Incident Overview
During a flight, the crew reported observing a discoloration of the carbon monoxide (CO) detector. They also experienced very mild symptoms and cognitive effects. Blood tests taken after the incident indicated mildly elevated carboxyhaemoglobin levels, which, together with the detector indication, were consistent with the presence of CO in the cabin.
Previous Incidents and Detector Replacement
The same aircraft had been involved in two other incidents nine months prior. Two days before the second incident, the CO detector was replaced with a new unit. Despite this, the possibility remained that CO card detectors could give an unreliable indication.
Post-Incident Investigation
Following the incident, the aircraft underwent ground runs and subsequent flight tests, but these were unable to detect elevated levels of CO in the cabin. An inspection revealed a non-standard gasket and an out-of-adjustment flapper valve. However, the Australian Transport Safety Bureau (ATSB) determined both to be unlikely sources of CO, citing the lack of smell from the foreign material and the absence of observed defects in the exhaust system. Consequently, the source of the CO leak was not identified.