Description

On 22 August 2019, a Boeing 737-800 (JA322J) being operated by Japan Airlines on a scheduled domestic passenger flight from New Chitose to Fukuoka as JL3512 stopped whilst taxiing for departure in daylight after encountering a sudden loss of visibility in unforecast heavy snow. After further taxiing and another stop, a cabin crew report of fumes and unusual smells in the cabin and flames coming from the right engine, it was decided to shut down and order an emergency evacuation of all 165 occupants. This was accomplished with one serious passenger injury and two minor ones.

Investigation 

After notification of the event the day it occurred, the Japan Transport Safety Board (JTSB) classified it as a Serious Incident and an Investigation was carried out. Data was successfully downloaded from both the FDR and the CVR and enabled a detailed review of the occurrence. It was noted that the 48 year-old Captain, who was acting as PF for the sector involved, had a total of 9,385 hours flying experience of which 3,386 hours were on type. The 36 year-old First Officer had a total of 4,731 hours flying experience of which 1,327 hours were on type. 

What Happened

After pushback and engine start, what had been almost imperceptible snowfall at an air temperature close to zero suddenly and rapidly intensified. After completion of checks and the selection of airframe and engine anti-icing on, taxi was requested but the flight was told to hold position. Whilst awaiting clearance, the request was modified to route to an airframe ground de-icing pad and a few minutes this was given to an intermediate position via a circuitous route due to taxiway snow clearance activity. After two minutes taxiing using a maximum thrust of 39% N1, the clearance limit was reached. After two minutes, a further taxi clearance was given and a similar maximum thrust was used but after three minutes and before reaching the clearance limit, the aircraft was stopped again when the deteriorating visibility in the continuing heavy snow made continued taxi impossible. 

Company were advised that a tow would be necessary to continue. The amount of engine oil in both engines began to decrease. The APU was selected on in preparation for shut down and towing but almost immediately, the First Officer recognised an unusual smell from the air conditioning as the cabin crew reported similarly, adding that “smoke” had also appeared. The APU was switched off as were both Air Conditioning Packs as the crew commenced the ‘Smoke or Fumes Removal’ Checklist. The Captain informed ATC that because of fumes in the cabin, there was a possibility that an emergency evacuation may be required. Completing the Checklist appeared to improve the air conditioning-related contamination although the indicated N1 on both (idling) engines had begun to decrease. However, after 14 minutes stopped, the snowfall had lessened and sufficient visual reference to continue was now available again so the thrust levers were moved forward to recommence taxiing as cleared but there was no response and the right engine then stopped.

As the engine failure or shutdown checklist was commenced, the cabin crew called to say that flames had been see coming from the right engine and so the checklist action was changed to the engine fire/severe damage one. Having heard the cabin crew report, the Captain then decided that an emergency evacuation was appropriate and gave the order (the First Officer subsequently stated that he had considered the possibility of a tailpipe fire but had “decided that it was better to conduct an emergency evacuation”).

The evacuation began immediately and the SCCM at the L1 door called out “no baggage” and instructed selected passengers to act as ‘helpers’ at the bottom of the slide to pull people up and off the slide to make way for more to come down. Similar action was taken at the other exit doors but in all cases, confiscation of cabin baggage was limited by the space available for it without obstructing access to the exits. The cabin crew at R2 was able deploy the slide there for safe use after checking that the flame from the right engine did not pose a hazard to its safe use. The mid cabin overwing emergency exits were not used. The serious injury occurred to a female passenger who exited through door 1R and fell forward at the bottom of the slide and sustained a serious fracture injury and was hospitalised. It was considered possible but not confirmed that this passenger may have allowed their upper body to lean forward rather than maintaining the recommended sitting position on the slide which would have increased the risk of injury. There were only two other minor passenger injuries during the evacuation.

Why It Happened

Both CFM56-7B24/3 engines were of similar age and flight cycles. They were examined soon after the accident and then subjected to teardown inspections at the OEM. The collective findings of these inspections were as follows:

• both engines had icing on the rear of the fan blades and on the LPC (low pressure compressor) inlets, blades and the periphery of the back steps of the LPC. Both engines had deposits of engine oil on the HPC (high pressure compressor) blades and on the downstream side of the PRSOV (pressure regulator and shutoff valve).
• only the right engine had accumulated engine oil inside the tailpipe and had soot containing engine oil in the tailpipe, at the front of the No. 1 bearing and at the back of the No. 5 bearing. Only this engine had traces of high temperature air flowing forwards inside of the Centre Vent Tube and soot and engine oil inside of the air duct.

The loss of oil was greatest from the left engine.

It was considered likely that loses of engine oil were caused by the leakage of oil from the inside of the bearing sump to the LPC when the normal air pressure on the outside of the bearing sump seal could no longer be maintained because of the reduced air inflow into engine due to ice accretion on the fan blades and at the LPC (see the illustration below).  

The flow of oil through the engines at the time of the malfunction. [Reproduced from the Official Report]

It was also considered that engine oil leaked into the LPCs of both engines would have mixed with the compressed air bled from the HPC as it then passed via the PRSOV and the air conditioning Pack and generated the abnormal smells and fumes/appearance of thin smoke. 

The initial decrease in engine N1 rpm would have been due to the to the fan blade and LPC icing which would have disturbed the normal fuel air ratio in the engine combustion chamber and caused engine rpm to decrease. Then, when the thrust lever was moved forward to recommence taxiing, the unstable condition of the engine would have been sufficient to cause the right engine to stop.

Finally, the flame seen at the rear of the right engine was caused by engine oil leaking into the tailpipe and accumulating after the engine stopped being ignited by its exposure to the heat of tailpipe. 

The Probable Cause of the investigated Serious Incident was described in narrative form as follows:

“In this accident, it is probable that while holding on the taxiway to taxi following the heavy snowfall, odd smells and smoke were generated within the cabin, following these events, because the flame from rear of No.2 engine was continued, the Captain ordered an Emergency Evacuation from the aircraft. During this evacuation, a passenger fell to the ground and sustained a serious injury.

Regarding the occurrence of unusual smells and smoke in the cabin and the continuation of the flame at the rear of the right engine, it is probable that when the heavy snow became intense due to the rapid weather deterioration, icing occurred on the fan blades and in the low pressure compressor, causing vapourised engine oil to leak into engine bleed air fed to the cabin air conditioning system leaked oil to accumulate within in the tailpipe and catch fire.”

Safety Action taken as a result of the Investigation findings included the following:

Japan Airlines:

• Amended procedures for engine operation on the ground in heavy snowfall.
• Reminded its ground personnel that any late and imminent change to the forecast weather must be communicated to potentially affected flight crew as soon as possible.
• Revised the contents of the passenger pre flight safety video to enhance the awareness among passengers on how to assist the fellow evacuees at the bottom of the slide and why baggage must not be carried when evacuating.
• Added new content to the periodic emergency training received by cabin crew on how to act when passengers rush to an exit with baggage in their hands and how to have passengers not to bring their baggage to an exit.
• Implementing improved training on emergency evacuation with regular simulator training on evacuation slide procedures and use.   

The Final Report was adopted by the JTSB on 22 November 2017 and published in both English translation and the definitive Japanese version on 21 December 2017. No Safety Recommendations were made.    

Related Articles

• Tailpipe Fire
• Cabin Fumes from Non-Fire Sources
• Engine Core and Fan De/Anti-icing
• Aircraft Ice Protection Systems
• Snow
• Ice Formation on Aircraft
• Emergency Evacuation on Land

Further Reading

• Emergency Evacuation of Commercial Passenger Aeroplanes (Second Edition), Air Pilots, RAeS, 2020