What happened
On the night of 22 November 2007, a Sikorsky S-76B (registration G-DPJR) was conducting a non-revenue positioning flight from Denham to Coventry. While cruising approximately 30 miles from the destination, the flight crew noticed an unidentified unusual smell. In an attempt to troubleshoot, the pilots deactivated the aircraft's auxiliary heating system.
As the aircraft approached Coventry, the situation escalated. The crew observed smoke beginning to fill the cockpit, and the second pilot identified a significant hotspot developing near the rear left of his seat. Despite attempts to isolate the issue, the smoke levels continued to rise. Upon crossing the airfield boundary at Coventry, the pilot slowed the aircraft for an expedited landing, which caused the smoke and heat levels to increase further. The commander declared a MAYDAY, and the aircraft landed directly in front of standby fire services. The crew evacuated the aircraft immediately following shutdown.
The investigation
Investigators examined the cabin and found extensive heat damage to the plastic ducting and cabin trim near the left-side auxiliary heater element. While resistance checks of the heater element and thermistor showed no defects, the vent blower circuit breaker had tripped.
Upon disassembly of the electronic control box by the manufacturer, several defects were identified on the power supply circuit board. These defects were found to be capable of causing a total system failure and shutdown of the controller. The investigation also looked into the cabin Environmental Control System (ECS), which utilizes 1,550-watt heating elements and electric fans to distribute heat.
Findings
- The primary cause of the incident was component failures within the electronic control box for the cabin ECS.
- These failures caused the controller to shut down, which prevented the overheat protection and the cockpit controls from functioning.
- The loss of the vent blower circuit breaker reduced airflow through the ducting, which contributed to the severity of the overheating.
- Because the cockpit switches lost authority following the controller failure, the crew was unable to isolate power to the heating element.
- The reduction in ram air as the aircraft slowed for landing exacerbated the heat buildup in the ducting.