What happened
On April 26, 2008, at 07:35 UTC, a Beechcraft 76 Duchess, registration CS-AVL, operated by GESTAIR Flying Academy, was taxiing toward the holding position for runway 35 at Cascais Municipal Aerodrome (LPCS). During the taxi maneuver, the pilot flying experienced a sudden loss of braking effectiveness through the rudder pedals.
Upon realizing the brakes had failed, the pilot immediately notified the second crew member, who took control of the aircraft. Without braking capability, the aircraft began to veer to the right, partially exiting the taxiway. As the nose wheel and the right main gear moved onto the sloped terrain adjacent to the taxiway, the propeller blades of the number two engine struck the ground. The incident resulted in damage to the propeller blades of the second engine, though the two crew members remained uninjured.
The investigation
The GPIAAF investigation focused on the mechanical state of the braking system and the operational procedures in place. Investigators identified hydraulic oil residue on the ground near the left main landing gear. Upon inspection of the aircraft, visible corrosion was found at various points within the braking system.
Technical analysis revealed that the hydraulic leak originated from the seal of the left main wheel. The investigation also noted that the aircraft had flown only 3.5 hours since its last inspection, during which no leaks had been detected, suggesting the failure occurred during the taxiing phase of this specific flight. Furthermore, the investigation reviewed the aircraft's flight manual and checklists, noting that the current procedures did not require a hydraulic fluid level check during pre-flight inspections, nor did they include a brake test during the taxi checklist or emergency procedures for brake failure.
Findings
- The primary cause of the brake failure was a hydraulic oil leak from the left main wheel seal.
- The loss of hydraulic pressure rendered the differential braking method—which allows either pilot to steer the aircraft on the ground—ineffective.
- The aircraft's braking system design lacks alternative braking options for scenarios involving leaks in common hydraulic lines, preventing the crew from maintaining directional control via the parking brake or the second pilot.
- Visible corrosion was present in several areas of the braking system.