National System B712 at Hobart on Jun 6th 2023, flight crew nearly incapacitated after chlorine like smell on flight deck

Last Update: April 16, 2024 / 10:56:03 GMT/Zulu time

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Incident Facts

Date of incident
Jun 6, 2023


Flight number

Aircraft Registration

Aircraft Type
Boeing 717-200

ICAO Type Designator

A National Jet Systems Boeing 717-200 on behalf of Qantas, registration VH-NXM performing flight QF-1541 from Sydney,NS to Hobart,TA (Australia), was on approach to Hobart descending through about 3000 feet MSL when the crew noticed a chlorine like odour. The captain, pilot flying, soon after assessed himself as unable to continue flying due to the onset of hypoxia like symptoms. The first officer assumed the role as pilot flying, but also felt the onset of deleterious symptoms, however, managed to continue for a safe landing on Hobart's runway 30.

Australia's ATSB reported: "Passing 3,000 ft on approach, the captain noticed a chlorine-type smell and shortly after assessed themselves as unable to continue as pilot flying due to the onset of hypoxia-like symptoms. The first officer took over as pilot flying. The first officer later reported that they started to feel the onset of deleterious symptoms on short final. The aircraft landed without further incident." and opened an investigation into the occurrence.

On Apr 16th 2024 the ATSB released their final report concluding the probable causes of the serious incident were:

- The flight crew noticed a chlorine odour during the approach to Hobart Airport. Shortly after, the captain experienced the rapid onset of symptoms of incapacitation, resulting in them handing over 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 of action.

- 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)

Other factors that increased risk

- 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)

The ATSB stated:

A comprehensive technical investigation was conducted to attempt to determine a source of foul air. This included cabin air quality tests during test flights, bleed air analysis during a test run of an engine with an identified oil leak in an engine test cell, and dismantling and inspection of the engine. Despite these efforts, a definitive explanation for the incident was not identified.

The ATSB analysed:

During climb, the 2 cabin crew members in the forward galley smelt a chlorine odour and experienced associated mild symptoms, which abated after a short time, before returning again on landing. The air-conditioning system was in the normal configuration so that bleed air from the left engine and pressurised air-conditioning kit was distributed to the flight deck and forward galley, while the right side supplied the passenger cabin. Although left-side air was also being distributed to the flight deck, the flight crew did not detect odour during the climb. The crew treated the odour as a cabin air quality (CAQ) event and conducted the associated reporting actions.

Later in the flight, on approach to Hobart Airport, the flight crew noticed a chlorine odour and the captain experienced the rapid onset of symptoms of incapacitation, including cognitive impairment. The captain handed over pilot flying duties to the first officer and assumed the responsibilities of pilot monitoring.

Just before landing, the first officer reported feeling some symptoms of incapacitation but due to the proximity to landing and perceived difficulties of donning the oxygen mask, the first officer assessed that supplemental oxygen was likely to increase the risk to safety of flight. A go-around was considered, but the first officer assessed that continuing the landing was the safest overall course of action. Both flight crew members were partially incapacitated during the landing and subsequent taxi to the parking bay.

The odour and symptoms were consistent with that experienced earlier by the cabin crew, albeit with the flight crew experiencing more serious effects. The reason for the captain experiencing more debilitating effects slightly earlier than the first officer and for the cabin crew’s symptoms being subjectively milder, could not be determined. Whether a stress response to a non-normal situation in a high workload phase of flight exacerbated or contributed to the symptoms experienced by the flight crew on the incident flight was also not determined. The same aircraft had a reported CAQ event 2 days prior to the incident, in which cabin and flight crew smelt chlorine and only the cabin crew reported any symptoms, which were mild. Following the 6 June 2023 occurrence, maintenance engineers conducted an inspection of the aircraft to identify a source of the odours, none were found, and the event was not able to be reproduced on the ground or during a subsequent test flight.

Cabin air quality events procedure

Based on occurrences recorded by the ATSB and the aircraft operator, most CAQ events are short-lived and benign, with exposed crew experiencing no or mild symptoms. However, in rare cases the effects can be more serious and affect the safety of flight. Crew incapacitation may be partial or complete, and symptoms can range from the obvious, such as vomiting or loss of consciousness, to the subtle, such as mild cognitive impairment, or difficulty executing a task. In accordance with the operator’s procedures, the appropriate response depended on the extent of the effects experienced by the crew.

The history of reported CAQ events not impacting safety of flight likely influenced the intent of the operator’s CAQ event procedure, which detailed steps to report an event to assist in post-flight maintenance activities, and to assess an affected individual’s fitness to continue or return to duty.

In response to the transient odour and symptoms reported during climb by the cabin crew, the flight crew completed the CAQ event procedure. Although the CAQ event procedure did not require consideration of supplemental oxygen, the captain reminded the affected cabin crew members to use supplemental oxygen if required. The CAQ event procedure did not require crews to consider actions in the event flight crews were affected by airborne contaminants.

The operator’s emergency procedure for smoke/fumes and the pilot handbook included that crew oxygen masks must be donned when concentration of fumes or odours were detected or anticipated on the flight deck. However, when the flight crew experienced the chlorine odour and more serious symptoms, they assessed it was a transient CAQ event rather than a fumes event and therefore were not required to use supplemental oxygen. Additionally, the captain’s first symptoms were likely cognitive impairment, which may have affected their judgement regarding the need for supplemental oxygen. If cognitive incapacitation is due to hypoxia or environmental contaminants, the window of opportunity to recognise and react to it can be very small before more significant cognitive effects impair decision making.

According to the operator’s medical officer, use of supplemental oxygen can have operational challenges but will not cause harm. Its use prevents further inhalation of contaminated air, preventing symptoms from worsening. In particular, it reverses the effects of hypoxia, including exposure to carbon monoxide and dioxide, and is also useful in the case of many medical emergencies.

National Jet Systems’ record of CAQ events showed 28 reported events in the 12 months prior to the incident flight, often during descent/landing preceded by events during take-off and climb. Eleven of the 28 events reported symptoms such as nausea/vomiting, headache, dizziness, or discomfort of the eyes and throat, but none of these effects impacted the safety of flight. The potential for a recurrence in the same flight was not considered in the CAQ event procedure. Additionally, the procedure did not include consideration of a CAQ issue leading to a fumes or incapacitation event.

As a result, the crew were unprepared for the subsequent fumes and incapacitation event, which occurred at a critical phase of flight, and did not use oxygen to significantly reduce the risk of both flight crew becoming partially incapacitated. The effectiveness of such preparation was demonstrated in a 2019 incident investigated by the United Kingdom Air Accident Investigations Branch. In that incident, following a mild transient CAQ event during the climb, the flight crew reviewed their smoke/fumes procedure, and discussed and planned to use oxygen if an event recurred. As a result, the flight crew quickly donned oxygen and declared a PAN when they detected fumes on approach.

Pilot incapacitation training

Multiple studies have identified gastrointestinal illness caused by food poisoning as the most common cause of pilot incapacitation. This finding has shaped pilot incapacitation training and procedures to ensure that the most prominent threat is adequately covered. The most salient features of food poisoning are obvious symptoms, the ability for the sufferer to communicate their distress, and ability to retain cognitive capacity. Training in recognition and response of pilot incapacitation has also been focused on the immediately hazardous effects of sudden and total incapacitation. The operator’s training in the use of supplemental oxygen for incapacitation focused on response to these events and did not encompass precautionary use of supplemental oxygen, or oxygen use during critical stages of flight.

The captain self-assessed as being unable to continue as pilot flying and handed over the duty to the first officer in accordance with the operator’s procedures, but did not communicate the severity of the difficulties experienced to the first officer. Although the captain knew and understood the symptoms and response to hypoxia, at the time, likely due to cognitive impairment, the captain did not recognise the hypoxia-like symptoms or identify supplemental oxygen as an appropriate response.

The National Jet Systems pilot incapacitation training focused on responding to overt or obvious incapacitation rather than the response to subtle incapacitation. The operator’s policy manual specified the flight crew’s absence of a standard call, or an acknowledgement of a standard call, as an indicator of pilot subtle incapacitation. In this case the first officer recognised that the captain had missed the stable approach call required at 1,000 ft altitude, and prompted the captain who then made the correct call.

Without the benefit of formalised training in the response to partial or cognitive incapacitation, and due to the absence of symptoms experienced by the first officer at that time, the first officer continued the landing without the use of supplemental oxygen. Additionally, the first officer assessed that the operator’s pilot incapacitation procedure and requirement to broadcast a PAN call were inappropriate in this event, as the captain was not completely incapacitated, and the aircraft was close to landing. While the proximity to landing limited the opportunity for the first officer to take any further action, the absence of training in management of subtle physical or cognitive incapacitation increased the risk of both flight crew members becoming impaired and unable to safely continue the flight.
Incident Facts

Date of incident
Jun 6, 2023


Flight number

Aircraft Registration

Aircraft Type
Boeing 717-200

ICAO Type Designator

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