The NTSB hasn’t yet released its probable cause finding concerning the Pilatus crash at Butte, Montana that killed the pilot and his 13 passengers.  But it has just made public its “docket.”  The docket sheds some light on what may have been happening in the cockpit in the minutes leading to the crash.

The flight was bound for Bozeman. Suddenly, the pilot diverted to Butte, which was only marginally closer.  Though the pilot never explained the reason for the diversion, the docket suggests that the

pilot’s change of course was of an urgent nature and that the pilot was under some sort of stress. 

For example, the pilot turned toward Butte without first asking the controller for clearance to do so. Rather, the pilot asked for a clearance only after he completed the turn. That’s a pilot “deviation.” It would have been justification for an FAA enforcement action against the pilot. Turning off course without a clearance is not, under normal circumstances, a mistake you’d expect this highly experienced pilot to make.

Shortly after belatedly asking for and getting the clearance to turn toward Butte, the pilot descended from his Pilatus Flight Pathassigned altitude. He asked for a clearance to do that only when he was well on his way down. That’s another deviation, and a considerably more serious one. According to the docket’s ATC report

At 1404:09, N128CM began descending out of his assigned altitude of FL250. At 1404:34, N128CM requested a lower altitude from ATC.

Later, the pilot asked the controller for a clearance to Butte. But as the controller pointed out to the pilot, he had already been issued one. Asking again for the clearance was not a deviation, but it’s another indication that the pilot was under stress.

Was the stress due to a mechanical problem, a medical issue, or something else? 

The NTSB materials discuss the possibilities of airframe icing as well as icing in the fuel system.  Airframe icing might explain the ultimate loss of control of the aircraft.  But it would not explain the diversion to Butte.  Ice in the fuel system might explain the diversion to Butte, but it wouldn’t explain the loss of control of the aircraft.

I wrote here more than a year ago that a medical issue such as a heart attack might explain both the diversion and the ultimate loss of control of the aircraft. It now seems that the NTSB is thinking about that too. From one of the reports:

A previous co-owner of the airplane . .reported that he had flown about “a dozen times” with the accident pilot sitting in the cockpit seat next to him. . .   He revealed that on one flight in 2008, the pilot broke out in a sweat that lasted for about a minute or less.

Sweating can be a symptom of heart trouble. Though the previous owner didn’t see any signs that pilot was having chest pains, a cardiac condition has to be a consideration. The coroner who performed the autopsy on the pilot found evidence of heart disease, but felt a heart attack was unlikely because the pilot never gave air traffic control any indication that he was having a problem.  

Pilots are quick to alert air traffic control of any mechanical problems they may be having.  Personal physical problems are a different matter.  But without actually telling air traffic control he was "having a problem," maybe the pilot did communicate just that. 

The NTSB’s entire docket can be found here.