The question on everybody's mind is: Just what happened to the Pilatus PC12 that crashed on Sunday while bound for Bozeman (but diverting to Butte)? The crash killed 14 people, including the pilot, six other adults, and seven children.

You've probably read the details elsewhere, but here are the basics: The airplane flew from Redlands (located in the eastern part of the Los Angeles Basin) to Vacaville, California, then on to nearby Oroville, where it picked up passengers and headed out for Bozeman. The families aboard were reportedly heading out for a ski vacation at an exclusive ski club near there. The flight should have taken about three hours.

When the airplane was nearing Butte, the pilot told controllers that he was changing his destination from Bozeman (around 80 miles away) to Butte. This is a common enough exchange--the pilot didn't offer a reason for the diversion (or simply change of plans) and the controllers didn't ask. If anything unusual was going on with the airplane, the pilot didn't mention it to controllers.

Though speculation abounds, just what happened next is a mystery. After the pilot changed frequencies, presumably to unicom at the uncontrolled field at Butte, Bert Mooney Airport, the airplane crashed, just short of the runway, apparently in an extreme nose down attitude, into a cemetery. All 14 aboard surely died instantly.

With this crash, as with the Buffalo crash of a Q400 last month, ice is a leading suspect. Unlike in Buffalo, the PC12, if it encountered icing conditions at all, encountered them for a short period of time, and the pilot spoke with controllers after he reported having the airport in sight. But ice could explain the loss of control at low altitude without, as in Buffalo, the pilot having made any reports of control issues.

There was a post crash fire, though speculation abounds over what that means in terms of fuel remaining. One blogger pointed out that it doesn't take much of a flame to ignite a fire in pine trees, which can be seen burning in post crash photographs. Did the pilot divert because he was low on fuel? Did the crash result due to fuel exhaustion? If so, the NTSB might be able determine that, by examining the post crash fire and the patterns of the flames, among other things.

Weight and balance issues are at the forefront of the investigation, too. And everyone has been pointing out that the PC12 is certified almost always for a maximum of 11 occupants, 10 passengers and the single pilot. With 14 aboard, there were almost certainly at least three too many occupants for the number of belts available.

While some in the FAA might like to think so, airplanes don't crash because they violate FAA regulations. However, CG issues are a possibility that the NTSB is looking at closely. How was the airplane loaded? Where were the bags? How much baggage weight was there? With that many people going on a ski vacation, one might safely assume there were a lot of bags. The PC12 has both a stick shaker and a stick pusher, and the PC12 lands slowly, at just above the normal 61-knot stall speed for singles--it gets credit for the pusher and shaker, and its extremely crashworthy seats. So, while the airplane will still stall, and spin, the airplane is designed to make that as unlikely a scenario as possible. 

Then again, if there were CG issues, most likely a too far aft CG condition, all bets are off in terms of control. A question that has been asked a lot is, will the CG shift aft dramatically as the PC12 burns fuel? The answer is, no, not dramatically. Could it have been a factor still? At this point, it's impossible to rule it out. That's impossible to say right now. In fact, it's impossible to say much authoritatively at all. The NTSB is trying to find out how much fuel the airplane took on and where, the number and weight of bags and passenger loading, seating arrangements and weights. 

Which leads to another question: Was the PC12 over gross? Depending on how much fuel the pilot loaded, the answer could be "yes" or "no." The PC12 has a long range and big tanks, so it can still fly a three hour flight without full fuel. The question remains though: Might a pilot who was willing to put 14 people into an airplane certified for 11 be willing to bend other rules too? That's a question investigators will try to answer as well. It's probably moot. By the time the airplane arrived near Butte, it had burned off around three hours of fuel. If it had been over weight for takeoff, it mostly likely wasn't by that point. 

And surely the NTSB will be looking at pilot incapacitation as a possible cause. Could it be that the pilot didn't report any problem with the airplane because there was no problem with the airplane? Incapacitation can happen to any of us, and the onset can be subtle. It's also one of the hardest things for investigators to determine, especially in a crash as violent as Sundays and especially when there's a post crash fire, as there was.

Or maybe there was a problem with the airplane. There was reportedly no flight data recorder or cockpit voice recorder on board, nor would one expect there to be, but the NTSB might be able to retrieve some valuable data from the engine monitoring system or the navigators, if that data is recoverable after the crash. Tracking down a mechanical problem, if there was one--and the PC12 has what appears to be an excellent safety record--would be a tough chore, but it's been done before.

In an accident as tragic as this one is and as mysterious as it's shaping up to be, any bit of evidence is critical. Because everyone wants to know what happened and why. Whether we ever learn those answers is an open question at this early stage of the investigation.