US investigators believe the pilot of a Cessna Citation V became incapacitated before the aircraft entered a tight spiral descent from 31,000ft and disintegrated on impact with terrain in Oregon’s Mutton mountain range.

The pilot was not type-rated on the executive jet – having discontinued training before completion – and the accident probably occurred on the first occasion he had flown it solo.

Although he was a private pilot, with experience in a number of aircraft types including jets, he had “struggled significantly” in high-workload environments and had “difficulty” operating the aircraft’s recently-installed avionics suite, according to the National Transportation Safety Board.

Air traffic control records indicate communication difficulties with the pilot after he took off from Troutdale airport in Portland.

FBO Citation V accident-c-NTSB

Source: NTSB

Surveillance video captured the jet (N3RB, on the left) taxiing for departure at Troutdale

Approach and tower controllers had to make repeated attempts to contact him after he failed to respond to certain instructions, and he also manoeuvred the jet onto an incorrect heading following clearance to a waypoint. An instruction to turn right was correctly read back, but the aircraft turned left.

When control was transferred to Seattle centre, the controller immediately issued a low-altitude alert warning, as the jet flew south of the peak of Mount Hood. After the pilot climbed, he was transferred to a new radio frequency – initially reading back the wrong one – and was then cleared to 37,000ft.

Some 4min later the aircraft had reached 27,000ft when it began deviating to the right, and the pilot did not respond to alerts from the controller. It continued climbing for about 2min, reaching 31,000ft before entering a spiralling right turn. The aircraft descended in a right bank of 60-70° at a rate of 2,500-5,000ft/min before striking the ground.

While the pilot had previously indicated that he preferred to hand-fly the aircraft, and the descent was consistent with the autopilot’s not being used, analysis of the flightpath showed the pilot was probably “not manipulating the controls” at the time, says the inquiry.

Citation V wreckage-c-NTSB

Source: NTSB

Such was the impact that only fragments of the Citation V remained recognisable

Evidence from the investigation shows consistency with pilot incapacitation. Examination of the oxygen system suggested it had not deployed, and the NTSB states: “The reason for [the] incapacitation could not be determined.”

Review of the pilot’s medical history uncovered conditions and medications that the pilot had not reported to the US FAA.

He suffered psychiatric conditions and regular medicines taken over the year prior to the accident included alprazolam – which the FAA classifies as a ‘do not fly’ medication – and sertraline, which cannot be taken without FAA authorisation.

“None of the pilot’s known medical conditions or medications would have directly caused incapacitation,” says the inquiry. “But the pilot may have had undiagnosed disease or had some acute event that would have incapacitated him.”

Little useful information could be obtained during the pilot’s autopsy. The inquiry into the crash, on 9 January last year, points out that his age – 72 years – and other factors, including high blood-pressure and hypertension, placed him at risk of conditions such as a heart attack.