Corporate Air Cessna 208B accident

The pilot of the cargo flight was aware of thunderstorms west of the destination airport, but he did not use onboard radar during the flight to track the progress of the thunderstorms. Shortly after the pilot checked in on the airport’s air traffic control (ATC) frequency, the controller made a transmission to all aircraft on the frequency to expect light-to-moderate precipitation when abeam the airport. The controller then made another transmission to all aircraft on the frequency about a pilot report (PIREP) regarding a windshear gain of 20 knots on an 8- to 9-mile final approach. The airplane was cleared for a visual approach, and the pilot was instructed to follow and land after a Boeing 757. The pilot reported that he encountered a left quartering headwind while on final approach. The pilot also reported that while the airplane was in the landing flare, the tower reported that the Boeing 757 had encountered windshear while landing. Shortly thereafter, the airplane, while still airborne, started drifting sideways across the runway. The pilot then initiated a go-around because he was unable to maintain directional control of the airplane due to the wind. After adding power and climbing to about 30 ft above ground level (agl), the pilot lowered the left wing to stop the airplane from drifting off the runway centerline. He encountered a “downdraft” that caused the airplane to descend and impact terrain, resulting in substantial damage to the wings and fuselage. Thunderstorms were forecast for the airport in several different weather forecast products, to include terminal aerodrome forecast (TAF), graphical forecast for aviation (GFA), and a convective Significant Meteorological Information (SIGMET). A convective SIGMET was issued after the pilot had departed and was valid for the accident site. The convective SIGMET forecast an area of thunderstorms with little movement and with cloud tops above flight level 450. A convective SIGMET implies the potential for wind shear, lightning, severe turbulence, among other things. This aviation forecast information was consistent with the weather conditions that the accident flight encountered. Upper air weather information for the destination airport about the time of the accident indicated an unstable environment from the surface to 25,000 ft. The Automated Surface Observing System (ASOS) located about 5,000 ft south-southeast of the accident site, about 2 minutes after the accident, reported the windspeed was 29 knots gusting to 48 knots with thunderstorms and light rain. Video imagery a few minutes before and through the time of the accident revealed that virga was descending toward the airport with gusting winds and rain. Microbursts can be found in and beneath convective clouds and can be embedded in heavy rain or virga. Video showed that a microburst encountered the ground about 2 minutes before the airplane landed with gusting surface winds and associated rainfall moving toward the landing runway. The National Weather Service issued three airport weather warnings that were valid at the time of the accident. The first warning was for outflow wind gusts of 30 mph/26 knots or greater. The second and third warnings were for lightning within 5 miles of the airport. Although the pilot was en route to the destination airport when these warnings were issued and would thus not have known about them, the pilot could have had better situational awareness of convective activity along and near the route of flight if he had operated the airplane’s onboard weather radar. The investigation determined that ATC tower at the airport used only the center field/airport wind sensor information for wind information (to be distributed to pilots) and runway configuration. About the time of the accident, the wind sensor that SLC ATC tower used reported a wind gust of 16 knots, whereas the airport’s ASOS was reporting wind between 25 and 29 knots with gusts to 48 knots. The ATC tower wind sensor did not report a wind gust above 25 knots near the accident time, and neither of the airport’s windshear display systems indicated a microburst alert until about 10 minutes after the accident. The circumstances of this accident showed that dangerous weather conditions can impact arriving and departing aircraft operating on the southern part of the airport. Additional wind sensors, if properly positioned on and around the airport and runways, could have identified the microburst activity in enough time for the controller to have provided the information to the pilot before the attempted landing. Wind information from only one sensor would not allow the comparison of wind from other sensors and provide information from the departure and approach ends of the runways.

Verbatim NTSB analysis and probable cause from the NTSB dataset