N167TM

HISTORY OF FLIGHTOn July 11, 2020, at 1500 Pacific daylight time, an Alexander Schleicher ASW 27 18 (or ASG 29) glider, N167TM, was destroyed when it was involved in an accident near Ely, Nevada. The pilot was fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

According to witnesses, the accident pilot was an experienced glider pilot who was part of a group that annually flew out of Ely due to its thermal soaring weather. A friend reported that the glider pilot had likely planned to establish new records in speed and/or distance when the accident occurred.

Data obtained from an onboard air data computer showed that the glider was released about 1124 on a southeast heading. The glider then turned to the north and began a series of right (clockwise) climbing turns. During the climb, the glider’s position gradually shifted northeast about 2.5 nm. At 1141, after having changed course to counterclockwise climbing turns, the glider made a left turn to the east and began a descent toward the Schell Creek Mountain Range and the location of the accident site. As it approached a group of mountains at an approximate field elevation of 8,500 ft msl, the glider made a slight left turn followed by a right turn toward the south. At 1144:25, the glider made its final right turn and climbed from about 9,400 ft msl to about 9,700 ft msl, which was followed by a steady descent and rapidly progressed into a steep descent. The final data point was captured at 1144:52 and showed the glider at an approximate altitude of 9,167 ft msl and about 900 ft north of the accident site.

Figure 1: Flight track from recovered data

AIRCRAFT INFORMATIONThe ASW 27 18 (also referred to as “ASG 29” in the flight manual) is a single-seat glider that can be configured with 15-meter or 18-meter-long wingspans, depending on which outboard wing panel is installed. The accident glider was being flown in the 15-meter-long wing configuration on the day of the accident.

METEOROLOGICAL INFORMATIONAt 1453, ELY weather reporting facility, located about 4.5 nm west of the accident site, reported wind 210° at 14 knots (kts), with gusts to 27 kts, visibility unrestricted at 10 miles or more, sky clear below 12,000 ft agl, temperature 33° C, dew point 1° C, and an altimeter setting of 30.25 inches of mercury. Calculated density altitude was 9,344 ft, with a relative humidity of 12%.

At 1508, a peak wind was reported from 220° at 31 knots.

A High Resolution Rapid Refresh (HRRR) numerical model was run for the accident coordinates for 1500 PDT. The sounding identified several low-level features, the stability, and low-level wind field over the area. The sounding depicted an elevation of 8,284 ft with a near temperature of 28° C, a dew point of -2° C, a relative humidity of 12%, and a calculated density altitude of 11,069 ft. The sounding depicted a dry low-level environment with the lifted condensation level and level of free convection (LFC) at 12,172 ft agl. The conditions were conducive to strong thermals from the surface to 21,000 ft with the atmosphere characterized as unstable below 5,000 ft agl. The maximum vertical velocity of potential convective updrafts was calculated at 24 kts.

A study of the potential of microbursts indicated outflow winds of 41 kts and the T2 Gust or outflow winds of potential thunderstorms was calculated at 72 kts.

The HRRR wind profile indicated a surface wind from 240° at 18 kts, with little variation in direction or wind speed through 15,000 ft. HRRR sounding indicated a predominate mountain wave near 15,000 ft with a maximum vertical velocity of 513 fpm.

The winds and temperature aloft forecast for Ely at 9,000 ft msl was for winds from 240° at 16 kts with temperature of 24° C, and at 12,000 ft msl at 17 kts with a temperature of 15° C. The Area Forecast Discussion (AFD) synopsis indicated a tightening of surface pressure gradient across the western portion of the area, expected to increase surface winds with gusts from 20 kts and to 30 kts in higher elevation areas.

According to wind data retrieved from the onboard air data computer, the wind was from the south for most of the accident flight at a magnitude of between 10 and 20 kts.

Few pilots chose to fly on the day of the accident on account of the wind conditions. Witnesses and other pilots who interacted with the pilot or flew that day noted that the wind conditions were strong, and the air was turbulent. A tow pilot noted the presence of strong thermals that were mixing with winds near the mountain where the accident occurred. The tow pilot also recalled that the buoyancy/shear ratio (B/S ratio), which is used to determine the usability of thermals, was low on the day of the accident. A low B/S ratio indicates that thermals are likely to be broken to unusable.

The GOES-17 visible satellite imagery surrounding the period depicted scattered to broken alto-cumulus clouds over the area during the period, which casted shadows on the ground below. The clouds were moving to the northeast and dissipating with time. A review of the NWS weather radar network indicated that the Ely, Nevada, area was located in a gap of radar coverage with coverage below 10,000 ft, to determine if precipitation was associated with the observed clouds over the region.

AIRPORT INFORMATIONThe ASW 27 18 (also referred to as “ASG 29” in the flight manual) is a single-seat glider that can be configured with 15-meter or 18-meter-long wingspans, depending on which outboard wing panel is installed. The accident glider was being flown in the 15-meter-long wing configuration on the day of the accident.

WRECKAGE AND IMPACT INFORMATIONThe accident site was located about 4.5 nm east of ELY at an elevation of 8,750 ft mean sea level. Photographs of the accident site provided by a witness showed a long and fragmented debris field. Multiple fragments from the left wing were collocated with the area noted as the first point of impact. The fuselage and empennage came to rest about 100 ft forward of the initial impact point and the right wing and most of the left wing came to rest about 60 ft forward of the fuselage/empennage.

A postaccident examination of the wreckage was completed after the wreckage was recovered to a secure facility. The outboard right wing had separated from the inboard wing about midspan along the top of the spar. The right wing exhibited some fractures but was otherwise intact as compared to the left wing, which retained most of its top skin, broken at 45° angles. The left while the lower skin had separated into multiple pieces and both the outboard wing panel and left-wing spar had fractured at the wing panel installation point.

The flight controls, comprised of the aileron, elevator and rudder, were traced from the cockpit to their respective control surfaces. A piece of intermediate control tube to the elevator control was not recovered; however, the recovered control tubes exhibited evidence consistent with overstress. The elevator trim was intact and unremarkable. The right- and left-wing airbrake systems were traced from the wings to the cockpit through separations that were consistent with overstress and both systems functioned normally at each wing.

The glider was equipped with a water ballast system comprised of a total of four ballast tanks: a fuselage tank, a tail tank, and a tank at each wing.

The water ballast control levers remained attached to the cockpit with their Bowden cables attached. Both the right and left water ballast tank Bowden cables had separated at the cockpit. Both the left- and right-wing Bowden cables had separated from their swaged tubes, which were both still connected to their plates. The tail water ballast control cable was traced from the cockpit lever to the tail water ballast tank through cable separations.

Both the tail and fuselage water ballast control systems were fractured in tension and the fuselage tank had broken but was otherwise unremarkable. The right-wing water ballast control system was traced from the cockpit to the right-wing water tank, which functioned normally when tested. Both the left- and right-wing water ballast control cables were continuous from the control lever at the cockpit to separations at the left and right swedged tubes. The left-wing water ballast tank valve was separated from the left wing during the accident sequence. The valve rod separated from the valve lever, which was intact.

ADDITIONAL INFORMATIONFlap Setting

According to the flap setting chart in the flight manual, for his weight at the time of the accident, in straight flight the flap setting should have been set at position 1 for speeds over 98 kts.

Stall Speeds

According to the flight manual, the stall speed for the glider at a gross weight of 1,322 lbs and flap position 1 was 52 kts. The stall speeds in circling flight are increased due to higher load factors. The flight manual showed that the stall speed would have increased by 107% in a 30° turn, 119% in a 45° turn, 141% in a 60° turn, and 200% in a 75° turn.

The FAA Glider Flying Handbook (FAA-H-8083-13A), published in 2013, discussed the handling characteristics of a glider carrying large amounts of water ballast. According to an excerpt, water ballast increases stall speed and reduces aileron response during free flight making quick banking maneuvers difficult or impossible to perform.

The flight manual contained procedures for recovery from a stall, spin or spiral dive, but did not suggest a minimum altitude for recovering from either of these conditions.

Microbursts

A microburst is defined as a powerful localized downdraft created by a column of sinking air through the base of a cumulus or cumulonimbus cloud that produces precipitation, with a downdraft core of 6,000 to 7,000 feet per minute, and usually impacts an area less than 2.5 miles with a wind differential of 50 knots or more. This meteorological...