BUTTE, MONT. — The investigation to unravel the cause of the plane crash that killed 14 Californians en route to a Montana ski trip focused Tuesday on the central mystery of the flight: Why did the pilot, not far from his destination in Bozeman, Mont., suddenly change his mind and attempt the fateful landing in Butte?
The pilot made the decision less than 20 minutes from Bozeman, where a vacation at the exclusive Yellowstone Club, near Big Sky, awaited the three families on board the small, single-engine turboprop.
Was there a medical emergency? Was the plane running low on fuel? Was there a mechanical problem? Had someone phoned one of the passengers and asked them to stop?
All possibilities will be looked at, federal investigators said as they prepared to conclude the on-scene recovery of the accident debris and truck it to a hangar in Bozeman for a painstaking reconstruction.
"There are some interesting attributes to this [investigation]. One is the question of the 25,000-foot request for diversion to come to Butte. There are a lot of questions, but it begins with that question," said Mark Rosenker, acting chairman of the National Transportation Safety Board.
"That's where we believe -- that's where we're beginning to look at things," he said.
On the plane were several doctors and dentists and seven of their children, all under 10. Prominent Redlands dentist Irving M. "Bud" Feldcamp III, whose two daughters along with their husbands and children were aboard, had driven with his wife to Montana, where they had planned to meet the group.
Feldcamp owned the Pilatus PC-12. The pilot, 65-year-old Bud Summerfield, was an Air Force flying veteran with more 8,500 hours of civilian flying experience.
Authorities said Tuesday that they had recovered a key engine-monitoring device, known as the engine instrument system trend card, which will allow them to determine if there were anomalies in the engine's operation.
Unlike commercial jetliners, the PC-12 was not equipped with a cockpit voice recorder and flight data recorder, which normally guide investigators in unraveling the causes of airplane crashes.
Investigators said Tuesday that the airplane's flaps were in the up position, possibly indicating the pilot was not immediately preparing to land, though the landing gear was extended.
They also said they have ruled out any problems with the plane's elevator cable tension controls, which are the subject of an impending federal airworthiness directive on PC-12s.
Complicating the investigation is that whatever caused the pilot to announce his plan to divert to Butte may be unrelated to what finally caused the plane to crash.
Many aviation experts have speculated that the plane's descent through the frigid cloud cover over Butte may have caused the plane's wings to ice over.
Ice on the tail could explain the plane's sudden, nose-over plunge.
"It looked like the tail-end lifted up, then it spun around at a 90-degree angle to the Earth and then it just dove right into the ground. When it hit, it exploded, in a big old fireball," Harley Howard, who saw the crash, said in an interview.
But Thomas Bosshard, CEO of Pilatus Business Aircraft Ltd., the manufacturer, said the PC-12 is equipped with inflatable de-icer boots on the wings and other equipment to control ice on the propeller, horizontal stabilizer and engine air intake.
"The PC 12 actually carries the ice very well because it has a very thick, slow airfoil," Bosshard said in a telephone interview. "Of course, in any case, if you have severe icing, you should try to get out of it."
Another puzzling aspect to the crash is that the plane appeared to be neither on a final approach to the airport's northerly runway nor in a standard traffic pattern.
The crash site's location well off to the side of the runway suggests the pilot may have attempted a dangerously steep turn to enter the downwind leg of the runway approach -- a maneuver that would be especially perilous in a plane heavy with passengers or ice.
Because airplane crashes are often the result of a series of events, the final answer may be a combination of factors.
In any case, it will probably not be arrived at for months, Rosenker said.
"I wish we had some really good working theories. In many times in investigations we work on, we can begin with a series of working theories. In this particular one, nothing is off the table yet."