We were talking about heavy rain on final approach — Page 186, Lesson 222

I want to pick up right where we left off, because we were in the middle of a very specific hazard. We were talking about heavy rain on final approach. Now, I want you to picture this: the aircraft suddenly enters a torrential downpour. The rain is so heavy it's physically driving the aircraft downwards. At the same time, the momentum of the aircraft is being lost, so the speed will decrease, and that requires you to increase thrust to compensate. The volume of rain in any given situation will vary, but the net effect is a loss of momentum and a decrease in altitude. And I want you to note this carefully: this is similar to the effect of microburst windshear. That's a cross-reference to Chapter 15, but the key point here is that heavy rain on final can mimic that dangerous sinking effect. Now, let's shift to a completely different topic: the stall and recovery characteristics of canards. This is a crucial contrast. In a conventional configuration, you have the tailplane at the rear. In that setup, the wing stalls before the tailplane. That's a deliberate design choice, and it means that at the stall, you still maintain longitudinal control and stability. The tailplane is still effective, so the nose drops and you recover. But on a canard layout, the foreplane is ahead of the wing. Here's the critical difference: if the wing stalls first, stability is lost. That's the dangerous one. But if the foreplane stalls first, then control is lost, and the maximum value of CL—that's the coefficient of lift, the measure of how much lift the wing can generate—is reduced. So you have two failure modes, and neither is good, but they're distinct. Wing-first stall on a canard means you lose stability; foreplane-first stall means you lose control and you cap the maximum lift you can achieve. Now, let's move on to spinning, which is a whole new section. When an aircraft is accidentally or deliberately stalled, the motion may in some cases develop into a spin. Let me give you the important characteristics of a spin, because these are the defining features you need to recognize. First, the aircraft is descending along a steep helical path about a vertical spin axis. That means it's corkscrewing downwards. Second, the angle of attack of both wings is well above the stall angle. Third, the aircraft has a high rate of rotation about that vertical spin axis. Fourth, viewed from above, the aircraft executes a circular path about the spin axis, and the radius of the helix is usually less than the semi-span of the wing. So the spiral is tight—tighter than the wing's half-span. And fifth, the aircraft may be in the "erect" or "inverted" position in the spin. Erect means right-side up, inverted means upside down. Now, the spin is one of the most complex of all flight manoeuvres. Here's the formal definition: a spin may be defined as an aggravated stall resulting in autorotation. Let me unpack that. Autorotation means the rotation is stable and will continue due to aerodynamic forces if nothing intervenes. So it's self-sustaining—it won't stop on its own. And during the spin, the wings remain unequally stalled. That's the key: one wing is more stalled than the other. Now, what are the primary causes of a spin? First and foremost, a stall must occur before a spin can take place. You cannot spin without stalling first. A spin occurs when one wing stalls more than the other. The wing that is more stalled will drop, and the nose of the aircraft will yaw in the direction of the lower wing. So the more-stalled wing drops, and the nose turns toward that dropped wing. The cause of an accidental spin is exceeding the critical angle of attack while performing a manoeuvre with either too much or not enough rudder input for the amount of aileron being used. That's what we call crossed-controls. So you're at high angle of attack, and your rudder and aileron inputs are mismatched. If the correct stall recovery is not initiated promptly, the stall could develop into a spin. And this is why co-ordinated use of the flight controls is so important, especially during flight at low airspeed and high angle of attack. Most pilots can maintain co-ordinated flight during routine manoeuvres, but this ability often deteriorates when distractions occur. That's the real-world danger—distraction breaks your coordination, and that's when a stall can turn into a spin.