Unfortunately, this disorientation crash profile is all too common. He got vertigo as I have many, many times but he didn't have the experience or instrument training and discipline to counter the vertigo. He lost control of his aircraft. You can see from the radar plot he was all over the place.
To put the final dive in perspective, for this type of aircraft, a 1000fpm (feet per minute) climb or descent rate would be on the high side of normal. In excess of 4700fpm is going to exceed the structural limits of the aircraft. Had he not slammed into the water, he would have torn the wings off the bird in short order.
"A performance study of the radar data revealed that the target began a descent from 5,500 feet about 34 miles west of MVY. The speed during the descent was calculated to be about 160 knots indicated airspeed (KIAS), and the rate of descent was calculated to have varied between 400 and 800 feet per minute (fpm). About 2138, the target began a right turn in a southerly direction. About 30 seconds later, the target stopped its descent at 2,200 feet and began a climb that lasted another 30 seconds. During this period of time, the target stopped the turn, and the airspeed decreased to about 153 KIAS. About 2139, the target leveled off at 2,500 feet and flew in a southeasterly direction. About 50 seconds later, the target entered a left turn and climbed to 2,600 feet. As the target continued in the left turn, it began a descent that reached a rate of about 900 fpm. When the target reached an easterly direction, it stopped turning; its rate of descent remained about 900 fpm. At 2140:15, while still in the descent, the target entered a right turn. As the target's turn rate increased, its descent rate and airspeed also increased. The target's descent rate eventually exceeded 4,700 fpm. The target's last radar position was recorded at 2140:34 at an altitude of 1,100 feet."
