I do accept that the cause may be something other than pilot incapacitation. However, I will relate this incident because it does involve a rare form of temporary incapacitation, and while that was miraculously resolved with a mostly intact airplane, had it not been, the very same type of thread would have been started, if they had forums in 1983.
I lived in Prince George at the time and I was Chief Pilot and Ops Manager there at a small charter company and flight school. I also owned, through a limited company, a Cessna 310Q. It was Christmas season and my wife and I had just returned from Zihuatenejo/Ixtapa and were in Salmon Arm, where our families were both from. A couple of days after, I had to go to PG on some matter and took a brother and a friend along for what was to be a day trip.
We took off and climbed to 8500. I noticed that I had flu symptoms, not unusual for me after a commercial airline flight. Bases were sometimes below the altitude and the trip did involve some maneuvering to stay VFR and avoid having to ask for a pop up to continue. I preferred to remain VFR given the stuffed head feeling I was having. However, about 25 miles short of PG, we encountered solid cloud and virga on most headings north. I called Center, got the pop up, along with a clearance for the approach in PG. The transition altitude was 7000 there, and prior to starting descent and completely in cloud, I closed my mouth and held me nose shut and forced my ears to pop. As soon as I did that, I experienced a fairly rapid closure of my field of view to a small circle, and then nothing. The last thing I saw was the artificial horizon start to move to a bank. Before I went black, I moved the yoke to correct. I think I moved it the wrong way. I went blind, and pulled back the throttles. I'm told that it was about 10 seconds and I heard a shout from one passenger, and my sight came back in reverse order to how it went. The first thing I saw was the back seat passenger up near the roof. I began to do partial panel stuff, with the airspeed indicator already well into the yellow. The guy came off the roof and down onto the seat. I could not rely on the AH, but I did manage to level the wings with the turn coordinator. I had started the technique of pulling back on the yoke till the airspeed stopped increasing, the make the thing back to level technique. Before that happened, we plunged out of the clouds, power off but engines running, wings level, and now about 70 degrees nose down. We had pulled some serious Gs. I was worried about the structural integrity of the aircraft. I called PG tower, said I had a medical emergency and had briefly lost my sight. As we were in bare VFR, and I wasn't going to lose sight of the ground again, I began discussing a highway landing option with ATC. As soon as we started that, I looked north and saw PG airport between virga to the ground. I told tower I could proceed VFR to the airport, but maintained the emergency as I had no idea if I was going to lose sight again, and I didn't know where we were structurally.
We landed and taxied to the Esso. I was shaken. I had barely spoken a word to the passengers as they could hear the situation in their headsets. I parked and went inside. I sat down, got a strange want to sneeze feeling in my nose, and then felt my chest get wet. I put my hand under my nose and the better part of a cup of water came out of my nasal cavity. Sea water.
About three days before, I and some friends, one of which went on to become Chief Pilot of Conair,,took scuba lessons there in the bay at Zihuateno. Part of that is to stand by the sea, take your mask, fill it with sea water, clamp it to your gob, and tilt your head back, and clear the mask by blowing. People were spluttering and puking all around. I called Transport Medical and told them all this. They figured it out. I had sea water trapped in my nasal cavity from the mask bs. It remained there right up to sitting down in the Esso. When I did the ear pop thing, that pressure transmitted through the water inside and affected the optic nerve, or nerves, not sure which. That led to the blindness, and that went away when the pressure stopped.
In the blind period, I do remember envisioning us splattered all over a mountainside, and people who knew me saying "Why in hell would Paddy do that?".But, the ones who knew me best would immediately presume pilot incapacitation. There are some types of incapacitation that can have you and your aircraft gone in seconds, and unlikely to come back.
I'm not just talking out of my arse when I express the opinion this Cirrus may have gone down as a result of pilot incapacitation. I never put out a Mayday through the whole thing until the airplane was back under control. Then I did.
For young pilots reading this, this is your lesson to take away...no matter that you may believe you are certain to die in the next few seconds, never stop being a pilot, never stop flying the machine.