Background
The pilot began flying the twin piston-engine airplane model for the cargo airline about 11 months before the accident. He subsequently upgraded to one of the airline's twin turboprop models but, due to logistical needs, was transferred back to the piston-engine model about one week before the accident.
Flight Planning
The flight originated at one of the airline's outlying destination airports and was planned to stop at an interim destination to the southwest before continuing to the airline's base. The late afternoon departure meant the flight would arrive at the interim destination about 10 minutes after sunset. The interim destination was situated in a sparsely populated geographic bowl just south of terrain that was significantly higher. Ceilings included multiple broken and overcast cloud layers near or lower than the surrounding terrain.
Although not required by FAA regulations, the airline employed dedicated personnel who performed partial dispatch-like activities, including providing weather information. Before takeoff, the pilot conferred briefly with dispatch personnel, and they agreed the flight would proceed under visual flight rules to the interim destination. Available information indicated cloud cover almost certainly precluded access to the airport without an instrument approach; however, the airplane was not equipped to conduct the only available instrument approach procedure for that airport. The pilot did not have in-flight access to GPS or terrain mapping/database information to assist in locating the airport or remaining clear of terrain.
The Flight
Although the airplane was not being actively tracked or assisted by air traffic control (ATC) early in the flight, ground tracking radar data showed the flight initially headed directly toward the interim destination but then began a series of turns, descents, and climbs. The airplane disappeared from radar due to radar coverage floor limitations from high terrain and radar antenna siting. It reappeared on radar about 24 minutes later and about 9 minutes after the FAA-defined beginning of night.
Based on the flight track, it is likely the pilot made a dedicated effort to access the airport while remaining clear of clouds and terrain by visual means. This task was made more difficult by attempting it in dusk conditions and then darkness.
About 15 minutes after the airplane reappeared on radar, at an altitude of about 13,500 ft, the pilot contacted ATC and requested and was granted an instrument flight rules clearance to the final destination. About 3 minutes later, the controller cleared the flight to descend to 10,000 ft, and the airplane leveled off about 6 minutes later. Upon reaching 10,000 ft, the pilot requested a lower altitude to escape “heavy” updrafts and downdrafts, but the controller could not comply because the ATC minimum vectoring altitude was 9,700 ft in that region. About 1 minute later, radar contact was lost. Shortly thereafter, the airplane impacted terrain in a steep nose-down attitude in a near-vertical trajectory.
Meteorological Conditions
Analysis of radar data indicated the airplane was above 10,000 ft for at least 41 minutes (possibly in two discontinuous periods) and above 12,000 ft for at least 18 minutes. Although the airplane was reportedly equipped with supplemental oxygen, the investigation could not verify its presence or use. Lack of supplemental oxygen at those altitudes could have contributed to a decrease in the pilot’s mental acuity.
Analysis of air mass data revealed mountain-wave activity and updrafts and downdrafts with vertical velocities of about 1,000 ft per minute near the accident site. The largest and most rapid transitions from up to downdrafts occurred near the accident site. The last radar target was located in a downdraft with a velocity between 600 and 1,000 fpm.
Other meteorological analysis indicated the airplane encountered icing conditions, likely in the form of supercooled large droplets (SLD), several minutes before the accident. No tools currently exist to detect airborne SLD, and tools and processes to reliably forecast SLD do not exist. SLD is often associated with rapid ice accumulation, especially on portions not served by ice protection systems.
Wreckage and Systems
Examination of the wreckage did not reveal any preimpact mechanical deficiencies that would have prevented normal operation and continued flight. However, the extent of the damage precluded any determination of the preimpact integrity or functionality of any systems, subsystems, or components, including ice protection systems, autopilot, and nose baggage door.
The investigation could not determine whether the pitot heat was operating during the final portion of the flight. Because of the pilot’s recent transition from the Beechcraft BE-99, in which pitot heat was always operating during flight, he may have forgotten that the accident airplane’s pitot heat procedures were different and that it had to be manually activated in icing conditions. If pitot heat is not operating in icing, airspeed information becomes unreliable and likely erroneous.
The investigation also could not determine whether the pilot used the autopilot during the last portion of the flight. If he was using it, it is possible he was forced to revert to manual flying due to the unit’s inability to maintain altitude in strong updrafts and downdrafts, which would increase workload. Another possibility is that the autopilot was unable to maintain altitude, and the pilot overpowered it via the control wheel. If the pilot overrode the autopilot for more than 3 seconds, the pitch autotrim system would activate opposite the pilot’s input, and when the pilot released the control wheel, the airplane could have been significantly out of trim, resulting in uncommanded pitch, altitude, and speed excursions.
Whether hand-flying or using autopilot, the encounter with strong updrafts and downdrafts and consequent altitude loss likely prompted the pilot to input corrective actions to regain altitude, such as increasing pitch and possibly power. Such corrections typically result in airspeed losses, sometimes significant depending on downdraft strength and climb capability. If climb capability is compromised by added weight, drag, and other adverse effects from airframe icing, the airplane may not be able to recover.