WPR09FA409

The pilot's failure to maintain adequate airspeed while maneuvering in terrain-induced turbulent wind conditions, resulting in an inadvertent entry into a stall/spin.

The pilot departed in his glider, as a group of three, with the intention of soaring in the area. During the flight the pilot reported via radio that he was unable to find favorable soaring conditions. He did not return to the airport at the predetermined time, and the wreckage was located on the steep slope of a summit the following day. Flight data recovered from the pilot's handheld GPS unit revealed a track that was consistent with ridge and thermal soaring. For about the first hour of flight the glider maintained ground clearances of between 1,600 and 5,000 feet. For the last 16 minutes of flight the glider continued ridge soaring with reduced ground clearances of between 250 and 1,400 feet. The glider then crested a peak and returned to a valley area performing a figure eight turn. During the last turn the recorded ground speed fell below the stall speed of the glider. Analysis of the weather at the accident site indicated the presence of turbulence and downdrafts, in addition to wind directions that would have indicated an airspeed that was even lower than the ground speed recorded by the GPS. Glider instrumentation, wreckage, and ground scars were consistent with a stall/spin event. The engine did not appear to be operating at the time of the accident and the damage to the propeller was consistent with it being stowed. The glider was having intermittent problems with its airspeed indicator a few days prior to the flight; however, impact damage prevented an assessment of the system's status. Such a failure in flight could have hindered the pilot’s ability judge stall speed. All major sections of the glider were accounted for at the accident site, and post accident examination of the engine and airframe revealed no anomalies that would have prevented normal operation.

**This report was modified on 5/3/2010** HISTORY OF FLIGHT On August 19, 2009, at 1702 mountain daylight time, a Stemme GMBH & CO S10-VT, N35MP, collided with mountainous terrain near Mackay, Idaho. The pilot was operating the glider under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The certificated commercial pilot, who held private pilot privileges in gliders, was killed, and the motor glider was substantially damaged by impact forces. The local personal flight departed Arco-Butte County Airport, Arco, Idaho, about 1525. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot departed Arco with two other gliders, with the intention of soaring in the area. The group planned to return to Arco by 1800. The two other pilots in the group reported that they were in radio contact with the accident pilot until about 1630, at which time he reported that he was close to the Mackay Reservoir, and was having difficulty gaining sufficient lift to climb out of the area. The pilots attempted to establish radio communications about 45 minutes later, but were unable to contact him. They stated that this was not unusual, and they were not concerned due to the limited range of their radio transceivers. The remaining members of the group decided to return to Arco about 1715 due to a lack of sufficient lift in the area. One pilot reported that soaring conditions were, "mediocre," the weather was "docile." He additionally stated that they had been warned by other pilots of the possibility of turbulence in the area. The accident pilot did not return by the planned regroup time; by 1915 the group became concerned, and contacted the Arco airport manager to report the pilot's late arrival. Local law enforcement and the Civil Air Patrol initiated a search, and the glider was located the following afternoon on the face of White Knob Summit, 6 miles southwest of the Mackay Reservoir. An impact damaged Garmin Rino 120 handheld radio transceiver/global positioning systems receiver (GPS) was recovered from the glider. The unit was sent to the National Transportation Safety Board Office of Research and Engineering for data extraction. The data revealed the entire flight sequence beginning at Arco, and continuing on an arching northwesterly track towards the accident location. The data showed that the glider departed Arco and began a series of 10 left climbing turns over Butte City to a GPS altitude of 9,888 feet. Over the course of the next 3 1/2 minutes, the glider maintained a course of about 20 degrees magnetic for 4 miles towards the southern peaks of the Lost River Range. By the time it had reached the peaks, the glider's altitude had dropped to 9,203 feet. Over the next 8 1/2 minutes it began a series of 12 climbing left turns until it reached an altitude of 13,242 feet. For about the next hour the glider tracked northwest, following the peaks of the range, and continuing a cycle of five groups of climbing turns, followed by meandering descents. Throughout the cycles the maximum altitude attained was 14,951 feet. During this portion of the flight the glider maintained clearance from the ground of between 1,600 and 5,000 feet. At an altitude of 14,182 feet the glider changed heading towards the southwest, descending and flying over Mackay Airport (U62). Ten minutes later, at an altitude of 11,539 feet, the glider cleared the peak of a ridge line adjacent to White Knob Summit by about 600 feet, and changed heading to the northwest. For the remaining 6 minutes the glider continued to track a descending northern path on the west side of the peaks of the mountain ridges. During this portion of the flight the glider maintained a terrain clearance of between 250 and 1,400 feet agl. The glider then turned to the east and entered a valley area, clearing the terrain by about 270 feet (10,501 GPS). The glider then began a left turn; about 270 degrees into the turn the glider had descended about 70 feet and changed course beginning a descending turn to the right. The glider continued the turn with the last recorded altitude at 10,326 feet on a heading of 101 degrees true. The wreckage was located in line with the final course, about 400 feet east of the last recorded track point, at an elevation of 9,730 feet msl. The ground speeds throughout the majority of the flight varied between 158 and 46 mph. The last 8 ground speeds, recorded over a period of about 25 seconds, varied between 59 and 62 mph, with the two final speeds indicating 56 and 44 mph consecutively. PERSONNEL INFORMATION The 78 year-old-pilot held a commercial pilot certificate with ratings for rotorcraft-helicopter, and instrument helicopter. He additionally held private privileges for airplane single-engine land, airplane single-engine sea, airplane multiengine land, instrument airplane, and glider. The pilot held a second-class medical certificate issued on July 1, 2009. It had limitations that the pilot must wear corrective lenses and possess glasses for near/intermediate vision. Recent pilot flight records were not available. The pilot reported on his most recent medical certificate application in March 20, 2009, that he had amassed a total of 20 flight hours during the 6 months prior to the application. The pilot was involved in an airplane accident on January 3, 2009 (NTSB accident number WPR09LA071), for which he submitted an NTSB Pilot/Operator Aircraft Accident Report. In the report he stated a total flight time for all aircraft of 11,000+ hours, 1,200 of which were in gliders. The NTSB determined the probable cause of this accident to be, ‘A loss of engine power due to fuel exhaustion as a result of the pilot’s inadequate preflight planning.’ In 2002/2003 the pilot flew the accident glider on a long distance expedition from Point Barrow, Alaska, to Cape Horn, Chile. GLIDER INFORMATION The dual seat, retractable gear motor glider was manufactured by Stemme AG (Germany) in 1998. Its wingspan was 75 feet 5 1/2 inches, and its maximum takeoff and landing weight was 1,874 pounds. A turbocharged, four-stroke, four cylinder Rotax 914 engine was mounted within the airframe aft of the main cabin, and powered a retractable, nose mounted composite propeller. The pilot purchased the glider in November 2001. The engine and airframe logbooks indicated that at the time of the last annual inspection, dated March 10, 2009, the glider had accumulated a total flight time of 428.2 hours. The March 2008 annual inspection entry reported a total glider time of 412 hours. A friend of the pilot stated that 2 days prior to the accident, the pilot reported that the airspeed indicator had failed. The pilot brought this to the attention of an A&P mechanic, but before the mechanic began troubleshooting, the airspeed indicator began to work again. The friend was not aware of the pilot making any further attempts to resolve the anomaly prior to the flight. Published documentation from the glider's manufacturer stated that the propeller blades extend into the airstream automatically by centrifugal force only when the engine is started. Additionally, the documentation details a 10-step checklist of items that need to be configured prior to changing from glider to powered flight. According to the flight manual applicable to the accident glider, the stalling speed with the flaps in the retracted position is 56 mph. The stalling speed in the flaps extended landing configuration is 53 mph. FLIGHT RECORDERS The glider was equipped with a Cambridge Aero Instruments GPS-NAV 25 Secure Flight Data Recorder, and a Magellan Blazer 12 handheld GPS. Both units were sent to the National Transportation Safety Board Office of Research and Engineering for data extraction; however, examination of the unit revealed that impact damage had destroyed all recorded flight information. WRECKAGE AND IMPACT INFORMATION The glide