Background
During post-incident ground runs, elevated levels of carbon monoxide (CO) were detected in the aircraft cabin. The issue was resolved after repairs to the exhaust stack and the scat hoses, indicating that the source of the CO was associated with these repaired components.
Findings
Investigators did not identify any obvious breaches that would have allowed CO to enter the cabin. Consequently, the exact pathway by which exhaust gases entered the cockpit on this occasion was not immediately clear.
Pilot Symptoms and Tests
The pilot reported physical symptoms and cognitive effects that were more likely associated with CO poisoning. This conclusion was supported by positive indications on a CO chemical spot detector and a blood test that showed a mildly elevated level of carboxyhaemoglobin (COHb).
COHb Levels and Variability
While the extent of the pilot's symptoms for the recorded COHb level was inconsistent with published literature, research has demonstrated that adverse effects on cognitive functions can occur at COHb levels as low as 3 percent. It is also known that individuals can react differently to COHb.