What happened
On May 14, 2025, a pilot operating a Flight Design CTLSi, registration SP-SMED, departed Katowice Muchowiec (EPKM) for a recreational flight. The flight plan involved navigating through the Kraków/Balice (EPKK) TMA, with an intended cruise altitude of FL150. During the climb, the pilot requested and was granted permission to climb to higher altitudes, eventually transitioning the flight from VFR to IFR status at the pilot's request.
As the aircraft climbed to approximately FL130, the pilot donned an oxygen mask. However, due to an error during the pre-flight inspection, the oxygen valve for the pilot's mask remained closed, while the valve for the passenger mask was opened. Consequently, the pilot was breathing exhaled air, leading to a buildup of carbon dioxide (hypercapnia) and a lack of sufficient oxygen (hypoxia).
By the time the aircraft reached FL248, the pilot's physiological state had significantly deteriorated. Air Traffic Control (ATC) noted that the pilot's radio communications became slow, slurred, and unclear. At one point, the pilot failed to respond to multiple attempts by the controller to establish contact, leading ATC to suspect hypoxia. The aircraft was observed descending at an extremely high rate, briefly exceeding the maximum operating speed (VNE) and reaching a vertical descent rate of approximately 7,000 ft/min.
Upon noticing low blood oxygen saturation on a handheld monitor, the pilot switched to the passenger's oxygen mask, which provided functional oxygen. This action restored the pilot's situational awareness and clarity of speech. The aircraft subsequently descended to a safe altitude and landed safely at EPKM without further incident.
The investigation
The PKBWL investigation examined the aircraft's oxygen system, the flight data from the Dynon onboard computer, and ATC communications. The investigation established that the aircraft was fully functional and maintained all required maintenance. The investigation focused on the sequence of events leading to the pilot's physiological impairment and the subsequent flight deviations.
Findings
- The primary cause of the incident was hypercapnia and hypoxia resulting from the use of an oxygen mask that was not delivering fresh oxygen.
- The physiological impairment was caused by an error during the pre-flight check, where the pilot incorrectly configured the oxygen supply valves.
- The pilot's impaired judgment led to an excessive descent rate that exceeded the aircraft's VNE.
- The pilot requested an IFR flight status despite not holding the required IFR ratings.
- ATC effectively managed the situation, providing assistance and monitoring the aircraft until the pilot regained situational awareness and landed safely.