Being a flatland pilot, I was unaccustomed to the very different cues which one could experience in mountains or canyons. When I had to send another flatland pilot on a contract in the Canadian Rocky Mountains, I insisted that he take a "mountain course" so he would enter the program with the necessary tools. My attempt was a miss - he died in a crash when he took the multi million dollar modified Cessna 207 for an unauthorized tour in a canyon. Indeed, he did have lots of room to maneuver, it was not tight. However, he lost reference, and crashed into a slope. The accident was intensely investigated, but little more detail was ever revealed, he just lost his sense of up. I did not understand, it was a perfect flying day, the sky and horizon were easy to see.
It was a few years later that I was taking training in a helicopter in a mountainous area not far away. We were about 3000' below the crest of the mountain ridge, and my instructor told me to "toe in" to the surface of the slope. This means flying the helicopter gently so the toes of the skids contact the slope firmly, and holding a hover there. I found this really difficult, because as I approached the slope, my instincts were to pull up, which in the hovering helicopter translates to backward. The only way I could mentally overcome this was to reference the small trees I saw, trees always point up. With that, I toed in, and sat hovering there, with the rotor disc whisping some grass.
As I perceived my environment, knowing that even toed in, I was still flying, I realized that my horizontal reference was 40 degrees up, 3000 feet away. That would at best be confusing, at worst useless as a reference while flying fixed wing in there. With a blur of green and rock, not perceiving any tree tops nor horizon, horizontal reference is lost. At that moment, I knew how my pilot had crashed in the canyon, he turned, saw only green and rock, with no horizon, and lost it.
I teach this now, very carefully!