In my March Aftermath column on the 2012 crash of a Pilatus PC-12 in Florida, I faulted the National Transportation Safety Board for mixing up indicated and true airspeeds. Actually, it was I who misread the report. I am indebted to reader Timothy Burtch, an accident investigator with the NTSB, for pointing out that the maximum speed of 338 knots that the airplane reached in a spiral dive before it broke apart was, in fact, an indicated airspeed, not a true one, and that the airplane did, therefore, exceed its maneuvering speed by
175 knots as the report stated.
The Pilatus, with a family of six aboard, was climbing through FL 250 in IMC. It was at 109 kias, in a 25-Âdegree right bank, deviating to avoid an area of rain, when the autopilot disengaged for unknown reasons. Presumably a chime sounded and a warning light illuminated, but the pilot seemingly did nothing to take control of the airplane. The baffling aspect of that accident was the pilot’s apparent failure to act even when the airplane was vertically banked and plunging downward at a horrific rate.
Within 10 seconds of the autopilot disconnect, the angle of bank had increased to 50 degrees and the airplane had begun to descend. After 30 seconds, the bank angle was 100 degrees and the airplane had lost 2,600 feet. In the next 13 seconds, it lost another 5,900 feet while the positive load factor increased to 4.6 G. At 36 seconds the indicated airspeed, having peaked at 338 knots — 430 true — dropped to zero, suggesting that the breakup — the airplane lost its horizontal stabilizer and portions of both wings — had taken place somewhere around 15,000 feet.
Any airplane will enter a spiral dive, sooner or later, if no attempt is made to control it. Some may fly hands-off for many minutes in smooth air, but if they are disturbed by a gust, or if they are banked in the first place, they will inevitably bank more and more steeply, the nose will drop, they will pick up speed, and the turn will continue to tighten without limit.
Builders of free-flight models will counter that their airplanes can remain right side up indefinitely, but that is because they have much more dihedral effect than piloted airplanes do. If your airplane had the lateral stability of a free-flight model, it would be very reluctant to bank and very fatiguing to fly. For the sake of maneuverability, therefore, we accept the necessary evil called “spiral divergence.”
In VFR conditions hardly anyone gets into a developed spiral dive unless he literally falls asleep at the wheel. It is the non-instrument-rated pilot who strays into a cloud who stands the greatest chance of experiencing a spiral dive. In this case, however, what made the sequence of events doubly puzzling was that the pilot was instrument rated and current.
The procedure for recovery from a spiral dive is simple: Power off, level the wings, slow down. But there are potential pitfalls, and the Pilatus may have encountered one of them.
The airplane was trimmed for 109 kias. The dynamic pressure of air at that speed is 39 pounds per square foot (psf). The wing loading of the airplane, which was probably close to its gross weight, was about 36 psf, and so the lift coefficient, which is the ratio between the lift force and the dynamic pressure, was about 0.9.
The dynamic pressure at 338 kias is 337 psf, and the lift coefficient in level flight, if level flight were permissible or even possible at that speed, would be about 0.1.
Normally we don’t talk much about lift coefficients in the context of piloting, as opposed to designing, an airplane, but they are of interest in this case because G-loads can be thought of as the ratio of two lift coefficients. If, for example, the Pilatus is trimmed for 109 kias and a lift coefficient of 0.9, it would experience an acceleration of over 9 G’s if its indicated airspeed were, by magic, instantly increased to 338. Another way to put it is that the acceleration is the square of the ratio of the high speed to the trimmed speed. Thus, if you were trimmed for 100 and you were suddenly indicating 300, the speed ratio would be 3 and the resulting G-load, as the airplane seeks to return to its trimmed speed, would be 9.
That is physically impossible, of course, because in the real world speed cannot instantly change from 100 to 300. But you don’t need 9 G’s to break an airplane, either. Six G’s will do it, five in some cases, and that would have occurred at 270 kias or less. In other words, if you are trimmed for 109 kias when some sort of upset occurs, you pick up a lot of speed in a spiral dive, and then you level the wings and the airspeed indicator is still showing over 270 knots, you are entering the neighborhood of a normal-category airplane’s ultimate load factor.
The ultimate load factor is not the one where you limp home with wrinkled wing skins. That’s called the limit load factor. The ultimate load factor is the one where big pieces break off.
So a caution has to be appended to the recovery procedure for spiral dives. After you roll the wings level, the airplane is going to try to return to its trimmed speed. Normally, this will not be a problem. If you are cruising at 150 kias and you look up from a chart or your iPad to find yourself in a 60-degree bank and the airspeed indicator winding through 220, you are not going to take the wings off the airplane by just leveling out. You’ll pull a couple of G’s, perhaps, but nothing will break.
It’s when you’re flying at low speed, for example climbing, as the Pilatus was, and then get into a dive at very high speed, as it did, that the recovery may involve dangerously high load factors, especially if you try to help it along with a pitch-up command. What is actually called for is nose-down trim and a forward push on the yoke. Otherwise the airplane may overstress itself even with no help from you at all.
Inflight breakups are rare, but they occur regularly enough to be worth thinking about. Sometimes an airplane emerges from the bottom of the overcast already in pieces, but I suspect that some breakups occur only after the airplane emerges into the clear, when the pilot sees the ground in some highly unfamiliar position and instinctively overcontrols while trying to level out. This is a danger even when the overcast is quite high and there is ample room for a more gradual recovery.
A common piece of advice for VFR pilots who have blundered into IMC is to lower the landing gear. As a young pilot I used to wonder what in the world the landing gear could do to keep the airplane upright; the authors dispensing the advice evidently considered it so obvious that they did not bother to explain that the purpose was to increase drag and thus to limit the speed gain in the inevitable spiral dive.
In the extreme situation in which the Pilatus found itself, lowering the gear could be part of the spiral dive recovery: Power off, level the wings while trimming nose down, lower the gear — never mind the doors — and don’t let the nose rise too rapidly. In a reciprocating-engine airplane, drag can be further increased by pushing the prop to high rpm.
The essential point, however, is to limit G while reducing speed. If the plane were going to come apart because of sheer speed, it would already have done so. It’s the G-forces that pilots apply in their urgency to stop pointing downward that end up breaking wings.
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