document.write(" serif">The Drag Curve
Why did I say, yesterday, that saying pull back was exactly the wrong thing to do?
This is a drag curve. Every airplane has one. This looks like the drag curve of a small airplane, say a C-172. The curve for the B-777 is the same, except stretched out in the speed axis, like this:
During descent a jet's engines are at idle. Descending at perhaps 320 knots, it is near the right-hand end of the curve. Because of the 250-knot speed limit below 10,000 feet, at about 12,000 feet the pilot will pull back on the control column and hold the nose a little higher, slowing the rate of descent and trading kinetic energy (speed) into potential energy (reducing the rate of descent). As he does so he is moving left on the speed axis, toward the low point of the drag curve. Notice that drag is reducing as he slows. This means the rate of descent will decrease.
As he slows further for approach, he moves toward the low point of the curve. As you can see, in a jet the curve at this point is pretty flat, so the transition from one side of the curve to the other is a subtle one, covering, let's say, from 170 knots to 220 knots. In this range the pilot can pull back, raise the nose, and slow down without much affecting the rate of descent. Subtle though it is, this low point on the curve, the minimum drag speed, is extremely important.
Why? Because as he slows further for final approach, say to 137 knots, he is on what pilots call the back side of the drag curve. (You will more often hear back side of the power curve, but it is the same thing.) Everything changes on the back side of the drag curve. Slower speed requires more power, not less. If you don't add power, pulling back increases the descent rate. Pilots know that to stretch a glide, you (counterintuitively) have to push, not pull.
In a jet Vapp (final approach speed) is quite a way up the left side of the curve. You can see that as you slow, the drag increases, causing you to slow further. The airplane is “speed unstable”, which means that a small change in speed from turbulence or control input becomes a larger change if left to itself. What the pilot has to do is “catch” the speed as he approaches Vapp by adding power. The pilots of Asiana 214 forgot this step, and continued to slide up the left side of the curve.
There is an old pilot saying: Pull back, you go up. Pull back more, you go down. Now you are closer to understanding why, but you may ask, What happens when you get to the end, where the curve stops? To answer, we have to look at the lift curve.
Instead of drag and speed, we are now graphing lift against Angle Of Attack (AOA, or alpha). (In 1G flight there is an equivalence between AOA and speed, so it's OK to think of the AOA axis as speed, except in this graph slow is on the right, with larger angles of attack.)
AOA is the angle at which the air meets the wing. What pulling back on the stick or the control column really does is increase the angle of attack. So if the pilot needs to increase lift, he pulls back, moving along the curve to the right. But the wing only “flies” in a narrow range of AOA, from 1 degree or less up to about 16 degrees. Above that the airflow starts to detach from the upper surface of the wing and the wing rapidly loses efficiency. That is called the stall. Now the wing is more like a board. It is still pushing air around, but not nearly as much air. The wing is stalled.
You can see in the graph that as you move to the right, increasing angle of attack, lift increases in a straight line and then suddenly goes over the top like a roller coaster track. Indeed, that's just what it feels like when an aircraft stalls. Now as you pull back more lift decreases, and the bottom drops out. That's what happened to Air France 447. When the autopilot kicked off the pilot panicked and pulled. For a brief period they went up. The pilot kept pulling, and for four minutes the aircraft fell, at angles of attack well above the stall, until it hit the sea.
So that's why the drag curve ends on the left-hand side. The end of the curve is the stall. Asiana 214 was almost there, and that's why the stick shaker went off. They weren't stalled yet, but they were within a whisker. They were sitting on top of the roller coaster. And when they pitched up even further in that last second before impact (you can see it clearly the YouTube Video and in this re-enactment) it did nothing to change their flight path. All it did was lower the tail and landing gear.
Now you know why.
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