I guess the guy that flew his plane into the IRS building in Texas today is making a lot of news. I read his entire manifesto and am still thinking it over. I might post on it in the future but for now I’ve seen the quote, “We’re at a delicate point in history, it’s too late to fix what’s wrong with the government and too early to shoot them.” I think that applies to flying airplanes into their buildings as well. Instead, I think we’ll talk about the media today.
Two of a pilots favorite past times (and there are many) are reviewing crash reports and mocking the media. Over the last few years the media has gotten better at reporting crashes but there are still a few points that they miss regularly. Two stories that surfaced today before the actual intent of the crash was known surrounded stalls. The media NEVER gets these right.
I popped the clutch and the airplane stalled..
The first one is easy. It was reported that the airplanes engine might have stalled and that it crashed. An airplane engine doesn’t stall. It’s not a car. There’s no clutch (for the purposes of this discussion anyway). Pilots, because they are geeks, will nod along with this and then say, “but, in the case of x,y, z you CAN actually get an engine stall or even mention that a prop stall (or a fan stall in jets) does happen”. That’s true, but those things are very very rare and if they did occur a reporter wouldn’t know enough to tell you about it. For the layman, if you hear a reporter tell you that an airplane engine stalled change the channel. It just doesn’t happen. Engines might quit running occasionally but they don’t stall.
Oh, and for the sake of clarity, when the spinny thing on the front of an airplane stops spinning it becomes a glider… it doesn’t fall out of the sky.
The airplane stopped flying and then flew…
When it does fall out of the sky is when the second bit of news from today comes into play. The report said, “A wing stall may have occurred as the plane was flying low and slow over the highway, which would cause the plane to bank sharply.” That sounds reasonable but it doesn’t work that way.
Here’s a bit of trivia: Wing stalls have nothing to do with speed. Think about that one for second because it’s one of the hardest concepts to grasp for people new to aviation. A wing stalls when the flow of air over the wing is interrupted. The flow of air is interrupted when the angle of attack gets too high. What that means is that the angle of the wing into the airflow becomes so great that the air doesn’t flow smoothly over the wing. So, in reality, a stall can occur at high speeds even easier than it can at low speeds (because it takes less angle of attack to interrupt the airflow).
In normal operation, an airfoil (wing) is curved on the top and flat (or mostly flat on the bottom) this means that air going over the top of the wing has further to travel than air going under the wing. In order to compensate for this increased distance the air traveling over the top of the wing moves faster than the air going under the wing. In a fluid (air is a fluid) internal pressures decrease as the velocity increases. This means that the air traveling over the top of the wing has less pressure than the air traveling under the wing. This difference in pressure causes lift and is what enables airplanes (and helicopters) to fly. In physics this is known as the “Bernoulli Principle”. The important thing to remember here is lift in airplanes is NOT created (to any significant extent) by “impact air” hitting the bottom of the wing when you point the nose up. It’s caused by an increased angle of attack into the relative wind (the air that the airplane is moving through) which causes the air going over the top of the wing to have even further to travel and creating more lift.
All of that is a bit foggy but the important thing to remember is that when an aircraft wing stalls it has no lift because the angle of attack has become so great that air cant flow smoothly over the wing. It’s important to note this because stalls can happen at ANY speed. But more importantly because a stalled wing has no lift. What that means is that it can’t turn. At all.
Everyone knows that airplanes tilt when they turn. Even a 5 year old knows this as evidenced by them leaning left and right as they zoom through the air. The reason that they tilt (airplanes, not five year olds) is very simply to change the lift vector. Think of it this way. When the airplane is flying straight and level the wings are mostly level and lift is created perpendicular to the wings. E.G. the wings are horizontal and the lift component is straight up.
When the amount of lift equals the weight of the airplane you float. When the amount of lift is higher than the weight of the airplane you go up. When the amount of lift is less than the weight of the airplane you go down. When the amount of lift is zero you go straight down.
When an airplane tilts in a turn, the lift stays the same. It’s still exactly perpendicular to the wings. This means that some of the lift that was keeping you in the air is now being used to “pull you around” your turn. I can show you the math to prove it but this post is already too long so, for now, just accept three things:
1) Lift is created by airflow over the wings or more specifically angle of attack into the airflow,
2) turns are created by using lift,
3) Stalls occur when airflow over the wings is interrupted and lift is reduced to zero.
Using those three rules it follows that if the wing stalls (meaning that it is no longer creating lift) the airplane stops flying and it won’t turn because there is no lift being created to “pull” it around the turn.
In most cases when a stall occurs the nose of the airplane points itself down and the airplane gains speed until the angle of attack is sufficient to produce lift again (yeah, speed… ignore that too for now).
Pilots practice this ALL of the time because, 1) it’s fun and 2) it’s best if you recognize stalls before they happen and prevent them from happening on accident and in close proximity to the hard flat stuff. However, what this reporter was probably confused by was the following. A well flown airplane will (almost) always stall both wings at the same time but it IS possible to stall one wing and not the other. When this occurs the lift is uneven and the airplane won’t turn…. at least not in the normal sense. The wing with lift will go up and the wing without lift will go down (hence “bank sharply) and the airplane will go into a spin. When an airplane spins it goes so very nearly straight down (discounting any forward inertia that you have left) as to not be considered in any way a turn.
All of that means that we can now create rule number four which is:
4) stalled airplanes do not turn. It’s physically (in the literal sense) impossible.
See? Easy! Why doesn’t everyone get this?
Yeah, right. That’s why people don’t like to sit by pilots at parties… because they like to talk about this stuff but in even deeper detail. Sometimes they even use slide rules. If you get two of them really going they can put an entire VFW post to sleep.
Airplane Pilots, have a ball ripping this apart.
Helicopter Pilots, get yer own stall rules… those things shouldn’t fly in the first place.
Sadly, the first thing that went through my mind when I found out that this was a purposeful crash was this question, answer it if you can:
Assuming that you’ve lost it to the point that you’re going to crash your airplane on purpose. Would you exceed Vne just this once?