What happened
On September 20, 2007, a Sikorsky S-76A, registration PT-YAZ, was performing taxi procedures at Pampulha Airport in Belo Horizonte, Brazil. The aircraft, operated by Helisul Táxi Aéreo Ltda, was preparing for departure after the crew decided to move from the parking apron to the taxiway.
During the taxi, the crew detected abnormal noises and parameters in engine number 2, including a rapid temperature increase into the redline range. The pilot attempted to shut down engine 2, but simultaneously observed that engine 1 had also shut down without command. Following the detection of smoke, the crew evacuated the five passengers and two crew members. Shortly after, a fire erupted in the area between the main gear box and the engines. Although the crew attempted to use the onboard fire extinguishers, they were ineffective. Airport firefighters eventually extinguished the blaze.
The investigation
CENIPA's investigation focused on the mechanical failure of the electrical-hydraulic rotor brake system. Investigators found that the crew had not performed the required cockpit checklist items for this system, nor had they performed the exterior check to verify the spacing between the brake pads and the disc, partly because the necessary measurement tool was not on board.
Technical analysis revealed that the rotor brake pads had been in contact with the brake disc, causing friction that raised the disc temperature to over 700 °C. This extreme heat caused the hydraulic oil seals to rupture, releasing pressurized hydraulic fluid onto the overheated disc. The fluid reached its auto-ignition temperature, triggering the fire. The investigation also noted that the rotor brake warning light was significantly dimmed because only one of its two bulbs was functional, making the warning nearly imperceptible in the daylight cockpit conditions.
Findings
- Mechanical failure: Friction between the brake pads and the disc caused temperatures to exceed 700 °C, leading to the rupture of hydraulic seals and the ignition of hydraulic oil.
- System design: The electrical-hydraulic rotor brake system contained three simultaneous failure points that could disable both the warning light and the automatic pressure relief (auto-dump) function.
- Human factors: The crew failed to perform specific pre-flight and cockpit checklist items regarding the rotor brake system due to an incorrect judgment that such checks were unnecessary because the aircraft had stayed at the airport overnight.
- Supervision and awareness: The crew was unaware of a Sikorsky Safety Advisory issued months prior regarding the importance of these specific checks, and the aircraft was not equipped with the necessary tool for the exterior brake disc inspection.