What happened
On August 25, 2012, a Grob Aircraft Club Astir II, registration LN-GHO, was performing a glider tow at Elverum Airport Starmoen. Shortly after takeoff from runway 15, the pilot noticed the canopy beginning to open. Due to the rear-hinged design of the canopy, aerodynamic pressure forced the structure backward, tearing it from its hinges and causing both the frame and plexiglass to fall onto the grass strip.
The sudden distraction caused the pilot to inadvertently pull back on the control stick, resulting in a steep climb. This maneuver caused the tow plane to fall below the glider, creating slack in the towline. Fearing that a sudden tensioning of the line could pull both aircraft into the nearby trees, the pilot chose to release the towline at an altitude of approximately 25 to 30 meters.
Following the release, the pilot attempted a right-hand turn to return to the grass strip. However, due to the extremely low altitude, the pilot realized a return to the runway was impossible and instead targeted an opening in the terrain. The glider landed on a golf course, where the right wing struck a tree and the aircraft eventually came to a halt after the left wing collided with a sign. The pilot escaped the wreckage without injury.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined the canopy's locking mechanism, which utilizes two spring-loaded bolts. Although the bolts were found in the locked position after the accident, investigators considered that the canopy might not have been fully seated in the cockpit frame before the bolts were engaged. This could have allowed the front of the canopy to sit slightly higher, potentially allowing aerodynamic forces to pull it out of position during flight.
Findings
- The canopy closure and locking were likely not performed correctly according to the intended safety procedures.
- The locking system lacks an integrated warning mechanism to alert the pilot of an improper lock.
- The pilot's attempt to return to the runway at a very low altitude carried significant risk, as the lack of altitude left almost no margin for maneuvering and could have led to a loss of control.
Safety action
- Pilots must ensure that all checklist items are performed according to the intent of the procedure, specifically verifying the visual engagement of canopy locking arms.
- Extra attention must be paid to the canopy locking mechanism during pre-flight inspections to ensure it is not susceptible to aerodynamic release.