What happened
On 10 September 2000, a Piper PA-25-235 Pawnee, registration G-ASLK, was completing its fifth glider towing mission of the day at Nympsfield Airfield. During the landing approach, the pilot chose to land further down the westerly grass runway to ensure the tow rope remained clear of cattle grazing near the runway threshold.
The aircraft touched down on a slight downslope at a speed of approximately 30 mph. As the tail settled, the pilot applied the brakes; however, the left brake suddenly failed without warning. This caused the aircraft to yaw between 30° and 40° to the right. Despite the pilot applying full left rudder and attempting to use both brakes, the continued operation of only the right brake caused the aircraft to slew further right and slide across the airfield. The aircraft eventually crashed through a low wooden fence and stopped near a car park, resulting in minor damage to the lower rudder.
The investigation
Investigators determined that the left brake pedal had become ineffective due to a total loss of hydraulic pressure. Interestingly, the pressure appeared to return to the system following repeated applications of the pedal.
The investigation revealed that the aircraft had a documented history of brake fluid seepage within the left brake system. These leaks had been noted in the aircraft's Daily Inspection logs on multiple occasions. In response to these reports, the hydraulic reservoir had been topped up with fluid to keep the system operational until the next scheduled 150-hour maintenance check.
During the 150-hour inspection, the pilot raised concerns regarding the brake defect. However, the pilot was informed that repairs would be deferred until the annual inspection, which was not scheduled until December 2000. The specific individual responsible for this decision could not be identified.
Findings
- The primary cause of the excursion was the sudden failure of the left brake due to a lack of hydraulic pressure.
- The aircraft had a known, ongoing issue with brake fluid seepage in the left brake system.
- Maintenance of the brake system had been deferred, with the fluid reservoir being topped up rather than the leak being repaired.
- The pilot noted that the absence of rudder-to-tailwheel interconnecting springs may have limited the ability to steer the aircraft during the initial swing.