What happened
On 3 March 2009, a Dornier 328-300, registration G-CJAB, was performing a non-revenue ferry flight from Biggin Hill to Southampton. The aircraft was being moved following an extended period of inactivity in a hangar. While cruising at 8,000 feet in instrument meteorological conditions (IMC), the crew experienced a rapid series of electronic failures.
Over a period of approximately 15 minutes, the cockpit instrumentation failed sequentially. The process began with the failure of the first Multi-Function Display (MFD), followed shortly by the second MFD. As the aircraft intercepted the ILS localiser for the approach to Runway 2-0, the Primary Flight Display (PFD) 1 failed. Shortly thereafter, PFD 2 also ceased functioning. Finally, during the short final phase of the landing, the Engine Indication and Crew Alerting System (EICAS) screen failed. The crew managed the situation by following abnormal procedures and completed the landing safely using standby instruments.
The investigation
The investigation focused on the technical cause of the simultaneous failure of five separate display units. Engineers examined the failed components and found that all five units were of the DU-870 type and had not been modified with recent service bulletins designed to improve high-temperature tolerance.
Detailed analysis by the manufacturer revealed that the High Voltage Power Supply (HVPS) unit had failed in every instance. Specifically, the transformer within the HVPS had suffered a failure in the epoxy encapsulation. This failure caused the potting around the secondary winding to break down, leading to a short circuit. While the aircraft's cooling system was inspected and found to be functional, the investigation noted that the display units are cooled by ambient cockpit air, which can run significantly hotter than the intake air.
Findings
- The primary cause of the failures was the breakdown of the transformer potting due to overheating.
- All five failed units were older versions that had not received recent modifications (J, K, or L standards) intended to provide better temperature resistance.
- The aircraft had recently undergone a period of extended storage without flying, and the units had not been returned for overhaul since their original installation.
- While the cockpit temperature did not exceed the maximum recommended 40°C, the internal temperature of the display units can be approximately 15°C higher than the ambient air.