What happened
Two serious incidents occurred at Paris Charles de Gaulle Airport involving large widebody aircraft. On March 18, 2010, a Boeing 747-400, registration HS-TGL, operated by Thai Airways International, arrived from Bangkok. Upon arrival, the crew maintained engine number 4 to provide power until a Ground Power Unit (GPU) could be connected. Shortly after the GPU was connected, ground personnel noticed intense heat emanating from the fuselage near the GPU connectors. Upon inspecting the avionics bay, flames were discovered. Ground crews used a 50 kg dry powder extinguisher to suppress the fire, which had reached temperatures near 800 °C.
On April 8, 2010, a second Boeing 747-400, registration B-HOV, operated by Cathay Pacific Airways, arrived from Hong Kong. During the connection of a GPU, the ground agent experienced difficulty establishing electrical power. To force the connection, the agent manually held the power switch in the 'on' position, noting a significant drop in the GPU engine speed. Shortly thereafter, intense heat was detected, and flames were observed inside the avionics bay. An airline agent used a dry powder extinguisher to extinguish the fire. In both incidents, passengers were in the process of disembarking the aircraft.
The investigation
The BEA investigation examined the damage to both aircraft and the technical specifications of the GPUs used. In the case of HS-TCL, the electrical cables connecting the neutral to the airframe were heavily damaged and partially severed due to extreme heat, and the fuselage structure required major repairs. For B-HOV, the cables were partially melted, and the fuselage also sustained heat-related deformation.
Investigators analyzed the GPU connection hardware, specifically the composite guides used to align the parking plugs. Testing revealed that the original guides could allow for an incorrect alignment where the phase C connection was made directly to the neutral line. Furthermore, the investigation looked into the training of ground personnel, noting that those handling GPUs often have limited technical knowledge of aircraft electrical systems.
Findings
- The primary cause of both fires was a short circuit in the avionics bay caused by the incorrect alignment of the GPU parking plugs.
- On B-HOV, one plug was misaligned, resulting in phase C being connected directly to the aircraft's neutral line.
- On B-HOV, the ground agent's decision to manually hold the power switch in the 'on' position prolonged the electrical short circuit.
- The original composite guides used on these aircraft were susceptible to misalignment, a design issue known to Boeing since 2001.
- Ground service agents often receive limited training regarding the complexities of aircraft electrical circuits.
- The intense heat from the short circuits caused significant structural damage and the ignition of insulation blankets.