What happened
On 6 March 2021, a Reims Cessna F4/06 performing a cargo flight from Sweden to the United Kingdom experienced an incident involving oxygen supply. While cruising at 18,000 feet over the North Sea, the pilot began to suffer from a sudden headache and noticed a decline in cognitive function, specifically struggling to use a function on an electronic flight bag.
Recognising these as potential signs of hypoxia, the pilot checked the aircraft's oxygen equipment. A finger pulse oximeter indicated that oxygen saturation levels were significantly lower than normal. The pilot attempted to increase the oxygen flow through the regulator, but when this failed to resolve the symptoms, he switched to a secondary oxygen bottle. This action successfully restored oxygen levels, and the flight continued to East Midlands Airport without further issues.
The investigation
Investigators examined the oxygen delivery equipment, including both the primary and secondary bottles and the regulator. The components were removed and tested at an approved maintenance facility, which found that the regulator functioned within all required parameters and no faults were detected in either bottle or the regulator.
The investigation also looked into the use of pulse oximeters. While these devices provide a rapid assessment of blood oxygen, the investigation noted that they can be unreliable. Specifically, hyperventilation—a symptom of hypoxia—can cause the device to show higher oxygen levels in the extremities than are actually present in the brain.
Findings
- The pilot experienced symptoms of hypoxia due to a reduction in the oxygen supply.
- The pilot's ability to identify and rectify the issue was aided by previous company training and the use of a pulse oximeter.
- No mechanical fault could be established in the oxygen bottles or the regulator following laboratory testing.
Safety action
- The operator has initiated a retrofit programme for all Reims Cessna F406 aircraft operating above 10,000 feet to install a fully integrated oxygen system using masks instead of cannulas.