Subtle incapacitation of an Air Canada Boeing 767-300 first officer during a transatlantic flight led the captain to divert the aircraft to Shannon, Ireland, landing with the assistance of a flight attendant who held a commercial pilot licence.
The 28 January 2008 incident is described as "serious" in a Irish Air Accident Investigation Unit synoptic report.
It says the captain's awareness that all was not well began pre-flight when the co-pilot, who had positioned into Toronto Pearson Airport from Montreal as a passenger, arrived with little time to spare and seemed "quite harried".
The captain told the co-pilot to meet him at the aircraft, assuring him that all flight preparations were complete. After the flight left on time and climbed to flight level 360 the captain became increasingly worried about the co-pilot's condition and behaviour, which was out-of-character.
More than once the co-pilot re-entered the cockpit using non-standard security procedures, and eventually began to speak in an unco-ordinated way, eventually becoming "belligerent and unco-operative", and failing to fasten his harness on returning to his seat.
The captain called for the assistance of the cabin crew to remove the pilot from his seat, identified two doctors on board who attended to him and, in the absence of any pilots among the passengers, enlisted the help of the CPL-qualified flight attendant.
Once in VHF radio contact with Shanwick Oceanic, the captain made a 'pan' call and requested diversion to Shannon, where the weather was good compared with the aircraft's destination, London Heathrow. The aircraft landed safely.
Medical staff met the co-pilot who was hospitallised for 11 days before being returned to Canada on a medevac flight, where his treatment continued.
The AAIU report does not describe the medical diagnosis, but lists Transport Canada advice about pilot subtle incapacitation for which, it says, the most common causes include hypoxia, hypoglycaemia, extreme fatigue, alcohol or toxic substances, and which can involve neurological causes like stroke or a brain tumour.
Irish investigators have not made any recommendations.