What happened
On 28 August 1999, an Aero Vodochody L39C, registration ZU-PRG, was performing a simulated aerial display at Matsapha Airport in Swaziland. The flight, involving a pilot and a co-owner, began with a takeoff from Runway 25. After climbing to approximately 4,000 feet, the pilot executed a dive toward the crowd line, reaching speeds of about 340 knots.
During this high-speed maneuver, the pilot heard a loud noise and a sudden increase in engine volume. The occupant of the rear cockpit notified the pilot via intercom that the rear canopy had flown open. In an attempt to manage the emergency, the pilot reduced power and prepared for an immediate landing. However, as the aircraft slowed to 144 knots, the pilot attempted to apply power only to find that the engine failed to respond. The pilot then executed a forced landing in an open field near the runway threshold. The aircraft struck the ground heavily, causing the undercarriage to collapse and the airframe to be destroyed. Both occupants survived with back injuries.
The investigation
SACAA AIID investigators examined the wreckage and the aircraft's maintenance history. The investigation focused on the canopy latching mechanism and the engine's failure. Physical evidence showed that fragments of the rear canopy had passed through the engine intake, and turbine blade fragments were recovered from behind the wreckage.
Analysis of the cockpit latches revealed that while the locking system was functional, the left-hand canopy pins had not been properly engaged in the latch claws. Instead, the pins were resting on top of the claws. The investigation determined that while the canopy appeared closed, the aerodynamic forces generated during the high-speed dive created enough lift to pull the unsecured canopy adrift. Furthermore, the investigation noted that the aircraft's maintenance logs used the term "Mandatory Periodic Inspection" instead of the standard "yearly inspection" required for its category.
Findings
- The rear cockpit canopy was not properly closed, allowing it to come adrift during the flight.
- Debris from the canopy entered the engine intake, leading to total engine failure.
- The aircraft was operating in the experimental category, which the investigator noted was potentially inappropriate for this type of ex-military aircraft.
- The canopy warning light failed to alert the pilot to the improper latching because the micro-switch was triggered by the closing handle rather than the actual position of the canopy.
Safety action
- It is recommended that the canopy micro-switch be relocated so that it is activated by the canopy pressing against its striker, rather than by the handle system, to ensure the canopy is truly secure.
- The classification and airworthiness requirements for ex-military aircraft should be reviewed to distinguish them from general experimental/homebuilt aircraft.
- Operational and training requirements for flying ex-military aircraft should be reassessed.