Description

On 6 June 2023, as a Boeing 717-200 (VH-NXM) operated by National Jet Systems for Qantas on a scheduled passenger flight from Sydney to Hobart was approaching destination in night visual meteorological conditions (VMC) when both pilots detected the same chlorine odour that had been reported by the cabin crew after takeoff but had disappeared. The captain sensed the onset of ‘hypoxia-like’ symptoms, and having self-assessed as unfit to continue as Pilot Flying, passed these duties to the initially unaffected first officer. However, just before touchdown, the first officer, too, began experiencing “cognitive difficulties” controlling the aircraft but safely completed the flight. After shutdown, both pilots reported persistent headaches. The captain experienced speech effects and lethargy, prompting ultimately inconclusive hospital medical tests.

Investigation

An investigation into the event was carried out by the Australian Transport Safety Bureau (ATSB). Relevant flight data was obtained from the quick-access recorder (QAR). It was noted that the captain had a total of 10,670 hours flying experience, which included 5,780 hours on type, and the first officer had a total of 6,984 hours flying experience, which included 1,614 hours on type.

What Happened

As the aircraft climbed on departure, the two cabin crew at the front of the cabin detected a “strong chlorine odour” in the forward galley. They called the flight crew and were advised that the flight deck was unaffected, as was the passenger cabin and the rear stationed cabin crew. Once established in cruise, the captain sought further information to complete a company cabin air quality event report. The forward cabin crew advised that the odour had disappeared after a while, but when it was present, they had experienced “dizziness, a feeling of pressure in the head and a metallic taste.” They also said the symptoms they had experienced were no longer sufficient to make them unfit to continue the flight. On hearing this, the captain determined that the intended flight should be completed as planned.

There was no further detection of chlorine odour anywhere on board until the aircraft was on base leg 10 nm from Hobart at about 4,000 feet. At this time, the captain noticed a chlorine odour, which he assessed as coming from the flight deck air-conditioning gasper vents. Soon after this, the first officer also recognised the odour, which was initially transient and disappeared after “about 10 seconds." However about half a minute later, whilst the aircraft was being configured for landing on runway 30 (see the illustration below), the captain reported having become aware of “quickly developing adverse effects on vision, mental capacity and movement." He subsequently described these as including “fogginess of thought, confusion, deteriorating situational awareness, weakness and tingling in the arms and legs and a narrowing of vision." He assessed himself unfit to continue in control of the aircraft and announced “your controls,” indicating that the first officer should take over. The first officer assumed PF duties and subsequently reported that the captain had “looked pale but did not appear incapacitated." 

The flight path to Hobart runway 30 with significant positions indicated. [Reproduced from the Official Report]

At this time, the aircraft was about 8.5 nm from the runway at approximately 3,250 feet. The captain continued to assist as pilot monitoring (PM) and correctly actioned instructions to select the flaps and lower the landing gear. The first officer was not experiencing anything abnormal, and the odour was no longer detectable so he decided that with landing imminent, use of emergency oxygen was not necessary. Shortly after this, the first officer reported noticing the captain was struggling to read the Landing Checklist and was “tripping over words,” although he was able to self-correct and had been able to respond correctly to all air traffic control calls.

As the aircraft passed 1,000 feet stabilised, the first officer reported noting that the captain had not made the required "stable" call and had therefore prompted him to do so, which achieved the expected response. The approach continued normally until about 100 feet above the runway elevation when the first officer experienced difficulty keeping the aircraft on the extended runway centreline. He subsequently stated that the aircraft began to drift to the right and that his corrective action was slower than normal. He also reported having felt “hazy” at this time but completed a normal landing and taxi in to parking.

Both pilots were “assessed by a company doctor via teleconference” during which the doctor observed that “the captain’s speech was noticeably affected, consistent with impairment” and advised blood tests at the local hospital. These showed slightly elevated levels of carbon dioxide and carbon monoxide. However, these findings were considered to be inconclusive due to the time which had elapsed since the flight.

The two forward-stationed cabin crew later reported that they had detected the same chlorine odour on landing and that they had again both felt similar mild symptoms. The first officer stated that he had developed a dull headache which began after the flight had been completed and had lasted “about two days." The following morning he had attended the same hospital for blood tests, which showed nothing abnormal.

Why It Happened

During the flight, the air conditioning and pressurisation system were operating in automatic mode, with the left pack supplying the flight deck and the forward galley area, and the right pack supplying the passenger cabin and the rear galley.

It was found that in the twelve months prior to the investigated flight, the aircraft operator had recorded 28 aircrew “Cabin Air Quality” (CAQ) events on their Boeing 717 aircraft. Two-thirds of these had involved mild symptoms such as nausea, headaches, or irritated eyes or throat, with over half of them occurring during take-off/climb and again during descent/landing. However, none of the events reported had affected flight safety and had therefore been categorised as “innocuous."

The investigation noted that in general, as opposed to the particular case of the Boeing 717, potential causes of CAQ events were usually considered to include:

• engine oil introduced into the air path by leaking internal engine or auxiliary power unit compressor seals 
• hydraulic oil or exhaust gases introduced into the air path from external aircraft sources
• dirty or defective water separator bags or components
• build-up of contaminants within the air-conditioning packs or ducting
• the use of aircraft cleaning or de-icing chemicals, inadvertently introduced into the air path 
• introduction of bio-effluents into the air path in flight
• external environment airborne fumes, gases or pollution
• fumes from internal cabin or cargo compartment sources

However, it was noted that the specific source of reported odours or fumes was frequently not determined. Despite a comprehensive maintenance-led series of inspections, this was the case for the investigated event as it had been for a similar event on a company 717 flight operated by the same aircraft two days earlier which led to a flight diversion. It was noted that four other flights by the aircraft between this diverted flight and one under investigation had not led to any reports of chlorine odour detected.

The investigation reviewed the aircraft operator’s flight crew procedures for CAQ events and found they were focused only on the post-flight reporting of issues and did not consider the potential consequences on crew fitness to complete a flight. However, whilst the vast majority of such events did not involve a risk of pilot incapacitation, the response to the degraded fitness to operate of one or both pilots, particularly with respect to subtle incapacitation, was inadequate. This was especially so in circumstances such as those encountered in the event under investigation where air conditioning system contamination was intermittent and cognitive impairment was delayed, in this case differentially.

It was concluded that the event had demonstrated that the absence of training in management of subtle physical or cognitive incapacitation could increase the risk of both pilots becoming impaired and unable to safely continue the flight.

Similar Investigated Events 

The investigation highlighted two relatively recently completed “notable” investigations of pilot incapacitation events which included:

• an A320 en-route over France in 2017
• an A320 in the vicinity of London Heathrow UK in 2019

Three Contributing Factors one of which was classified as a ‘Safety Issue’ (and identified as such) were formally documented based on the findings of the investigation as follows:

• The flight crew noticed a chlorine odour during the approach to Hobart Airport. Shortly after, the captain experienced rapid onset of incapacitation symptoms, resulting in handover of pilot flying duties to the first officer.
• Just before landing, the first officer experienced symptoms of incapacitation, specifically, slowed reaction time. Due to the impending landing, and perceived difficulties of donning the mask, supplemental oxygen was considered but not used. A go-around was also considered but rejected following an assessment that continuing the landing was the safest course.
• National Jet Systems’ cabin air quality events procedure focused on the recording/reporting of odours, post-flight care of crew and maintenance actions. However, it did not consider the possible application of the smoke/fumes procedure, or incapacitation procedure. As a result, there was an increased risk of flight crew being adversely affected by such an event during a critical stage of flight. [Safety Issue AO-2023-026-SI-01]

One Other Factor that Increased Risk was also identified and classified as a Safety Issue:

• Although National Jet Systems had procedures for recognition and management of pilot incapacitation, the associated training did not include the identification and response to subtle physical or cognitive incapacitation. [Safety Issue AO-2023-026-SI-02]

Safety Action taken by National Jet Systems in respect of the event and the two safety issues identified during the Investigation, which were both closed as a result, was noted as follows:

• A Safety Action Notice was issued reminding flight crews to consider supplemental oxygen use and the declaration of a PAN in response to a cabin air quality event. 
• The flight crew recurrent simulator training syllabus had been updated to include in-seat instruction specific to pilot incapacitation, followed by a practical exercise donning oxygen masks and conducting an approach, landing, and taxiing to the gate while using oxygen. 
• A review of smoke/fumes checklist and simulator training was undertaken.
• Lessons from this occurrence have been incorporated into the newly introduced company A220 aircraft smoke/fumes, hypoxia, and incapacitation procedures. 
• A review of hypoxia training material to include specific reference to histotoxic hypoxia was undertaken, with draft content awaiting input from the Qantas chief medical officer. 
• Aircrew emergency procedures documentation and training pertaining to the recognition and management of crew incapacitation have been updated to include subtle and cognitive incapacitation.

In Conclusion, a Safety Message based on the investigation findings was as follows:

Airborne contaminants may result in the rapid onset of incapacitation, which although possibly subtle, can significantly affect the safety of flight. Physical or cognitive incapacitation can occur for many reasons and may be difficult for others, or even the sufferer of, to detect and respond to. Flight crews should therefore be alert to the potential hazards posed by odours and fumes and not hesitate to use supplemental oxygen. The use of oxygen is a proven mitigating action in the case of environmental hazards and its rapid use ensures flight crews’ physical and mental capacity is maintained.

The Final Report was released on 16 April 2024. 

Related Articles

• Pilot Incapacitation
• Fumes Detection
• Fire, Smoke and Fumes
• Hypoxia
• Cabin Air Quality and Contamination
• Aircrew Oxygen Mask Utilisation
• Aircraft Bleed Air Systems
• Aircraft Pressurisation Systems
• A320, vicinity London Heathrow UK, 2019

Further Reading

• Final Report: A320 en-route over France in 2017, UK AAIB Bulletin 9/2020, 30 July 2020