Salt Lake City, UT, USA
N877FE
CESSNA 208B
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.
HISTORY OF FLIGHTOn July 13, 2022, about 1852 mountain daylight time, a Cessna 208B airplane, N877FE, was substantially damaged when it was involved in an accident at Salt Lake City International Airport (SLC), Salt Lake City, Utah. The pilot sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 135 cargo flight. The flight was operated as CPT 7727, a Federal Express Corporation feeder flight, which departed from Friedman Memorial Airport (SUN), Hailey, Idaho, about 1729 and climbed to 15,000 ft mean sea level (msl). The en route portion of the flight to SLC was uneventful. About 1817, the airplane began a descent into SLC. About 1823, when the airplane was at 12,000 ft msl, the pilot checked in with Salt Lake Terminal Radar Approach Control (TRACON), and the controller cleared the airplane to descend to 11,000 ft msl and provided waypoints for the approach. About 1835, the controller issued a transmission to all aircraft near SLC to expect light-to moderate precipitation once abeam the airport. About 1844, the controller instructed the pilot to descend to and maintain 9,000 ft msl. About 1845, the pilot reported that the airport was in sight, and the controller told him to expect to land on runway 34R. About 1846, the controller instructed the pilot to follow a heading of 150° and to descend to and maintain 8,000 ft msl. The controller also provided a PIREP to all aircraft in the area, indicating a windshear gain of 20 knots on an 8- to 9 mile final approach. The controller then instructed the pilot to follow a Boeing 757 on final approach for runway 34R, cautioned him about wake turbulence, and cleared the airplane for the visual approach to runway 34R. The controller then instructed the pilot to contact the SLC tower, and the pilot acknowledged this instruction. About 1848, approach controller and the local controller both transmitted, to all aircraft on frequency, a windshear alert for runway 34L, which was parallel to runway 34R. The alert cited 15-knot loss 2 miles out for arriving airplanes. The pilot contacted the SLC tower, and the local controller instructed the pilot that he was following a Boeing 757 on a 5-mile final approach, reported the wind from 300° at 18 knots with gusts to 29 knots, and cleared the airplane to land on runway 34R. The controller then asked the pilot of an airplane that just departed from runway 34R if they encountered windshear on climbout. The pilot reported no wind shear on climbout but stated that the midfield wind gusts were strong. The controller solicited a PIREP from the Boeing 757 that was ahead of the accident airplane. The 757 crew reported that they encountered windshear of ± 10 knots on final approach. The controller then transmitted to all aircraft on frequency, a windshear alert for runway 34R arrivals and departures, indicating a 15-knot loss over the runway. The pilot reported that he was flaring the airplane when the local controller reported that the Boeing 757 had encountered windshear on landing. The pilot stated that, during the flare, the airplane started going sideways across the runway. The pilot also stated that he “elected to go around” because he was unable to maintain direction control of the airplane due to wind from the left and that he informed the local controller about the go-around. The pilot reported that, after adding power and climbing to about 30 ft agl, he “lowered the left wing” to counteract the drift from the runway centerline. Subsequently, he encountered a “downdraft” that was “pushing” the airplane toward the ground. Surveillance video showed that the airplane impacted terrain off the right side of the runway in a left-wing-low, nose-down attitude. The airplane’s tail came up to a near vertical position and then settled back down. The airplane came to a rest on its belly and right main landing gear; the nose and left main landing gear had collapsed. PERSONNEL INFORMATIONThe pilot was hired by Corporate Air in June, 2001, and was current and qualified in accordance with FAA regulations and Corporate Air requirements. AIRCRAFT INFORMATIONAccording to the Corporate Air operations specifications, the CE-208B airplane was authorized to conduct cargo operations in instrument flight rules (IFR) and visual flight rules (VFR) and during the day and night. The accident airplane’s weight and balance were within operational limits for taxi, takeoff, and landing. METEOROLOGICAL INFORMATIONThe pilot reported that he checked the weather on his phone before the flight using Aviation Digital Data Service. The pilot also reported that he reviewed the terminal area forecast, meteorological aerodrome reports, and the radar for the route of the flight. The pilot stated he did not use the onboard weather radar during the flight. He stated that, as the airplane approached SLC, “some weather” was occurring over the mountains to the west of the airport that was moving slowly to the east. He recalled that the automatic terminal information service (ATIS) information for SLC changed a few times, mostly due to barometric pressure updates, and that the wind was from the northwest at 20 to 30 mph. The pilot stated that he was not provided with any wind information after being cleared to land. SLC had an ASOS that was augmented by a certified weather observer. At 1833 (19 minutes before the accident), the following observations were reported: wind from 340° at 16 knots with gusts to 21 knots, visibility 10 miles or greater, thunderstorm, few cumulonimbus clouds at 11,000 ft agl, and broken ceiling at 24,000 ft. The thunderstorm began at 1827, with cumulonimbus clouds southwest moving northeast. At 1854 (2 minutes after the accident), the following ASOS observations were reported: wind from 300° at 29 knots with gusts to 48 knots, visibility 9 miles, thunderstorm with light rain, and broken ceiling at 14,000 ft agl. The thunderstorm began at 1827, ended at 1842, and began again at 1850 with cumulonimbus cloud overhead and moving northeast. The National Weather Service issued three SLC airport weather warnings between 1800 and 1900. The first warning, at 1817, was issued for outflow wind gusts of 30 mph/26 knots or greater. The second warning, at 1826, was issued for lightning within 5 miles of the airport through 1845. The third warning, at 1845, extended the time to 1900 for lightning within 5 miles of the airport. The GFA applicable to the accident site that was valid at 1800 (before the airplane’s departure) indicated VFR surface visibilities, scattered thunderstorms, and a southwest surface wind at 10 knots with gusts to 15 knots. The GFA cloud forecast indicated clouds above the accident site. At 1354, the National Weather Service Storm Prediction Center issued a convective outlook with areas of general thunderstorms forecast for the accident site. Weather radar base reflectivity images between 1836:37 and 1854:11 depicted reflectivity values between 20 and 40 dBZ moving from southwest to northeast over the accident site at the time of the accident. An increase in windspeed toward the accident site was noted at the accident time. In addition, the base reflectivity core, or the main portion of the rainfall from the thunderstorm, descended toward the accident site at the accident time. Fifty lightning flashes were reported within 25 miles of the accident site in the 10 minutes before and after the accident time, with the closest lightning flash occurring 1 mile west of the flight track at 1850:46. Airman’s Meteorological Information (AIRMET) advisory Sierra was issued at 1445 and was valid for the accident site at the accident time. The AIRMET forecast mountain obscuration conditions due to clouds and precipitation. Convective SIGMET advisory 66W was issued at 1755 and was valid for the accident site at the accident time. The SIGMET forecast an area of thunderstorms with little movement and with cloud tops above flight level 450. Convective SIGMETs indicate the potential for windshear, lightning, and severe turbulence, among other hazards. Rain showers and convective clouds can produce downdrafts, microbursts, outflow boundaries, and gust fronts, which can create an environment favorable for unexpected changes in wind direction and speed at the surface. According to the FAA Aeronautical Information Manual, section 7-1-24, microbursts are typically relatively small in area (less than 1 to 2 1/2 miles in diameter) and produce strong divergent winds and downdrafts that can reach 6,000 ft per minute. Microbursts can be found in and beneath convective clouds, usually embedded in heavy rain or in “benign-appearing” virga, (trails of precipitation that fall from the underside of a cloud but evaporated before reaching the surface). If little or no precipitation is occurring at the surface, a ring of blowing dust may be the only visual cue of the microburst Review of video imagery around the accident time revealed that virga was descending toward the accident site with gusting winds and rain. About the time of the accident, the wind sensor that the SLC ATC tower used reported a wind gust of 16 knots. The sensor did not report a gust above 25 knots between 1852 and 1915. In addition, the Integrated Terminal Weather System and windshear displays that the tower used provided no microburst alerts until about 10 minutes after the accident. AIRPORT INFORMATIONAccording to the Corporate Air operations specifications, the CE-208B airplane was authorized to conduct cargo operations in instrument flight rules (IFR) and visual flight rules (VFR) and during the day and night. The accident airplane’s weight and balance were within operational limits for taxi, takeoff, and landing. WRECKAGE AND IMPACT INFORMATIONPostaccident examination of the airplane revealed that the outboard 6 ft of the right wing was bent up about 30° and that the outboard 5 ft of the left wing was bent up about 5°. The left side of the fuselage was crumpled aft of the cargo loading door. The cockpit was undamaged. The flap lever was in the “UP” position, and the flap jack screw correlated to the flaps “UP” position. The left main strut was bent inward about 45°. ADDITIONAL INFORMATIONFAA Advisory Circular 00-54, “Pilot Windshear Guide,” dated November 25, 1988, was in effect at the time of the accident but has since been canceled. (The FAA’s website stated that all ACs dealing with weather haven been consolidated into a new handbook in order to streamline access to the FAA’s weather documentation) The guide provided, the following information: Wind variations at low-altitude have long been recognized as a serious hazard to airplanes during takeoff and approach. These wind variations can result from a large variety of meteorological conditions such as: topographical conditions, temperature inversions, sea breezes, frontal systems, strong surface winds, and the most violent forms of wind change--the thunderstorm and rain shower. This document defined windshear as – “any rapid change in wind direction or velocity.” Severe windshear was defined as “a rapid change in wind direction or velocity causing airspeed changes greater than 15 knots or vertical speed changes greater than 500 feet per minute.” Text that followed these definitions stated the following: The primary focus of the Pilot Windshear Guide is directed toward windshear associated with convective weather conditions: thunderstorms, and in particular the most hazardous form of wind shear, the microburst. An appendix addressed the microburst as a windshear threat and stated the following: Identification of concentrated, more powerful downdrafts--known as microbursts--has resulted from the investigation of wind shear accidents and from meteorological research. Microbursts can occur anywhere convective weather conditions (thunderstorms, rain showers, virga) occur. Observations suggest that approximately five percent of all thunderstorms produce a microburst. Downdrafts associated with microbursts are typically only a few hundred to 3,000 ft across. When the downdraft reaches the ground, it spreads out horizontally and may form one or more horizontal vortex rings around the downdraft…. The outflow region is typically 6,000 to 12,000 ft across. The horizontal vortices may extend to over 2,000 ft AGL. A review of the National Aeronautics and Space Administration Aviation Safety Reporting System (ASRS) found that, during the 10 years preceding the accident, none of the reported windshear events involved a Cessna 208 airplane. The most recent Cessna 208 windshear event recorded in ASRS was in 2006. During that event, the controller instructed the airplane to go around due to windshear, but the airplane had already landed. A search of ASRS revealed that, during the 10 years preceding the accident, three windshear-related events at SLC were reported. One of the reports, from an SLC local controller, stated that two go-arounds had to be performed due to windshear and that at least one of them created a “potentially dangerous situation,” with departure traffic. The second report stated that, while on the downwind leg to land on runway 34L, an airplane encountered windshear and experienced a loss of 2,000 ft. The third report indicated that, while on approach to runway 34L inside the final approach fix, an aircraft entered an area of precipitation, and the wind “immediately sheared” from a tailwind of 40 knots to a crosswind of 38 knots. The aircraft’s airspeed subsequently increased by 40 kts which resulted in a flap overspeed. Both of the pilot submitted ASRS reports did not indicate any windshear alert from the air traffic controller. ORGANIZATIONAL AND MANAGEMENT INFORMATIONCorporate Air was founded in 1981 and was headquartered in Billings, Montana. The company owned and operated maintenance facilities and pilot bases in Montana, Wyoming, North Dakota, Utah, and Hawaii. The company offered feeder airline cargo service and operated a fleet of about 34 Cessna 208 Caravans on a regularly scheduled basis in the United States and Canada. The Corporate Air Flight Operations Training Manual showed that special operations was a training topic provided during recurrent ground training. Windshear was considered to be a special operation The Corporate Air General Operations Manual, section 3, “Operations,” provided the following about airborne thunderstorm detection equipment: a. For the record, in Corporate Air's…airborne thunderstorm detection equipment is not required by regulation. 1. However, in the event this equipment becomes inoperative, and thunderstorms or other potentially hazardous weather can be expected along the route of the flight, the following actions will be taken by the crew: i. Select a route of flight that will avoid the weather in question. ii. For day operations, select a route of flight to maintain VFR conditions and that allows avoidance of this type of weather visually. iii. Delay the flight until such weather conditions no longer exist along the route of flight. A. Advise Flight Following of your intentions. The Corporate Air Flight Operations Manual, volume 1, “Operating Bulletins,” provided the following guidance about windshear: Wind never blows from a constant direction or speed for more than short periods, even in the most stable conditions. Shear is nothing more than a change in wind component, and since a change in either wind direction or speed changes the component, the airplane is always affected by shear. It is fascinating to observe the continuous display of wind speed and direction on an INS, IRS, or GPS equipped airplane and see the airplane’s response to just half a knot change in wind component. This ever-present kind of shear is benign; it is the sudden and large changes in component that cause the mischief. Shear causes a loss (or gain) of airspeed every time the wind component changes. While shear is often thought of as a horizontal wind effect, vertical wind components are involved as well. Aside from the ’downward’ effect of a downdraft (’microburst,’ ’downburst’), as it nears the ground, it spreads out horizontally, affecting airspeed, thus lift. The manual also provided the following
The pilot’s inability to maintain control of the airplane when it encountered a microburst during a landing attempt and a go-around near known thunderstorm activity. Contributing to the accident was an inadequate amount of wind sensors and wind shear detection equipment used by the ATC tower to detect microburst activity at the airport.
Source: NTSB Aviation Accident Database
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