What happened
During a recent flight, the crew of an aircraft observed a color change on the carbon monoxide (CO) detector card, indicating the presence of the gas. Along with this visual cue, the crew members began experiencing very mild physical symptoms and cognitive impairment. Following the event, medical examinations performed on the crew revealed mildly elevated carboxyhaemoglobin levels, confirming that carbon monoxide had entered the cabin.
The investigation
Investigators examined the aircraft to identify the origin of the gas. While the CO detector card was noted as a potential source of unreliable readings, the investigation found that the same aircraft had been involved in two similar incidents nine months earlier. Furthermore, the detector had been replaced with a new unit only two days prior to the second previous incident, making a faulty detector unlikely to be the sole explanation.
During the inspection, investigators identified a non-standard gasket and a flapper valve that was out of adjustment. However, these components were ruled out as likely sources because there was no detectable smell from the foreign material and no visible defects were found within the exhaust system. Subsequent ground runs and flight tests conducted on the aircraft failed to replicate the elevated CO levels in the cabin, and the specific source of the leak remains undetermined.
Findings
- The crew was exposed to elevated levels of carbon monoxide within the aircraft cabin.
- Post-incident blood tests confirmed the presence of carbon monoxide in the crew's systems.
- An inspection of the aircraft identified an improperly adjusted flapper valve and a non-standard gasket, though neither was deemed the primary cause of the leak.