Sounds like he was partially blinded. We used to routinely take pilots in normal flight and have them cover their eyes and they would within seconds stray from straight and level flight. Within seconds of that, they would depart controlled flight, and the other pilot would recover. The other senses are tricked during this type of flight, and the only way to control the aircraft is via crosschecking of the instruments. A blind pilot would not be able to do this, so I'd say he was experiencing partial blindness which would throw off his depth perception, making a landing very difficult. It's pretty cool that the RAF pilot was able to be his depth perception and calming force to get him down. Pretty amazing.