First, let's anchor the definition — Page 98, Lesson 111

I want to walk you through what happens to the tailplane when the airflow around it changes — and then we'll take that straight into ground effect, because that's where this really bites in the real aeroplane. First, let's anchor the definition. Angle of attack is the angle between the chord line and the relative airflow. Now, relative airflow has three defining characteristics, and you need all three. Magnitude — that's the speed of the aircraft through the air, which we call the True Airspeed, TAS. Direction — parallel to, and in the opposite direction to, the aircraft flight path. And condition — unaffected by the presence of the aircraft. That last one is the key. Relative airflow is airflow the aircraft hasn't touched yet. Now here's the trap. Air flowing off the wing trailing edge — that's downwash — cannot be called relative airflow, because it fails that condition test. It has been influenced by the aircraft. So we can't strictly talk about a tailplane angle of attack in the normal sense. Instead, airflow that has been influenced by the presence of the aircraft — both its direction of flow and its dynamic pressure — we call Effective Airflow. And the angle between the chord line and that effective airflow is the Effective Angle of Attack. Keep those two terms straight: relative airflow is the undisturbed stream, effective airflow is what the tailplane actually sees. Now look at Figure 5.25. Airflow from direction A gives the tailplane zero effective angle of attack. Airflow from direction E, F, or G would be an increase in effective angle of attack. Now flip it — if airflow from direction G is considered as the reference, then flow from F, E, A, B, C, or D would be a decrease in effective angle of attack. And one important convention: the term "negative angle of attack" is not used. We talk about decreases, not negatives. So let's draw the conclusion. Increasing downwash — that's going from G toward D — gives a decrease in tailplane effective angle of attack. And decreasing downwash — from D back toward G — gives an increase in tailplane effective angle of attack. That relationship is the whole game. Now, it's necessary to understand how changing downwash affects tailplane angle of attack, but it's vital to understand the influence of downwash on the aircraft's pitching moment. That's where we're headed. Let's enter ground effect. Picture an aircraft descending into ground effect, holding a constant CL and a constant IAS. As it descends, several changes happen, and I'll walk each one. First, the decreased downwash gives an increase in the effective angle of attack. That means the wing needs a smaller angle of attack to produce the same lift coefficient. So if you hold a constant pitch attitude as you enter ground effect, you may feel a "floating" sensation — that's because CL has increased and CDi, the induced drag, has decreased, which lowers the thrust requirement. Second, look at the induced drag. The decrease in induced drag causes a reduction in deceleration. Any excess speed may lead to a considerable "float" distance. And because thrust required has dropped, the aircraft may have a tendency to climb above the desired glide path — we call that "ballooning" — if you don't reduce the throttle setting. Third, the stall risk. If airspeed is allowed to decay significantly during short finals, and you arrest the resulting sink rate by increasing angle of attack, then upon entering ground effect the wing could stall — and that results in a heavy landing. So the very thing that saves you on short finals can bite you in ground effect. Fourth, the pitch input. The pilot may need to increase pitch input — more elevator back-pressure — to maintain the desired landing attitude. Why? Because the decreased downwash increases the effective angle of attack of the tailplane. The down load on the tail is reduced, producing a nose-down pitching moment. So you have to pull more to hold the attitude. That's the full picture: downwash changes effective angle of attack at the tail, which changes the tail's down load, which changes the pitching moment — and ground effect is where all of it shows up on the landing.