Flight Instructor’s Failure To Maintain Adequate Airspeed And His Exceedance Of The Glider’s Critical Angle Of Attack

Location: Gregory, Michigan Accident Number: CEN23LA254
Date & Time: June 21, 2023, 15:30 Local Registration: N10BR
Aircraft: LET L-23 Super Blanik Aircraft Damage: Substantial
Defining Event: Aerodynamic stall/spin Injuries: 1 Serious, 1 Minor
Flight Conducted Under: Part 91: General aviation - Instructional

Analysis: The student pilot reported that he was flying the glider until about 10 minutes before the accident, when the flight instructor took over control and started looking for thermals to gain altitude. He said the flight instructor was mostly quiet until he said they were going to perform an off-field landing. The glider flew across a tree line about 150 ft above ground level and turned to land in a bean field. Neither the student nor the flight instructor had any recollection of the accident.

Postaccident examination showed that the forward fuselage was crushed upward, and rearward and the outboard 6 ft of the left wing was separated. The separation point had signatures indicating upward bending of the tip. The damage to the glider was consistent with a nose-low, left-wing-low impact. Flight control continuity was verified from the cockpit to all control surfaces. No anomalies were detected.

Based on the available information, the glider likely had insufficient altitude to return to the departure airstrip and the flight instructor attempted an off-field landing. The nose-low, leftwing-low impact suggested that the instructor likely exceeded the glider’s critical angle of attack and encountered an aerodynamic stall at low altitude during the landing attempt.

Probable Cause and Findings: The National Transportation Safety Board determines the probable cause(s) of this accident to be -- The flight instructor’s failure to maintain adequate airspeed and his exceedance of the glider’s critical angle of attack, leading to an aerodynamic stall during the off-field landing.

FMI: www.ntsb.gov