What happened
On March 19, 2016, a Czech Sport Aircraft PS28, registration F-HSAB, was conducting a flight instruction mission near the Muret airfield in France. While flying south of the aerodrome, the student pilot noticed a draft of air entering the cockpit. Upon inspection, the instructor discovered that the rear left locking mechanism of the canopy was not properly engaged.
To rectify the issue, the instructor took control of the aircraft and instructed the student to unlock the canopy to allow for a proper re-engagement of the lock. During this procedure, the aerodynamic forces caused the canopy to lift abruptly, allowing high-speed airflow to enter the cabin. This sudden change in aerodynamics caused the aircraft to pitch down sharply. The instructor managed to reduce power and stabilize the aircraft in level flight, and the occupants were able to close the canopy.
Following the event, the instructor noticed vibrations in the control column and observed that a jacket, which had been resting behind the seat, had become caught on the edge of the right horizontal stabilizer. The jacket eventually detached and fell to the ground. Upon returning to Muret, the crew discovered minor damage to the aircraft, specifically a dent on the leading edge of the right fixed stabilizer.
The investigation
The investigation focused on the canopy locking mechanism and the cockpit warning systems. The Czech Sport Aircraft PS28 is equipped with a canopy that pivots forward, secured by two locks connected to the fuselage. While the aircraft features a switch to detect the position of the locking lever, the investigation established that this system only monitors the lever position and does not detect whether the actual locks are engaged in their housings.
Investigators found evidence of friction near the left lock housing, suggesting the lock was not consistently aligned with its receptacle. Furthermore, the club operating the aircraft noted that the canopy could sometimes rest against the protruding lock even when the lever was in the open position, creating a gap between the canopy and the fuselage.
Findings
- The primary cause of the incident was the abrupt opening of the canopy due to aerodynamic forces acting on the partially unlocked structure during an attempt to correct a locking error.
- The sudden influx of air into the cockpit caused a significant aerodynamic disturbance, which likely reduced elevator effectiveness and altered the aircraft's longitudinal stability, leading to a momentary loss of control.
- The instructor's belief that the forward-opening design of the canopy would keep it closed against the relative wind contributed to the decision to attempt a manual re-locking in flight.
- The aircraft's warning system was unable to alert the crew to the improper engagement of the locks, as it only monitored the lever position rather than the physical state of the locking pins.