Aftermath: An Ambiguous Accident

Was the "probable cause" really probable?

Crash Icon

Crash Icon

As regular readers of this column know, I at times cavil at the NTSB’s published findings of “probable cause.” Sometimes that “probable” is there from force of habit, the conclusions being beyond dispute; but often probable causes sound as if their components had been picked from a menu, without careful consideration of the nuances of the particular case.

Here is the “probable cause” of an accident that took place in 2012:

The non-instrument-rated pilot’s continued flight into instrument meteorological conditions, which resulted in the design limits of the airplane being exceeded and an in-flight breakup.

The airplane was a classic Bonanza, manufactured in 1947. Originally it would have had a 165 hp Continental E185 engine — what later continued life as the O-470 — but at some point in the airplane’s 65-year, almost 5,000-hour career, the engine had been upgraded to 205 horsepower. There were three people — the pilot (who was not a doctor, by the way), his wife and his mother — and three dogs aboard. All died instantly on impact.

The flight began in Minneapolis, bound for Colorado. The NTSB dutifully observes that there was no record of the pilot’s having obtained a weather briefing, but that fact has become meaningless in the era of online weather and does not imply recklessness any more than that perennial shibboleth of reporters, the lack of a flight plan, should. The 250-hour pilot had an iPad, and there is no reason to doubt that he checked the weather before the flight. The forecast to the southwest called for 2,500-foot ceilings, with visibilities occasionally as low as 3 to 5 miles in mist.

Now, if you are planning a flight through an area of low ceilings and you call a flight service station for a briefing, a good briefer will look over the metars for airports along your route and tell you whether their recent reports differ substantially from the outlook. You can do that online as well, but chances are you may not be so familiar with the reporting stations en route, and so it might not have occurred to you, after seeing that encouraging prediction of 2,500-foot ceilings west of Minneapolis, to check what the automated reporting station at little Glencoe, 35 nm away, was seeing. If you had done so, you would have discovered that Glencoe had not read the forecast and was showing a 900-foot ceiling and 5 miles.

So we know that at the time of the accident, which took place 5 miles north of Glencoe, the ceilings were slightly below VFR standards, but the visibility was pretty good, the wind was calm, and there was no convective activity. In other words, one could be airborne around there without danger of anything but violating a few FARs.

The brief version of the accident report states that “Witnesses in the area reported hearing the airplane flying from east to west but could not see the airplane due to the low clouds and reduced visibility.” The long version mentions “several” witnesses hearing the airplane but makes no mention of anyone seeing it. Having discovered that witness statements can often be mined for information that the published NTSB report omits, I went to the NTSB “dockets” site, where they store the documents, photographs, maps and other records on which reports are partially based. There I found two witness reports that had actually been transcribed; it’s possible that others were received but not formally recorded. One of these came from a point about 20 nm east of the accident site; the witness heard an airplane traveling from east to west, did not see it, and reported that its engine was revving up and down “like a lawnmower running out of gas.” Assuming that the airplane could cruise at something under 3 miles a minute, this would have been seven or eight minutes before the crash.

A second report preceded the crash by only 30 seconds and was much more informative. This witness saw the airplane traveling east to west at high speed — over 200 mph, he believed — and low altitude, just below the bottoms of the clouds. It was drizzling or raining at the time. The airplane flew in a straight line and the engine “was purring just fine.” Thirty seconds after the airplane disappeared from view, the witness heard “a loud bang that sounded like a shotgun going off, and right after that a loud explosion.” Looking to the west, he saw black smoke rising.

So we have, both from the probable cause and from the implication that “several” witnesses heard but did not see the airplane because it was in cloud, the initial impression that the noninstrument-rated pilot flew into cloud and sooner or later lost control of the airplane, perhaps in a spin or a spiral dive. But it turns out that according to this detailed and reliable-sounding witness statement, he was not in cloud but in the clear below the clouds, and seemingly not out of control at all.

Setting aside for the moment the question of what happened between that sighting and the shotgun-like report, let us visit the accident site. Photographs show a large furrowed field upon which fragments of an airplane, few of them recognizable, lie widely scattered. From the linear arrangement of certain parts, it can be deduced that the airplane broke up in flight and that its “energy vector” — in other words, its heading — was 108 degrees at the time of impact. The ground track of the flight up to the last witness sighting would have had a bearing of about 250 degrees. It follows that during the 30 seconds or so preceding the crash, the airplane turned 140 degrees.

The conclusion drawn from the arrangement of the broken parts along the ground is that first the left wing failed upward at midspan, and then the V-tail and part of the tail cone broke away. The sequence of events is important. The notorious spate of V-tail Bonanza breakups that ended two decades ago with an AD-mandated tail modification involved the tail failing initially and the wings then breaking downward, because without balancing download on the stabilizer the airplane would pitch nose-down abruptly.

Interestingly, the balance weights from both elevators had separated and were found at a distance from them. This fact might suggest ruddervator flutter, to which V-tail Bonanzas were not immune. But if the wing failed in upward bending, prior failure of the empennage would be ruled out, and a more probable explanation might be that the disintegrating left wing struck the tail and somehow severed both balance weights.

Why did the left wing fail? The evidence suggests that the pilot, perhaps at the urging of his wife and mother or perhaps because the weather looked as if it would get worse before it got better, decided to turn back. He banked fairly steeply, to judge from the short time it took to reverse course. Possibly, if he was flying just below the clouds, he found himself heading toward a low-hanging cloud and tightened the turn to avoid it. Possibly he found himself losing altitude at some point and overcontrolled in a sudden effort to arrest the descent.

It appears that the pilot did overstress the wing. We don’t know why. Whether the airplane was still as strong as it had been in 1947 is unknowable; there is no mention in the accident report of corrosion or previous damage that could have weakened the left wing.

I think, at any rate, that the sequence of events in this accident may not have been the familiar one suggested by the probable cause. The phrase “into instrument meteorological conditions” strongly suggests that the pilot was in cloud at the time of the breakup, became disoriented, and overcontrolled. I think it is at least equally possible that he was in the clear and under control, but overcontrolled for some other reason — quite a different picture.

This article is based on the NTSB’s report of the accident and is intended to bring the issues raised to our readers’ attention. It is not intended to judge or to reach any definitive conclusions about the ability or capacity of any person, living or dead, or any aircraft or accessory.

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