Good item in Slate on Cory Lidle, with one crucial error
Slate's "hot documents" feature has an informative item about the sad Cory Lidle crash. (Disclosure: "hot documents" was created, and most of the time is written, by my close friend Tim Noah, although not this item.) Unfortunately the item has one innocent but major error of logic, or of understanding how airplanes work.
The good part of the item is surveying what's increasingly known, and not known, about the crash. The bad part is its "hot document" aspect -- the link to a description of an unusual design feature of Cirrus airplanes, like the one in the crash.
The feature is commonly known as "wing cuffs," and is evident in the lower of the two photos shown in the Slate item. (It also is shown in the slideshow you get if go to this link on the Cirrus Design site, choose Exterior, and see the #3 photo. Or, you could just trust me.) Conceptually, it means that the wing of the airplane has two different shapes, or "angles of attack." For reasons spelled out in a footnote below, these wing cuffs make it easier to avoid stalling the plane, and to avoid the worst consequences if a stall begins. An aerodynamic "stall," again, has nothing to do with the engine stalls of normal life. It means that the air is no longer flowing smoothly enough or quickly enough over the top of the wing to create lift. Its effect is to make the plane stop flying and simply fall.
The Slate item asks whether this special safety design somehow failed, or if the pilots didn't know how to make use of it. With the caveat that much about the crash is still unknown and may never be known, for now it appears as if these wing cuffs and similar anti-stall features had nothing whatsoever to do with the crash.
Why? If the anti-stall cuffs had failed, or if the pilots had in some other way gotten into a stall, the airplane would have been heading down nose-first. That's what happens in a stall. Airplanes are designed to be nose-heavy. (Explanation below.) When the wings "stall" because the airspeed has gotten too slow, the airplane becomes a big falling object and goes more or less straight down, led by the propeller. Yes, it has some forward momentum, and falls in a diagonal line. But so-called "stall-spin crashes," which typically happen as a pilot is preparing for landing but makes too sharp a turn toward the runway and loses too much airspeed, are typified by straight-down impact with the ground. I won't add links to aviation sites with videos of this "auguring-in" process, but everyone who has learned to fly has seen them, as reminders of the importance of avoiding a stall.
Did the Cory Lidle airplane fall into that building on the Upper East Side? Anything could prove to be true when all the facts are in. But on first exposure, it looks as if it flew into the building. For instance, the damage was relatively contained to the floor or floors where it hit. This phenomenally useful graphic from the New York Times, which tops any other newspaper graphic I can think of at the moment, gives a plausible reconstruction of events and suggests that the fundamental problem was the difficulty of making the "box canyon" U-turn in the narrow space available. All is hypothesis at this point, but this one fits the evidence better than other available theories -- certainly better than the idea that the wing cuff design had anything to do with the crash.
Yes, the concept of stalling is related to this wide-U-turn problem. As explained yesterday, if a pilot didn't have too worry about losing too much airspeed with too steep a bank, he could try to turn on a dime. So it appears as if whoever was flying the plane was trying to solve several problems at once: reverse direction fast enough not to head into LaGuardia's airspace; not do it so fast that he risked a stall; keep the turn tight enough to stay over the river; but again not so tight that the airplane stalled instead. If that is what actually happened, it was an impossible equation to solve -- but not because of "wing cuffs."
Footnote equivalents start here:
1) How do wing cuffs prevent stalls? Technically a wing stalls when its "angle of attack" -- the angle at which the wing meets the oncoming flow of air -- becomes too high. Up to a certain point, a sharper angle of the wing into the wind produces more lift. Then at that critical point, the wind stops flowing over the wing altogether, the wing "stalls," and the airplane falls.
The function of "wing cuffs" is to ensure that the angle of attack on the outboard part of the wings -- the part nearest the tips, and the part where the ailerons are also located -- is lower than the angle of attack of the inboard part of the wings. So when a stall begins, the inside part of the wing stalls first. While the pilot is already feeling the signs of an incipient stall -- which range from a warning horn blaring to a shaky feeling in the controls -- half of the wing's surface is still flying, and still able to control the plane (because the ailerons are in the "working" part of the wing). It is possible to ignore these signs and force the airplane into a full-wing stall -- everyone is made to do it in training. But any prudent pilot in any normal circumstance has extra protection and warning because of the wing cuff design. Also, for reasons not worth going into, wing cuffs prevent stalls from becoming the more deadly stall-spins. Anyone who cares about this probably already understands why.
2) Why is an airplane nose-heavy? That's how they're designed, and largely for safety reasons involving stalls. When an airplane has stalled, its airspeed has become too low. (Or, technically, its angle of attack has become too high.) To get the airspeed back up, the nose of the airplane has to be pointed down (so gravity will speed it up, meaning more and faster air over the wings, meaning that the wings can start lifting again and the pilot can recover from the stall). If the plane were tail-heavy, it would have no way of recovering from a stall. The desirable trait -- an airplane turning nose-downward when it starts to fall -- is connected to the undesirable trait of a nose-down crash if the pilot fails to correct the stall.
3) Why does the VFR flyway over the East River exist at all, if getting out of it requires this demanding U-turn? Mainly it is used by helicopters, to get out of town from locations on the Upper East Side. They obviously have no turn-on-a-dime problems like those that affect airplanes. I will let New Yorkers say more about the sociology of helicopter use from Manhattan.
4) What did the Cirrus airplane have to do with this crash? Based on available evidence, about as much as a Toyota would if a driver took a curve too fast and went off the road. Yes, the Toyota allowed him to go too fast; yes, the Toyota shows up in a lot of crash statistics, because it's so popular a car. But fundamentally whatever happened was not about the Toyota, or the Cirrus. That is the cautionary reality of almost all small airplane crashes.