Express Freighters B733 near Canberra on Aug 15th 2018, loss of cabin pressure, first officer incapacitated

Last Update: June 24, 2021 / 05:32:28 GMT/Zulu time

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Incident Facts

Date of incident
Aug 15, 2018


Aircraft Registration

Aircraft Type
Boeing 737-300

ICAO Type Designator

An Express Freighters Australia Boeing 737-300 on behalf of Qantas, registration VH-XMO performing freight flight QF-7335 from Brisbane,QL to Melbourne,VI (Australia) with 2 crew, was enroute at FL260 about 110nm northwest of Canberra,AC (Australia) when the crew received wing body overheat warning, which resulted in a reduction of cabin pressure. The crew donned their oxygen masks and descended the aircraft to FL200, the first officer became incapacitated however. The captain descended the aircraft to 8000 feet, advised they were diverting to Canberra, requested emergency services on stand by, an embulance to meet the aircraft on arrival and stairs for medics to enter the aircraft. On final approach, already in contact with tower, the captain advised the first officer was still unconscious. The aircraft landed on runway 35 without further incident. The first officer was taken to a hospital.

Australia's TSB opened an investigation into the occurrence, the blackboxes have been secured.

On Jun 24th 2021 the ATSB released their final report concluding the probable causes were:

Contributing factors

- Faults with the right wing-body overheat detection system likely led to intermittent illumination of the master caution and wing-body overheat annunciator.

- The crew were unable to isolate the right wing-body duct due to a faulty isolation valve.

- In consultation with Sydney Line Maintenance Operations, the flight crew conducted troubleshooting activities that resulted in the right air conditioning pack being selected to AUTO. This resulted in a reduction of cabin airflow.

- Due to the reduction in cabin airflow, along with a higher than normal cabin leak rate, the aircraft was unable to maintain required cabin altitude. This resulted in the flight crew electing to conduct an emergency descent.

- During the descent, the captain selected the emergency setting on the oxygen mask resulting in an inhalation of pressurised oxygen. This caused a gagging reflex leading to a temporary incapacitation.

- Throughout the diversion and landing the first officer experienced incapacitating symptoms consistent with hyperventilation from a heightened state of anxiety, leading the captain to declare the first officer incapacitated.

Other findings

- Post-occurrence medical testing and assessments were carried out on the flight crew with no lasting effects from the flight identified.

The ATSB described the sequence of events:

The aircraft departed Brisbane at 1206 Coordinated Universal Time
(UTC)1 and climbed to a cruising altitude of Flight Level (FL) 260.

At 1300 the flight crew observed a momentary illumination of the master caution light within the flight deck of the aircraft. The master caution then began to flicker with increasing frequency. After about 2 minutes of continuous flickering, the flight crew identified illumination of the right wingbody overheat annunciator, indicating a possible bleed air leak from the ducting of the right wing.

The flight crew consulted the quick reference handbook (QRH) and conducted the non-normal checklist5 procedure for a right wing-body overheat annunciator alert. This procedure removed bleed air pressure from the right wing bleed air duct by closing the isolation valve, switching off the right air conditioning pack, and turning off the bleed air source from the right engine.

Following completion of the non-normal procedure, the right wing-body overheat annunciator remained illuminated and the master caution light continued to flicker. Additionally, the right wing-body duct pressure gauge in the cockpit displayed a residual pressure of 14 pounds per square inch (psi), indicating to the crew that the duct remained pressurised despite that system being isolated. The flight crew, concerned with the potential for leakage of high temperature bleed air into the right wing, contacted Sydney Line Maintenance Operations by VHF radio for technical assistance.

In consultation with a Sydney Line Maintenance Operations engineer, the flight crew began a process of troubleshooting with the intention of isolating the bleed air duct and extinguishing the right wing-body overheat annunciator light. The flight crew advised of the already completed steps and discussed the non-normal checklist with the engineer. The engineer advised the flight crew to cycle components of the system and monitor the right bleed air duct pressure.

After noting that the residual pressure was lowest when the right pack was switched to AUTO the engineer assessed that the right wing-body overheat annunciator light was likely an erroneous warning. However, the engineer recommended that the right pack remain in AUTO for the remainder of the flight. Following receipt of that advice, contact with the engineer ceased due to the aircraft proceeding out of VHF radio range.

While continuing the flight toward Melbourne, the first officer conducted an instrument scan and identified the cabin pressure differential at 5 psi. The cabin altitude registered 6,000 ft and was increasing at 600 feet per minute. The flight crew discussed the rising cabin altitude, also commenting to each other that they both felt slightly unwell. The first officer recalled that the physical symptoms were not consistent with hypoxia9 and became concerned about the possibility of incapacitation due to fumes within the cockpit. As the cabin altitude continued to climb and anticipating that it would exceed 10,000 ft, the flight crew elected to don the emergency oxygen masks.

At 1325, the captain contacted air traffic control requesting a descent to FL 200. Air traffic control subsequently cleared the aircraft for a descent to FL 250. The captain immediately responded with a PAN PAN11 call and advised air traffic control that they were descending the aircraft to FL 200.

At the commencement of the descent, the first officer switched the right air conditioning pack from AUTO back to OFF as per the original non-normal checklist instruction. This commanded the left pack into high flow mode, increasing and restoring air flow into the cabin.

During the initial stages of the descent the captain reported that after manipulating the mask settings an ingestion of pressurised oxygen occurred. This resulted in a choking and gagging response from the captain. The first officer, cognisant that they had previously discussed feeling unwell, observed the captain slumped forward, gagging, and gasping for air. The first officer recalled checking on the captain during this episode but not receiving a response.

At 1326, approximately 19 km north of Narrandera Airport, New South Wales, the first officer broadcast a MAYDAY to air traffic control advising that they had issues with the aircraft and had commenced an emergency descent.

Air traffic control acknowledged the MAYDAY and advised the first officer that there was no other reported Instrument Flight Rules aircraft traffic for their descent to 10,000 ft.

During the emergency descent, the captain recovered and subsequently removed their oxygen mask. Both the captain and first officer noted that oxygen could be heard continuously flowing from the captain’s mask. Due to their ongoing concern of fumes within the cabin, the first officer elected to remain on oxygen.

Air traffic control contacted the flight crew regarding their intentions to either continue to Melbourne or divert to an alternate airport. The first officer responded with a request for information on the lowest safe altitude for the area, and a heading for an immediate landing at Canberra Airport, Australian Capital Territory. After receiving direction from air traffic control, the captain elected to further descend the aircraft to 8,000 ft and tracked the aircraft toward Canberra.

During the diversion, the oxygen supply from the fixed onboard system depleted. In response, the first officer, who had elected to remain on breathing oxygen, retrieved and connected to the portable oxygen unit that was mounted on the bulkhead at the rear of the flight deck. While transferring to the portable oxygen system, the first officer also attempted to operate an iPad in preparation for the instrument approach to Canberra Airport. The captain recalled being unable to gain the first officer’s attention during that sequence. The first officer eventually responded to the captain, advising they were feeling unwell and had severe back pain.

The captain then instructed the first officer to transition to an observer-pilot role for the remainder of the flight. The captain subsequently declared a PAN PAN radio call to air traffic control informing of the first officer’s incapacitation and requested the attendance of emergency services on arrival at Canberra Airport.

At 1404, the flight landed at Canberra Airport and once the aircraft had been shut down, aviation rescue fire fighters entered the aircraft cabin and conducted an air sample test. No fumes were detected. Both the captain and first officer were transported to hospital via ambulance for medical assessment.


Post occurrence, both flight crew were transported to hospital. Assessments and blood tests were carried out with no markers or indicators of a fumes event detected. The first officer recalled still feeling ‘dizzy’ and ‘foggy’ with a dry throat and lips during this time. The flight crew were subsequently released from hospital with no ongoing effects from the occurrence.

The flight crew were individually interviewed and medically assessed by their company doctor. It was reported to the ATSB that the first officer’s symptoms of tingling in the fingers and lips, along with the muscle tension were consistent with hyperventilation, although the symptoms can be confused with hypoxia or fume inhalation. An additional assessment of the symptoms was provided to the ATSB by an aviation medicine specialist, who similarly concluded that the first officer had developed hyperventilation as a response to the severity of the emergency.

The ATSB analysed:

Aircraft depressurisation

While cruising at Flight Level 260, the flight crew observed the illumination and rapid flickering of the master caution light. Approximately 2 minutes later, the first officer identified the annunciator light for the right wing-body overheat was also illuminated. The concern from such an alert for the flight crew was the potential leakage of hot bleed air gases within the wing cavity damaging wiring and the internal structure of the wing. While various recommended actions were attempted to extinguish the warnings, the flight crew efforts were unsuccessful, and the flickering continued forapproximately 10 minutes.

Though the flickering master caution and overheat annunciator light eventually extinguished, it is likely that these alerts were due to an electrical fault with the aircraft detection system rather than the result of elevated temperature within the wing body. After landing, maintenance engineers established that the centre conductor from a sensor within the overheat detection system was out of specification, leading to the observed erroneous system behaviour.

Actioning the non-normal checklist associated with the right wing-body overheat annunciator alert required isolating the bleed air systems from the right engine. Those actions were intended to remove high temperature bleed air, from the right wing-body duct and shut down that part of the air conditioning system.

However, following completion of the checklist the flight crew observed that the right duct remained partially pressurised, indicating that it was not isolated. Technical examination following the occurrence identified that a faulty isolation valve prevented complete closure of the duct.

The residual pressure after completion of the non-normal checklist operations led to the flight crew conducting further troubleshooting activities outside of that checklist in consultation with the Sydney Line Maintenance engineer. This resulted in the right air conditioning pack switch being selected to AUTO from OFF, which then transitioned the left (operating) pack from high flow to low flow, leading to a significant reduction of air available to pressurise the aircraft.

As the right air conditioning system remained partially pressurised, the captain’s decision to conduct further troubleshooting outside the non-normal checklist instructions was appropriate and completed in conjunction with assistance from Sydney Line Maintenance. Despite the attempts to extinguish the overheat alert by isolating the right bleed ducting, the effects of those actions unknowingly contributed toward a gradual depressurisation of the aircraft.

Following the occurrence, the aircraft was found to have significant air leaks from the auxiliary power unit duct bellows, cabin drain valves that were found stuck open and minor leaks from the aircraft door seals. In combination, these leaks were significant however, when the aircraft was operating with both the left and right air conditioning packs operating, cabin pressure was still able to be maintained.

Single pack operation was configured as a result of the completion of the non-normal checklist.

The additional troubleshooting actions by the flight crew reduced the air supply into the aircraft cabin. This, in combination with the excessive leakage pathways, resulted in a gradual depressurisation of the aircraft. That, in turn, resulted in the flight crew electing to don oxygen and conduct an emergency descent.

Pilot incapacitation

While wearing an oxygen mask during the descent, the captain selected the emergency purge setting on the oxygen mask resulting in the inhalation of pressurised 100 per cent oxygen. The flight crew operations manual stated that the emergency position of the oxygen regulator was to be used to supply positive pressure within the mask to remove contaminants such as fumes or particles from within the mask. Selection of the emergency position on this occasion produced a gagging reflex and led to the captain becoming temporarily incapacitated.

In response, the first officer declared a MAYDAY and continued the descent to 10,000 ft. While there was no enduring effect from the captain’s temporary incapacitation, had the system remained in normal operation mode administering an on-demand supply setting of regulated oxygen, it is unlikely to have occurred.

After landing at Canberra Airport, the flight crew were transported to hospital for medical assessment. Blood tests showed no markers or indicators that the crew were exposed to disabling or hazardous fumes generated from within the aircraft. This was supported by emergency services not detecting any hazardous gases during their post-landing check of the aircraft flight deck. Additionally, no medical problems were identified.

Despite that, the first officer recalled feeling ‘dizzy’ and ‘foggy’ with a dry throat and lips. They also reported feeling unwell and having severe pain during the descent. On balance, the ATSB concluded that the accumulation of stressors associated with the:

- troubleshooting activities

- use of the oxygen masks

- temporary incapacitation of the captain

- concern for hazardous fumes

- emergency decent and subsequent diversion probably led the first officer to a heightened state of anxiety, hyperventilation, and the development of further incapacitating symptoms.

ATSB research into pilot incapacitation occurrences showed that the majority of cases were due to gastrointestinal illnesses, such as food poisoning or gastroenteritis. There were no reported instances involving incapacitation due to hyperventilation. The report noted that once declared incapacitated, the pilot was often removed from active flying duty and the flight was sometimes diverted, such as occurred in this instance.
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Incident Facts

Date of incident
Aug 15, 2018


Aircraft Registration

Aircraft Type
Boeing 737-300

ICAO Type Designator

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