Description

On 16 March 2020, a Bombardier DHC8-400 (VH-QOE) being operated by Sunstate Airlines for owner Qantas Airways on a scheduled Qantas Link international passenger flight from Port Moresby to Cairns as QF713 had only just taken off when abnormal fumes and later visible ‘smoke’ became apparent in the cabin and the flight deck. When these continued to intensify as the climb continued, it was decided to declare a ‘PAN’ and return to Port Moresby and the climb was stopped at FL190. The return was without further developments but once clear of the landing runway, a precautionary rapid disembarkation was carried out because of some passengers’ breathing difficulties.

Investigation

The event was reported to the Papua New Guinea Accident Investigation Commission (AIC) by the Civil Aviation Safety Authority (CASA) of Australia later the same day after they had received notification of it from the Operator. Relevant recorded data from both the SSFDR and the SSCVR were successfully downloaded.

It was noted that the 35 year-old Captain, who had been PF for the flight, had a total of 7,366 hours flying experience including 2,289 hours on type and the First Officer had a total of 1,614 hours flying experience which included 363 hours on type.

What Happened

Before takeoff, one of the cabin crew detected an abnormal smell in the passenger cabin described as reminiscent of ‘dirty socks’ for which no source was apparent but this was not reported to the flight crew at that time. Early in the climb, the flight crew then noticed a similar smell entering the flight deck but as it “did not appear strong to them at the time” it was initially decided to continue the flight as planned. However, climbing through 10,000 feet, the Captain considered that the smell had begun to intensify and after consulting the cabin crew and learning of the presence of a similar smell there, he responded by instructing them to standby whilst the situation was assessed and to anticipate a return to Port Moresby.

Both pilots then donned their oxygen masks, levelled the aircraft at just under 19,000 feet QNH and actioned the initial items on the QRH ‘Fuselage Fire, Smoke or Fumes’ Checklist. Whilst maintaining their outbound track, a ‘PAN’ was declared to Moresby Radar and a priority return requested. The acknowledgement was accompanied by an instruction to turn right and descend to 10,000 feet. At this stage there was no ‘smoke’ in the flight deck. The Captain then briefed the cabin crew about the turnback due to the fumes but advised them to expect a normal approach and landing. He subsequently instructed them to brief the eight passengers about the emergency situation and the intention to return to Port Moresby.

Once transferred to the airport radar frequency, the crew requested to complete their descent in the Bayview hold prior to making an RNAV approach to runway 32R and entered this hold at 15,000 feet QNH with clearance to descend to 6000 feet. Whilst descending, further relevant ‘Smoke and Fumes’ checklist items were actioned and, after only partially completing the actions to remove smoke or fumes, the cabin crew reported that  ‘smoke’ was entering the cabin. The flight crew then completed the remaining actions on the Smoke and Fumes Checklist but contrary to the procedure switched off bleed air 2 without first restoring bleed air 1.

Shortly after descending through 8,000 feet in the hold, the crew advised that they were now turning inbound for the runway 32R approach and subsequently reiterated their request for a priority landing “due to smoke and fumes in the cabin” which was accepted. Shortly afterwards, the SCCM opened the toilet compartment door to see if the smoke was coming from there but this allowed smoke to enter the compartment from the cabin which activated the photosensitive aural smoke alarm in the compartment. This alarm simultaneously triggered flashing lights on the passenger compartment ceiling and a single audible warning chime on the PA system. 

The remainder of the approach and the landing were without further event but when ATC requested the crew intentions after the aircraft had cleared the landing runway, the Captain advised that they intended to carry out a precautionary rapid disembarkation at that position because of smoke in the cabin. Once the engines had been shut down, this was accomplished and with the RFFS remaining present, the passengers and their luggage were eventually taken to the terminal by bus and the aircraft towed to the parking area. The Cabin Crew stated that none of the passengers appeared to have been physically affected by the in-flight fumes and smoke.

Why It Happened

The origin of the smoke/fumes was found by means of a borescope inspection to be an oil leak in the right engine. It was removed and a subsequent disassembly and detailed examination found that the No.3 bearing carbon seal had fractured into multiple pieces on the air side - see the illustration below. Once the air side carbon element had begun to disintegrate, oil was able to leak from the No.3 bearing cavity and into the gas path and eventually, as in this case, into contact with hot surfaces inside the engine causing fumes/smoke which then contaminated the bleed air used by the air conditioning system. 

The No.3 Bearing Carbon Seal removed from the right engine. [Reproduced from the Official Report]

The seal which failed was subject to a ‘soft’ in service life of 10,000 hours since new or since the last engine overhaul (such a limit is one chosen by an operator to be done at a specific interval but may be adjusted to fit their maintenance schedule). Engine manufacturer Pratt & Whitney Canada (P& WC) stated that such carbon seals were not assigned a specific ‘wear-out’ life, and that the intention was that they could remain in place as long as no significant wear was detected at the required normal engine inspection intervals. Any evidence of minor ‘wear and tear’ found during such inspections would require the replacement of the seal.

The view of P&WC was that the high local operating temperature and humidity which the aircraft involved was routinely subject to “meant there was a probability that the seal could experience a rather sudden mode of fracture not necessarily preventable by scheduled inspection”. Such ‘premature’ wear was believed to be caused by “exudation of salt and oxidation of the air side carbon element”.   

P&WC also stated that similar carbon seal failure events had been reported on PW150A engines in the past at or above 8,500 hours since new and that in order to reduce their prevalence, such seals were already being automatically replaced at every engine shop visit and more recently (in accordance with SB 35341) an improved seal was being substituted at replacement. At the time of the investigated event, the engine involved had been in service for 9,211 hours since new and on the basis of the engine performance based on routine Engine Condition Trend Monitoring (ECTM) and  borescope inspection, overhaul of the engine had been scheduled for four months after the seal failure.

In the light of these findings, the Investigation was concerned that in practice, the degree of exposure to environmental conditions “cannot be monitored or measured” and also the seemingly low likelihood that any secondary signs would indicate that a failure of the seal in service was imminent. Further concern derived from the fact that, according to the applicable Service Bulletin, the conditions for the replacement of the carbon seal were that the engine has been disassembled which can only occur during overhaul. This fact meant that continued use of the subject seal could occur without implementing the Service Bulletin and without any mention of the possible minimum in service failure time.  

Discussion - Non Airworthiness Issues

• It was noted that although the Operator’s ‘Aircrew Emergency Procedure Manual’ stated that “fumes caused by oil contaminated bleed air have been described as having a strong odour similar to ‘dirty socks’ and possibly visually a blue smoke, haze or mist”, this association had not been made by the SCCM or apparently recognised by the Captain although they had distributed surgical masks they had on board for other purposes to three passengers who had requested masks and had also distributed some wet towelettes to all passengers as per the smoke inhalation prevention procedures in the same Manual. 
• The intentional decision of the Captain not to follow the bleed air fumes tracing procedure by not selecting the left engine bleed air on before also turning the right bleed air off meant that the pilots were “not able to isolate the origin of the fault and continue with the applicable steps required by the checklist to avoid unnecessary effects on safety”.  
• An assessment of relevant ATC communications found that although Moresby Radar, who had received and acknowledged the flight’s PAN call had informed Airport Radar to whom the flight was subsequently transferred of this status, the latter had not done the same when transferring the flight to the TWR frequency and when the TWR then asked for “more information about the emergency situation” such clarification had not been provided. Although this had not added to the flight time, it had led to the PAN traffic having to insist that they receive priority for landing rather than be delayed to allow other traffic to join final ahead of them.   

 The Contributing Factors to the event were, in summary, as follows:

• The smoke/fumes that entered the cabin through the bleed air system originated from oil liberated at the No.3 bearing carbon seal of the PW150 engine coming into contact with hot surfaces inside the engine.
• The wear-out and early fracture of the air side carbon element was believed to be caused by exudation of salt and oxidation from the airside carbon element, processes aggravate by high operating temperatures and humid environments.  
• Pratt and Whitney Canada were aware that the No. 3 bearing carbon seal in the PW150 engine was likely to fracture earlier than its first scheduled overhaul as the earliest reported failure case was about 8,500 hours. At the time of the investigated event, the seal had been in use for 9,218 hours and had not reached the time for its first engine overhaul shop visit. The recorded life of the failed seal was 718 hours more than the known minimum failure life.
• Service bulletin SB35341, issued by Pratt and Whitney Canada on the conditions for replacement of No. 3 bearing carbon seal did not include the wear trend of the component they had determined as a relevant consideration for its replacement.  

Three Safety Recommendations were made as a result of the findings of the Investigation as follows:

• that Pratt & Whitney Canada ensure the operators of aircraft fitted with the engine PW150A which have the No.3 bearing carbon seal, PN: 3053630-01, are fully aware of the sudden failure trend of the seal and that the earliest probable fracture time can be earlier (as early as about 8,500 hours) than its first overhaul shop visit. [21-R01/20-2001]
• that Sunstate Airlines ensure that its flight and cabin crews are fully aware of the relevant information relating to unusual odours that can be indications of potential sources of smoke/fumes in the cabin and the applicable company procedures in place, to timely and adequately identify, report and react to such conditions, facilitating the adequate administration of the in-flight operation. [21-R02/21-2001]  
• that ANSP NiuSky Pacific should ensure that effective and appropriate communication and coordination is maintained with aircraft in an emergency or urgency situation. [21-R03/21-2001]

Safety Action in accordance with the above Safety Recommendations and advised to the AIC prior to the completion of the Investigation Report was noted to have been taken as follows: 

• Pratt & Whitney Canada have confirmed that aircraft operators have been informed as recommended in [21-R01/20-2001]
• Sunstate Airlines have advised that they will include in the next cycle of crew recurrent training enhanced awareness of the fact that the nature of unusual odours detected in the cabin can indicate potential sources of smoke/fumes.
• NiuSky Pacific intend to update refresher training to ensure that all their Air Traffic Services operational staff fully understand the nature of these types of incidents to ensure that inappropriate questions are not asked of crew of emergency-status aircraft in future.  

The Final Report was approved on 21 September 2021 and subsequently published. 

Related Articles

• Cabin Fumes from Non-Fire Sources
• Bleed Air System
• Emergency and Abnormal Checklist
• Standard Operating Procedures
• Service Bulletin