What happened
During the initial stages of a scheduled flight from Calgary International Airport to Los Angeles, an Alaska Airlines Boeing 737-900, registration N317AS, experienced an abnormal engine start. While the left engine started normally, the right engine discharged significant flames and smoke from the tailpipe. This phenomenon, known as engine torching, resulted in smoke entering the rear of the passenger cabin.
As the aircraft was being pushed back from gate 2 and the crew began the start sequence, the flight crew was alerted to the fire by a nearby aircraft's crew via radio. Although the pilots could not clearly see the flames from the cockpit, reports from the pushback operator and cabin crew confirmed the presence of fire and smoke on the right side of the aircraft.
Following the assessment of the situation, the captain ordered an emergency evacuation using the left-side doors. All 113 passengers were successfully evacuated via emergency slides without injury. Aircraft rescue and firefighting services arrived during the evacuation process.
The investigation
Investigators examined the engine components and the cockpit instrumentation. The investigation focused on the fuel flow mechanics and the communication between the flight deck and cabin crew.
Technical analysis revealed that the engine's electro-hydraulic servo valve (EHSV) contained excessive solder on a jet pipe nozzle. This manufacturing defect reduced the clearance area, allowing small particles to cause the nozzle to bind. This binding forced the valve into a position that commanded a fuel flow ten times higher than normal parameters. Because the excess fuel ignited outside the area monitored by the Exhaust Gas Temperature (EGT) probes, the cockpit fire warning system was not triggered.
Findings
- Excessive solder on a jet pipe nozzle within an overhauled EHSV caused particle-induced binding, leading to excessive fuel flow.
- The high fuel flow resulted in unburned fuel igniting as it exited the tailpipe, creating the torching effect.
- The manufacturer's quality assurance process failed to detect the solder anomaly during overhaul.
- The flight crew's ability to detect the abnormal flow may have been hindered by the digital and analog hybrid format of the fuel flow display.
- The cockpit door remained closed during the event, which limited direct communication and prevented the pilots from visually assessing the smoke in the cabin.
- During the evacuation, the left engine had not yet been shut down according to the required checklist, causing the deployed slide to momentarily flap due to engine airflow.