Federal Aviation Administration requires pilots to use supplemental oxygen when flying over 14,000 ft msl; however, no evidence was found that supplemental oxygen was available to or used by the pilot. The duration of flight at these altitudes without supplemental oxygen could have resulted in the pilot experiencing hypoxia, which can cause various symptoms, including headache, decreased reaction time, impaired judgment, euphoria, visual impairment, and drowsiness. The pilot's decision-making was likely significantly impaired during this period, and the radar track showed that the airplane turned away from and flew northwest beyond the intended destination. However, the data then showed the airplane descend and remain below 10,000 ft msl for the last 17 minutes of recorded data as the flight track changed direction back toward the original destination. During this period, it is likely that the symptoms of hypoxia would have largely abated.

Text from the NTSB. Open the NTSB docket for WPR13FA312 for the full record.