Inhofe, 51, the son of U.S. Senator Jim Inhofe, died on November 10, 2013, when his Mitsubishi MU-2B-25 twin-engine turboprop airplane crashed while on approach to Tulsa International Airport.
The investigation found that Inhofe was known as a careful and experienced pilot with 2,874 of flight hours. He had only 12 hours of time in the MU-2 and the crash happened the first time he flew it alone.
The NTSB says the airplane was properly certified, equipped, and maintained and that it showed no evidence of any pre-impact structural, engine, or system failures.
"The investigation also determined that the pilot was properly certificated and qualified in accordance with applicable federal regulations, including Special Federal Aviation Regulation (SFAR) No. 108, which is required for MU-2B pilots and adequate for the operation of MU-2B series airplanes," the NTSB report says.
In fact, Dr. Inhofe was returning to Tulsa from Kansas where he had just completed the special training when the crash happened.
The NTSB found no evidence of any preexisting medical or behavioral conditions that might have adversely affected his performance on the day of the accident.
The plane flew normally from takeoff in Kansas until the beginning of the approach to Tulsa International Airport, the NTSB says, when the airplane strayed off course.
The airplane then turned 360 degrees o the left at an unusually low altitude. When a controller asked the pilot what was going on, he said he had a "control problem" and then stated he had a "left engine shutdown."
Investigators say they found the airplane's flaps extended 20 degrees and the propeller blades on the left engine in the feathered position. The left engine's fuel shutoff valve was in the closed position, which would indicate the engine was not operating.
They say the airplane was not configured in accordance with the airplane flight manual engine shutdown and single-engine landing procedures, which state that the airplane should remain in a clean configuration with flaps set to 5 degrees at the beginning of the final approach descent and the landing gear retracted until landing is assured.
The NTSB says the fire that started after the crash damaged the cockpit instruments and prevented investigators from determining the pre-impact position of fuel control and engine switches.
They also report that the airplane was not required to have any kind of crash-resistant recorder, so investigators have no way of knowing precisely how the pilot had the controls configured.
As a result, the NTSB says the probable cause of the crash was the pilot's loss of control of the airplane while flying with one engine shut down. It can't determine what caused the loss of control and why Dr. Inhofe shut down one engine.