B733, en-route, north of Narrandera NSW Australia, 2018

B733, en-route, north of Narrandera NSW Australia, 2018


On 15 August 2018, a Boeing 737-300SF crew concerned about a small residual pressure in a bleed air system isolated after a fault occurred en-route then sought and were given non-standard further troubleshooting guidance by company maintenance which, when followed, led directly and indirectly to additional problems including successive incapacitation of both pilots and a MAYDAY diversion. The Investigation found that the aircraft concerned was carrying a number of relevant individually minor undetected defects which meant the initial crew response was not completely effective and prompted a request for in-flight assistance which was unnecessary and led to the further outcomes.

Event Details
Event Type
Flight Conditions
Not Recorded
Flight Details
Type of Flight
Public Transport (Cargo)
Flight Origin
Intended Destination
Actual Destination
Take-off Commenced
Flight Airborne
Flight Completed
Phase of Flight
19 km north of Narrandera Airport, New South Wales
En-route Diversion, CVR overwritten
Flight Crew Incapacitation, Inappropriate crew response (technical fault), Procedural non compliance, Stress
Significant Systems or Systems Control Failure, Flight Crew Incapacitation
Emergency Descent, MAYDAY declaration, PAN declaration
Bleed Air
Damage or injury
Non-aircraft damage
Non-occupant Casualties
Off Airport Landing
Causal Factor Group(s)
Aircraft Operation
Aircraft Technical
Safety Recommendation(s)
None Made
Investigation Type


On 15 August 2018, the crew of a Boeing 737-300SF (VH-XMO) being operated by Qantas-owned Express Freighters Australia on a non-scheduled domestic cargo flight from Brisbane to Queensland in unrecorded night meteorological conditions were concerned that after responding to a bleed air leak indication, the system concerned was not completely isolated. A series of further ad-hoc actions were then followed by circumstances which prompted an emergency descent and a MAYDAY diversion to Canberra with temporary incapacitation of the Captain and then incapacitation of the First Officer. Both pilots passed all heath checks after completing the flight.


An Investigation into the event was carried out by the Australian Transport Safety Bureau (ATSB). Relevant data was obtained from the FDR but the CVR was not isolated before being powered up by maintenance personnel at Canberra engaged in rectifying the reported fault and relevant data was overwritten. 

It was noted that the Captain had a total of approximately 4,500 hours flying experience which included 3,400 on type and had been a company employee for about 8 years, initially as a First Officer before gaining his command in 2018. He was also a simulator and a 737 ground training instructor with the company. The First Officer had a total of approximately 5,000 hours flying experience which included 700 hours on type and had been a company employee for almost 5 years. 

What Happened

Whilst in the cruise at FL 260 almost an hour after departure from Brisbane, the right wing-body overheat caution illuminated which indicated a possible bleed air leak in the right wing. The corresponding QRH procedure which 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 was completed. However, the fault illumination was not extinguished and the right wing-body duct pressure gauge displayed a residual pressure of 14 psi which meant that the duct remained pressurised despite its system being isolated. 

Concerned with the potential for leakage of high temperature bleed air into the right wing, the crew contacted Maintenance Control by radio for advice. They were then guided by an engineer through a troubleshooting exercise to resolve the observed system inconsistency. Just after being told to switch the right air conditioning pack to ‘AUTO’ contrary to the QRH procedure previously completed, radio contact with the assisting engineer was lost as the aircraft continued out of VHF radio range.

The First Officer noticed that the cabin pressure differential was only 5 psi and the cabin altitude was increasing at 600 fpm. The crew discussed the rising cabin altitude and reported having commented to each other that they both felt slightly unwell. The First Officer stated that “the physical symptoms were not consistent with hypoxia and that he had become concerned about the possibility of incapacitation due to fumes. As the cabin altitude continued to climb and seemed likely to exceed 10,000 feet, both pilots decided to don their emergency oxygen masks. The Captain then requested a descent to FL 200 and when ATC subsequently cleared the aircraft for a descent to FL 250, he immediately responded with a ‘PAN PAN’ call and without waiting for a response advised that they were descending to FL 200.

As the descent commenced, the First Officer switched the right air conditioning pack from AUTO back to OFF as per the original QRH procedure which led to the left air conditioning pack returning to high flow mode which restored increased airflow into the flight deck. The Captain subsequently stated that during the initial stages of the descent, his manipulation of the mask settings had resulted in an ingestion of pressurised oxygen which had caused him to begin choking and gagging. The First Officer saw that the Captain had “slumped forward, gagging and gasping for air” and stated that on checking if he was alright had not received a response. He therefore made a MAYDAY call with the aircraft approximately 10 nm north of Narrandera advising that they had issues with the aircraft and had commenced an emergency descent with ATC acknowledging and adding that there was no other IFR traffic conflicting with a descent to 10,000 feet. FDR data showed that at no time did the recorded cabin altitude actually exceed 10,000 feet.

During the descent, the Captain recovered and subsequently removed his oxygen mask. Both pilots reported noting that oxygen could be heard continuously flowing from the Captain’s mask. Due to their ongoing concern about the possibility of fumes, the First Officer decided to remain on oxygen. A diversion to Canberra was commenced and the Captain decided to continue the descent to 8000 feet. During the diversion, the mask oxygen supply was exhausted and the First Officer responded by connecting his mask to a portable oxygen unit which was mounted on the rear flight deck bulkhead. While transferring to the portable oxygen system, the First Officer also attempted to operate an iPad in preparation for the instrument approach at Canberra and the Captain stated that he had been unable to gain the First Officer’s attention during that time. 

The First Officer then said that “they were feeling unwell and had severe back pain” so the Captain instructed him to take up an observer-pilot role for the remainder of the flight. The Captain then made a second PAN call, this time informing ATC of the First Officer’s incapacitation and requested the attendance of the emergency services on arrival at Canberra. The diversion was completed without further event and once the aircraft had been shut down, RFFS personnel boarded the aircraft and conducted an air sample test in the flight deck which did not detect any fumes. Both pilots were taken to hospital for medical assessment.

Why it happened

The Investigation considered both the apparent in flight airworthiness issues and the response to them in flight and its consequences.

(a)    Airworthiness Issues

  • A minor bleed air leak from the right-wing leading-edge ducting was found and rectified by tightening of the duct clamp.
  • The isolation valve between the left and right systems was not completely closing, resulting in leakage and hence the right bleed air duct remaining partially pressurised when supposedly isolated.
  • During single-pack operation in normal flow, the aircraft was unable to maintain differential cabin pressure due to a combination of leakage from multiple cabin drain valves, minor door seal leaks and damage to the APU bleed air duct bellows and clamp which together were causing a loss of cabin air through the pressure bulkhead.  
  • The centre wing-body overheat detector element was found to be unserviceable and together with an associated electrical connector was replaced.
  • No evidence of fumes or other abnormality was found during air quality testing. 
  • Both pilots’ oxygen masks with tested with faults found. However, the Captain’s mask was found set to emergency flow.

(b)    Crew Response

It was noted that use of non-normal procedures in the QRH required the Captain to “assess the situation to determine the safest course of action” noting that “if further troubleshooting is determined to be the safest course of action, taking steps beyond published non-normal checklist steps may cause further loss of system function or a system failure”. The operator’s guidance to their pilot’s was that “troubleshooting should only be considered when completion of the published non-normal checklist results in an unacceptable situation”. The Flight Administration Manual was found to further state that “crew troubleshooting of system problems should not normally be attempted in flight unless there is an overriding operational requirement to do so, or where it may assist subsequent rectification” and required that “before attempting any troubleshooting, flight crew were to attempt to contact Maintenance Control for further guidance”

(c)    Crew Incapacitation 

  • The Captain’s brief incapacitation after donning his oxygen mask was attributed to his attempt to use it with the emergency purge setting selected which results in the inhalation of pressurised 100 per cent oxygen. The FCOM was noted to state that this position was intended only for the removal of contaminants such as fumes or particles from within the mask.
  • The First Officer’s incapacitation was attributed to an “accumulation of stressors associated with troubleshooting activities, the use of oxygen masks, temporary incapacitation of the Captain, concern for hazardous fumes and the emergency descent and subsequent diversion”. It was concluded that collectively these had probably resulted in “a heightened state of anxiety, hyperventilation and the development of further incapacitating symptoms”.     

Six Contributing Factors were identified as follows:

  • 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.

Safety Action taken by Express Freighters Australia as a result of this event and known to the Investigation was reported as having included:

  • the implementation of an enhanced inspection regime to ensure timely detection and rectification of faults compromising the operation of the wing-body overheat detection system. 
  • an amended maintenance program to incorporate an enhanced pressurisation system check and scheduled functional checks of the fuselage drain valves. 
  • a specific task during scheduled maintenance of the aircraft to verify the integrity of the auxiliary power unit duct bellows seal. 
  • The amendment of their Flight Administration Manual in respect of their policy for system failures and in-flight troubleshooting which now reads:

 “A component or system failure inflight should be dealt with according to established procedures, including completion of any associated checklist. Systems troubleshooting by crew is prohibited, except in accordance with the FCOM/QRH. Actions contrary to the FCOM/QRH cannot be authorised by Maintenance Watch.”

The Final Report was released on 24 June 2021. No Safety Recommendations were made.

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