Airspeed, Angle of Attack (AOA), pitch attitude, and stall are all related, but it’s complex.
So, let’s break it down.
AOA is the angle between the relative wind, the air moving over the airplane, and a reference line ascribed to the wing, known as the mean chord line. The precise definitions aren’t as important as this: lift increases with increasing AOA, until you hit the stall AOA.
When the wing hits stall AOA, airflow over the top of the wing becomes turbulent, lift is lost, you usually have buffet, and usually a big increase in drag (this last bit is important). The relationship between AOA, lift, drag, and stall, all depend on the shape of the wing. A 172 has a simple wing, with clearly defined stall, and a very sharp drop in lift. A jet liner, for example, with a swept wing, aerodynamic twist, and a supercritical shape, has a much less defined stall. But it will still begin to stall at some AOA.
A wing will always stall at precisely the same AOA. Always. Keep that bit in mind.
So, the relationship between AOA and lift, then, is pretty simple - more AOA equals more lift up to stall, then more AOA equals less lift.
But we need enough lift for the airplane to fly. So, we need lots of air moving over the wing. Without enough air, there just isn’t enough lift. Think of a jetliner on takeoff, they get going pretty fast, so that there is enough lift to carry the weight of the airplane.
More airspeed equals more lift. In fact, it’s a square relationship, but again, the precise nature isn’t as important as the fact that more speed equals more lift, and you need enough lift to keep the plane in the air.
So, you need both airspeed and AOA to get enough lift to counter gravity. The force of lift acting on the airplane has to be equal to or greater than the force of gravity for it to fly.
If I’m low on airspeed, I need more AOA to get enough lift. That works, right up to stall AOA. If I’ve got lots of airspeed, I don’t need much AOA to create the lift.
It gets more complicated if the airplane is in a bank, and part of that lifting force is now pointing away from the ground. Some of the lift is used to counteract gravity. Some of the lift is now pointing to the inside of the turn and being used to turn the airplane to a different heading. In this case, I need more lift. It’s a physics vector thing. The vector of lift is always perpendicular to the wing. So, in a bank, there is a lift vector, which means that lifting force has a direction as well as a magnitude.
Which means, in a bank, we need a bit more total lift. So, we can increase our AOA just a bit (if we aren’t close to stall) and get a bit more lift. So, in a turn, at the same speed, we must be at a higher AOA than we would be for the same weight aircraft in wings-level flight. Rolling into a turn at low airspeed can create a stall as the pilot increases AOA to stay aloft. This is important for understanding but not critical to understanding this accident, since this accident happened when the airplane was wings level.
So, what about pitch? For small adjustments in pitch attitude, you get small changes in AOA and small changes in the airplanes flight path. For big changes in pitch, you get big changes in AOA, and you can stall an airplane, because you‘re making a big AOA change that might get you to stall AOA.
How about climb? Climb is a matter of power. A 172 doesn’t have a lot of power. If we pitch the nose up for a steep climb, the airspeed can, and will, decrease. It has to. The engine can’t make enough thrust to maintain airspeed in a steep climb.
Uh oh. Don’t we need enough airspeed to keep the wing flying at an AOA below the stall AOA?
Yeah. We do. That’s exactly what happened here. Steep climb reduces airspeed - as airspeed decreases, lift decreases.
Lift decreases, so, we have to increase AOA for more airspeed in order to stay in the air, right? Yeah, we do, but the wing will always stall at the same AOA. Once it stalls, we lose lift, and we increase drag, which slows our airspeed even more.
Recovering from a stall requires controlling the AOA. Lowering the nose to reduce AOA below the stall AOA. At 100 feet above the ground, there isn’t enough time at that lower AOA for the relatively weak engine to increase airspeed enough to get back the point where we can achieve level flight.
Continuing to pull the nose up (a visceral, amygdala reaction to impending crash) keeps the AOA above stall, high AOA, where there is not enough lift, and which keeps the drag high enough that airspeed won’t increase*.
Once that airplane got to an excessive nose up pitch, and airspeed was bleeding off (trading kinetic energy for potential energy, or airspeed for altitude ), they were doomed.
If, at 50 feet, the nose was lowered to a normal pitch, they might have maintained enough airspeed that they could fly in level flight without exceeding stall AOA.
But once they got slow, they couldn’t recover. The drag from the high AOA, caused by the deceleration from the high pitch, took away their airspeed, and when it stalled, there wasn’t enough room to recover.
* By the way, this is what Air France 447 did from 35,000 feet until impact. Nose up, full thrust, stay stalled. Even the powerful engines on that particular airliner couldn’t overcome the increased drag of a fully stalled wing.
