Accident Sequence
The airplane collided with mountainous terrain after the flight crew terminated the instrument approach and proceeded visually at night. The crew had failed to brief or review the instrument approach procedure prior to takeoff. During the approach, they exhibited various cognitive task deficiencies, including selection of the wrong frequency for pilot controlled lighting, concluding that the airport was obscured by clouds despite contrary weather information, stating inaccurate information regarding instrument approach headings and descent altitudes, and descending below appropriate altitudes. These deficiencies resulted in a lack of awareness regarding terrain in the approach path.
Visual Illusion
The dark visual scene on the approach path and the absence of a visual glideslope indicator were conducive to producing a false perception that the airplane was at a higher altitude. Pilots approaching a runway over dark, featureless terrain may experience an illusion that the airplane is at a higher altitude than it actually is, a phenomenon referred to as the featureless terrain illusion or black hole phenomenon. In response, a pilot may fly a lower-than-normal approach, potentially compromising terrain clearance requirements.
Flight Crew Actions
The investigation noted that a ground proximity warning device may have alerted the crew prior to impact, but the amount of advanced warning that could have been provided was not determined. Additionally, although the flight crew's performance was consistent with fatigue-related impairment, based on available information, the Safety Board staff was unable to determine to what extent the cognitive task deficiencies exhibited by the flight crew were attributable to fatigue and decreased alertness.
Probable Cause
The official probable cause was inadequate crew coordination, which led to the captain's decision to discontinue the instrument approach procedure and initiate a maneuvering descent solely by visual references at night in an area of mountainous terrain. The crew failed to review the instrument approach procedure, and the copilot failed to provide accurate information regarding terrain clearance and let down procedures during the instrument approach.