Incident
On an unspecified date, an Alaska Airlines Boeing 737-900 (registration N317AS, serial number 30856) was scheduled for its first flight of the day from Calgary International Airport, Alberta, to Los Angeles Airport, California. After pushback from gate 26, the left engine started normally. During the right engine start, the engine discharged a large quantity of flame and smoke from the tailpipe, with smoke eventually entering the aft cabin. The engines were shut down, and all 113 passengers were evacuated using the emergency slides on the two left-side main doors, away from the right engine tailpipe fire. There were no injuries to passengers or crew. Initial examination determined that the fire was contained within the engine flow path (CFM 56-7B26, serial number 890392). There was no damage to the engine or the aircraft structure.
Evacuation
The flame and smoke were not readily visible from the cockpit. At 0701:17 mountain standard time, 31 seconds after the right engine start was initiated, the crew of a nearby aircraft informed the pilots by radio of the engine torching. Eleven seconds later, the captain shut down the right engine; the left engine remained running. Both the pushback tug operator and the cabin crew reported smoke and fire on the right side via the aircraft interphone. At 0702:28, the captain informed the in-charge flight attendant that the pilots were aware and working on the problem. The flight attendant reported smoke in the aft cabin. Both air conditioning packs were off for the start, but the auxiliary power unit (APU) was operating, allowing smoke to enter the aft cabin via the APU intake and air conditioning system.
Passengers in the rear became anxious due to visible flame and smoke irritation. The in-charge flight attendant called the flight deck at 0702:56 to inquire about an evacuation. The captain deferred the decision. At 0703:01, the captain instructed the first officer to run the tailpipe fire checklist. At 0703:28, the in-charge flight attendant reaffirmed smoke and anxiety levels and queried again. The first officer released the cockpit door lock, but flight attendants did not hear the lock release, and the door remained closed. The captain requested aircraft rescue and firefighting (ARFF) response. At 0704:28, 3 minutes 11 seconds after the first warning call, the captain ordered an emergency evacuation using left doors only. The evacuation checklist required engine shutdown before opening doors. When the aft left door was opened, the checklist had not been completed to the point of left engine shutdown. The left engine was shut down 15 seconds after the evacuation order, and the deployed slide flapped off the ground momentarily in the coasting airflow from the spooling-down engine.
Evacuation was orderly, assisted by the pushback crew and two passengers. Passengers were escorted across the apron to a terminal holding area. The flight and cabin crews remained on board until all passengers evacuated and then deplaned via portable airstairs. A responding fire truck passed through a gap in the passenger line to access the right side; a construction area prevented access from the right. The evacuation was complete at 0708:00, 6 minutes 43 seconds after the first warning call.
Investigation
The Transportation Safety Board of Canada investigated. Examination of the right engine's electro-hydraulic servo valve (EHSV) found excessive solder on a jet pipe nozzle, reducing clearance so that particle contamination caused binding. This resulted in the nozzle commanding excessive fuel flow, exceeding normal start parameters by a factor of 10. The very rich fuel-air mixture caused most excess fuel to be expelled and ignite beyond the area of the exhaust gas temperature (EGT) probes; EGT remained normal. The fire warning system did not activate because the fire was not under the cowl. Cockpit indications included an Engine Control annunciator light, Master Caution light, reduced N1, and rapidly changing digital fuel flow display. The abnormal fuel flow was likely missed due to crew focus on N2, EGT, and oil pressure, and the digital display format.
The manufacturer's quality assurance did not detect the solder defect, allowing the valve into service. Subsequent inspection of other overhauled units found no similar manufacturing flaws, suggesting an isolated event.
Findings
The TSB findings as to causes and contributing factors:
- Excessive solder on a jet pipe nozzle in the overhauled EHSV reduced clearance so that particle contamination allowed binding, resulting in a nozzle position commanding excessive fuel flow.
- The manufacturer's quality assurance monitoring did not detect the excessive solder, allowing the nozzle back into service.
- Excess unburned fuel, caused by the excessive fuel flow, ignited as it exited the engine and tailpipe, resulting in severe external torching.
Findings as to risk:
- When the evacuation order was given, the evacuation checklist was not complete; the left engine was not yet shut down. When the aft left door was opened and the slide deployed, engine airflow could have resulted in injuries.
- The closed cockpit door likely reduced communication effectiveness and prevented pilots from directly assessing smoke in the cabin.
An additional finding noted that the digital fuel flow display format may have affected the crew's ability to detect abnormal fuel flow.
Safety Actions
Honeywell International Inc. inspected all returned EHSV units; no anomalies were detected. The overhaul process underwent an internal quality review, and changes were made: training for soldering operators and inspectors, quarterly peer audits, added quality inspection point for spring solder joints, annual process verification audits, and complete segregation of rejected components during disassembly.
Alaska Airlines Inc. revised training to ensure flight crews complete the emergency evacuation checklist before ordering an evacuation.
Calgary Airports Authority discussed hazards and mitigations for ground vehicle operation near deplaning passengers during emergencies.