Note: the following information is based mainly on the statements made by the pilot and the maintenance shop. This information has not been independently validated by the BEA.
This is a courtesy translation by the BEA of the Final Report on the Safety Investigation published in May 2020. As accurate as the translation may be, the original text in French is the work of reference.
1 - History of the flight
The pilot, who was accompanied by a photogrammetry operator, took off under VFR at about 09:10 from Moulins-Montbeugny aerodrome bound for Colmar-Houssen (Haut-Rhin).
The pilot reported that, during the initial climb, five to six minutes after take-off, the carbon monoxide (CO) detection patch turned black. It then changed to a brown colour for the remainder of the flight. The small lateral windows were opened to ventilate the cockpit with fresh air and mutual surveillance was instituted through to arrival at their destination.
Before arrival, while in radio contact with Basel Information Service, the pilot informed the controller of his intention to leave the frequency to switch to the Colmar control tower frequency. The controller replied that he had previously tried three times to ask him to change frequency without getting an answer from him. The operator, who was a student pilot at the time of the incident, confirmed that he had not heard anything either.
Once on the ground, after fuelling the aircraft, the pilot and operator reported experiencing headaches, dizziness, nausea and difficulties concentrating.
At the A&E, the pilot and operator were put on oxygen for three hours. The hospital doctor issued a work accident report with a 48-hour sick leave certificate.
2 - Additional information
2.1 Pilot information
The pilot holds an aeroplane commercial pilot licence (CPL (A)) issued in 2015. He stated that, on the day of the occurrence, he had logged 490 flight hours, including 80 hours on P 68s and 80 hours in the previous three months. He also had an IRME (A) rating. He held a class-1 medical certificate, which was issued in 2018.
2.2 Aeroplane searches
An initial visual inspection of the engines by maintenance shop personnel did not find any faults, leaks or holes in the exhaust systems.
During a first test flight to check CO levels in the cockpit, the rate measured was 200 ppm.
After the first test flight, SB 257 (see paragraph 2.3), which was issued on 4 June 2018 following the serious incident to the P 68 registered F-HVEY which experienced CO contamination in flight on 10 April 2018, was complied with.
During the second test flight, the CO rate was initially 50 ppm in all flight configurations.
After 30 minutes of flight, the rate suddenly rose to 250 ppm at the instrument panel and then to above 2000 ppm on the right side. It was thus suspected that there was a leak between the right engine exhaust and the heat exchanger on the heating system. After inspecting the exhaust pipe with an endoscope, no visible faults were found.
After immobilising the heating system on the right side, on the third test flight, the CO rate was a maximum of 10 ppm on the ground and 4 ppm in all flight configurations.
After removal of the exhaust pipe, barely detectable signs of leakage were observed at the rear end. The welds in this area were found to be porous.
After changing the exhaust pipe and reassembling the heating system on the right-hand side, on the fourth test flight, the CO rate was confirmed to be between 10 and 4 ppm in all configurations of the test flight with or without the heating switched on.
2.3 Measures taken by the manufacturer
On 23 May 2018, service letter SL 52 was issued to all operators to inform them to "be aware to seal opportunely the photogrammetric hatch cover (if installed) inside the cabin, located between fuselage frames No.6 and 8, and furthermore to leave all the air vents present inside the aircraft free from obstruction, in order to avoid any possible access of carbon monoxide inside the cabin and guarantee its correct ventilation".
On 4 June 2018, Service Bulletin No 257 was published.
It informs operators that "Vulcanair has been advised of CO contaminations on a little number of aircraft of the entire flying fleet.
CO contamination into the cabin was caused by poor aircraft maintenance status related to inadequate cabin and firewall sealing conditions, and/or cracks or holes or poorly fitting components in the exhaust system, which contaminate the cabin air.
Because of the above, Vulcanair recommends maintaining the aircraft always in a good condition and also recommends the embodiment of this Service Bulletin supplying the information for improving cabin CO contamination prevention by installing additional defences and air way-outs.”
3 - Lessons learned and conclusion
Although a one-off communication transmission problem could not be completely ruled out, the failure of the pilot or his operator to read back the three messages from the Basel Information Service controller was probably due to carbon monoxide poisoning.
The poisoning of the pilot and his operator in flight was caused by a gas leak from the right engine exhaust pipe, which contaminated the aircraft cabin through the heat exchanger on the heating system. This leak was difficult to detect during maintenance operations. Failure to embody the modifications recommended in SB No 257 may have contributed to an increase in the contamination level.
The presence of a CO detector in the cockpit and its verification by the pilot in all probability mitigated the consequences of the incident.
Use of an onboard detection system must serve to encourage the pilot to land as soon as possible when contamination is detected.
 The back of the detector mentions three colours: yellow = normal; green = caution, ventilate; midnight blue = danger, evacuate.
 The engines are naturally aspirated.
 By way of an indication, the World Health Organization (WHO) has defined reference values that are considered harmless based on the duration of exposure : 26 ppm for 1 h; 52 ppm for 30 min; 90 ppm for 15 min. Headaches, dizziness and nausea occur from 200 ppm onwards. An exposure of 1,000 ppm kills in one hour.
 An SB is not mandatory.
 See the F-HVEY serious incident report via the following link: www.bea.aero/en/investigation-reports/notified-events/detail/event/serious-incident-to-the-plane-vulcanair-p68-registered-f-hvey-on-10042018-en-route/
 The heating system on this P68 Victor model is of the conventional type with a hot air exchanger on each engine, unlike the electrical system fitted to the P68 TC (F-HVEY) involved in the serious incident on 10 April 2018.
 The rate is always higher on the ground than in flight because the laminar flow of gases around the aircraft during ground manoeuvres is not so smooth.