What happened
On 22 March 2021, a Cargolux Airlines Boeing 747-8F, registration LX-VCD, departed Al Maktoum International Airport in Dubai for Luxembourg. While cruising at flight level 280, the flight crew observed electrical sparks and smoke emanating from the right-side windshield heating terminal block.
The crew declared a PAN PAN emergency and requested to return to Al Maktoum International Airport. To manage the situation, the commander deactivated the electrical supply to the right window heater, which caused the sparks to cease and the smoke to dissipate. The aircraft performed a fuel jettison to reach an appropriate landing weight and returned to the airport, landing without further incident.
The investigation
The investigation focused on the failure of the windshield anti-ice heating system. Technical records showed no prior defects for this aircraft, which had recently undergone scheduled maintenance. Because the flight data recorder (FDR) did not monitor the specific electrical power switch position for the window heaters, investigators relied on physical examinations and laboratory testing.
The right R1 windshield was removed and sent to a Boeing facility in Seattle for an intensive teardown. Using X-ray digital radiography and CT scans, engineers examined the terminal block assemblies. The investigation also reviewed historical data regarding similar incidents involving Boeing aircraft windshield heating terminals.
Findings
Investigators determined that the primary cause of the smoke and sparks was a defective windshield anti-ice heating electrical pin/socket terminal block. Specifically, the failure originated from the integrated power braid wire crimped at the base of the connector pin. Fractures in the wire strands at this crimp point led to increased electrical resistance and heat buildup, which triggered the arcing event.
Contributing factors included:
- Inadequate manufacturer actions to eliminate manufacturing defects in electrical blocks during the assembly process.
- Existing FAA airworthiness directives and Boeing service bulletins were not effective in detecting these specific weak or poor thermal terminal connections before an incident occurred.