Stemme GMBH & CO S10-VT accident near Mackay, Idaho, August 19, 2009
On August 19, 2009 at about 11:02 pm local time, a Stemme GMBH & CO S10-VT (glider), registered N35MP, was substantially damaged in an accident during maneuvering near Mackay, Idaho (Arco-Butte airport). It was a personal flight under general aviation rules (Part 91). 1 person was killed. The weather was visual conditions (good weather).
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The NTSB's probable cause their words, unchanged
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.
Source: NTSB aviation accident database, copy made October 4, 2026. Docket and reports at the NTSB.
What the record shows
- Date
- August 19, 2009 · about 11:02 pm local time
- Place
- Mackay, Idaho · Arco-Butte · map
- Type
- Accident
- Injuries
- 1 person was killed.
- Weather
- visual conditions (good weather)
- Aircraft
- Stemme GMBH & CO S10-VT · all S10-VTs on the register
- Registration
- N35MP · no longer on the register · serial 11011
- Damage
- Substantial damage
- Flight
- Personal flight · general aviation rules (Part 91)
The NTSB's narrative final · quoted from the NTSB record
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.
The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.
The factual record from the NTSB's investigation tables, in plain English
What happened, in order
- Loss of control in flight during maneuvering defining event
- Aerodynamic stall/spin during maneuvering
- Collision with terrain or object (not controlled flight into terrain) during uncontrolled descent
The NTSB's findings
- cause Aircraft › Aircraft oper/perf/capability › Performance/control parameters › Airspeed › Not attained/maintained
- cause Personnel issues › Task performance › Use of equip/info › Aircraft control › Pilot
- Environmental issues › Conditions/weather/phenomena › Turbulence › Terrain induced turbulence › Effect on equipment
Pilot
- Certificate: commercial pilot, private
- Ratings: multi-engine land; single-engine land; single-engine sea; instrument: airplane; instrument: helicopter; rotorcraft: glider; rotorcraft: helicopter
- Flight time: 11,000 hours in all; 11,000 as pilot in command
- Last flight review: October 18, 2008
- Medical certificate: Class 2 (with waivers/limitations)
- Seat: unk
- Injury: fatal
The aircraft
- Airframe total time: 428 hours
- Last inspection: annual inspection, March 10, 2009
- Maximum gross weight: 1,874 lb
- Seats: 2
- Landing gear: retractable
- Engine: Rotax 914 (piston); 0 hours total
The flight
- Departed from: AOC Arco ID at 9:25 pm
- Destination: AOC Arco ID
- Flight plan: none
Weather at the time
- Light: daylight
- Wind: from 200° at 4 knots
- Visibility: 30 statute miles
- Sky: clear
- Temperature: 81°F (27°C), dew point 41°F (5°C)
- Altimeter: 30.06 inHg
- Observation at 10:45 pm from SUN, 42 miles away
Injuries
| Fatal | Serious | Minor | None | |
|---|---|---|---|---|
| Flight crew | 1 |
Documents from the investigation the NTSB's docket: the evidence folder behind the report
The docket, the folder of records gathered during an investigation (maintenance records, photographs, witness statements, examinations), has not been read from the NTSB for this case yet; the list appears here once it has. Open the docket at the NTSB.
About this page
Everything above comes from the NTSB's public accident database: the narrative, probable cause and findings are the NTSB's own words, and the coded tables behind its report (pilot, aircraft, flight, weather, injuries, sequence of events) are written out in plain English. Pilots' ages, home towns and medical details are left out on purpose. The documents and photographs are the NTSB's docket, the folder of records gathered during the investigation, shown as the NTSB released them (the NTSB redacts some personal details first) and listed under the NTSB's own titles. This site's own text never names pilots, passengers or anyone else involved. A preliminary report describes what is known soon after the event and can change; the final report, with the probable cause, usually follows one to two years later and this page is refreshed when it does. Case number WPR09FA409.