What happened
On the morning of 3 June 1999, a B737-448, registration EI-BXI, was at Stand 34 at Dublin Airport preparing for a scheduled public transport flight to Amsterdam. During pre-flight procedures, the flight crew encountered difficulties pressurizing the hydraulic "B" system electric pump. While maintenance personnel were troubleshooting the fault and resetting a circuit breaker, the pump was activated, resulting in a visible cloud of smoke and vapor exiting the cockpit window.
As the aircraft was in the process of boarding, between 114 and 119 passengers had already taken their seats. The air traffic controller observed smoke emanating from the front of the aircraft and alerted the fire services. Noticing heavy fumes in the cabin, the captain ordered an immediate evacuation. Passengers exited through the forward door via the airbridge, leaving their hand luggage behind. The evacuation was completed in an orderly fashion with no injuries reported.
The investigation
Technical examinations following the incident revealed that the metal braid of a hydraulic flexible pipe had caused the insulation on a phase "C" electrical supply wire to chafe away. This exposure allowed the voltage to arc to the metal braid, which simultaneously punctured the hydraulic pipe. The resulting high-pressure escape of hydraulic fluid created a spray that vaporized, leading to a brief flash of flame and significant smoke.
Investigators also looked into the maintenance history of the component. While the manufacturer had previously issued service letters regarding the positioning of the acoustic filter elbow to prevent such chafing, evidence suggested that the specific requirement to "clock" the fitting at a precise angle had not been strictly followed during a previous pump installation.