BURKHART GROB G103C TWIN III ACRO near Gregory, MI — 2009-06-14

Probable cause

The pilot's failure to maintain sufficient airspeed during the turn to return for landing, which resulted an inadvertent stall/spin. Contributing to the accident was the failure of the ground winch tow cable shortly after takeoff.

Contributing factors

Contributing to the accident was the failure of the ground winch tow cable shortly after takeoff.

NTSB narrative

The pilot reported that the takeoff and climb were normal during the ground launch of the glider until about 400 feet above ground level when the winch cable broke. The pilot stated that he lowered the nose and established a speed of at least 60 knots. He continued the upwind pattern until the glider was in a position to begin a right 180-degree turn in order to line up with the landing area. He stated: “I could feel in the controls that something was not right and the glider was not responding in the manner that I am accustomed. There were none of the signs of a stall and the glider did not behave like it has any time that I have practiced stalls. Before I could determine the cause or take any action, the nose abruptly dropped and we dove toward the intended runway.” The glider impacted the ground nose down and in a right bank. A postaccident inspection confirmed flight control continuity from each control surface to the aft cockpit control stick with two exceptions. The inspection noted two separation points in the flight control push-pull tubes at the aft fuselage and empennage. Both were consistent in location and appearance with overload failures due to impact forces. No anomalies consistent with a pre-impact failure or malfunction were observed. Metallurgical examination of the winch cable revealed features consistent with overstress fracture.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Pilot could have aborted the flight after the winch cable broke.

NTSB coding

• Aerodynamic stall/spin · Maneuvering
• Maneuvering Stall

Evidence available

• 7 docket documents

Docket documents7

• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• Pilot Statementinterview
• Witness Statementsinterview
• Accident Site Diagramother
• Conversation Record -- FAA Inspectorother
• Glider Maintenance Records -- Excerptsother
• Materials Laboratory 15 - Factual Report 09-097report

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