What happened
On 4 November 2022, a Saab 340B, registration VH-ZRC, was preparing for a charter flight from Flinders Island Airport, Tasmania. During the take-off roll, the flight crew noticed the aircraft veering left of the runway centerline. As the aircraft accelerated, the crew detected a noticeable drop in acceleration rate. At approximately 96 knots, the crew decided to reject the take-off.
Following the aborted departure, the aircraft continued to veer, eventually pulling to the right. The crew noted a sensation of the brakes applying themselves and felt pressure in the brake pedals. An airport operations officer reported seeing smoke emanating from the aircraft's wheels. Upon inspection, the crew found that all four main landing gear tyres had deflated and one tyre exhibited a flat spot. There were no injuries to the 25 passengers or 3 crew members on board.
The investigation
The investigation focused on the mechanical state of the braking system and the sequence of events leading to the heat buildup. Investigators examined the parking brake mechanism, which is operated by a push-pull handle on the pilot's side panel. The system relies on an unrestricted hydraulic return line to relieve pressure when the brake is released.
Analysis of the runway surface revealed significant tyre marks. These marks transitioned from a leftward veer to a rightward movement, and eventually changed from solid marks to parallel lines, which indicated that the tyres had deflated due to the activation of fusible plugs. The investigation determined that the heat generated by the dragging brakes caused the fusible plugs to melt as a safety measure to prevent tyre explosions.
Findings
- The parking brake handle was likely not completely seated in the panel after being released by the pilot.
- This improper positioning caused a partial restriction of the hydraulic return line, leaving residual pressure in the brake system.
- This residual pressure caused the brakes to drag during the taxi phase, generating significant heat.
- The heat buildup led to a continuous increase in brake application during the take-off roll, eventually triggering the activation of the main landing gear fusible plugs and resulting in tyre deflation.