Let me unpack that — Page 235, Lesson 286

Let's start with rain ice, because it's the most dangerous form and it sets up everything else we're about to discuss. Rain ice is caused by rain which becomes supercooled by falling from an inversion into air which is below 0°C. Let me unpack that. An inversion is a layer where the temperature increases with altitude instead of decreasing — so you have warmer air sitting on top of colder air. Rain starts as liquid in that warmer layer above. As it falls through the inversion into the freezing air below, it becomes supercooled. Supercooled means the water drops are still liquid even though their temperature is below 0°C — they haven't frozen yet. The key danger is that this rain does not freeze immediately on impact. Instead, it flows back over the surface before it freezes, so it forms considerable flow back, and it builds up very quickly. That's why rain ice is so insidious — it spreads and accumulates fast. Now let's look at the general effect of frost and ice on the aircraft. The formation of ice and frost on the airframe will do three things. First, it will modify the profile of the aerofoil — that's the shape of the wing cross-section. Second, it will increase the roughness of the aircraft surface. Third, it will increase the weight of the aircraft. The main effect of frost specifically is to increase the surface roughness. Why does that matter? Because roughness increases the energy loss in the boundary layer. The boundary layer is the thin layer of air that clings to the surface as the aircraft moves through it. When that layer loses energy, two things happen: the skin friction drag increases, and the boundary layer will have an earlier separation. Earlier separation means the airflow detaches from the wing sooner than it should, and that gives a reduced CLMAX. CLMAX is the maximum lift coefficient — the peak lift the wing can generate before it stalls. So a reduced CLMAX means the wing stalls earlier. Here's the practical consequence: take-off with frost on the wings could result in a stall after lift-off if the normal take-off speed is used. Because the wing can't produce the lift it normally would at that speed, you could stall right after becoming airborne. Tests have quantified this. Frost, ice or snow with the thickness and surface roughness of medium or coarse sandpaper reduces lift by as much as 30% and increases drag by 40%. That's a massive performance hit from something that looks like sandpaper. Now, ice itself — as opposed to frost — will normally form on and behind the leading edges of wings and tailplane. The leading edge is the front edge of the wing. Ice can result in severe distortion of the leading edge profile. That gives a large increase in drag and a substantial decrease in CLMAX. Let me show you what we're dealing with here. That's the profile drag picture. Now, the reduced CLMAX of the wing gives a higher stalling speed. And here's a subtle one: the decreased CLMAX of the tailplane could cause it to stall when the aircraft is flying at low speed, particularly if the wing downwash is increased as a result of flap extension. Downwash is the air deflected downward behind the wing. When you extend flaps, that downwash increases, and it can push the tailplane into a stall. Tailplane stall will result in loss of longitudinal control. Longitudinal control is your pitch control — your ability to raise and lower the nose. Losing that is serious. Clear ice and rain ice especially can add considerable weight to the airframe. That extra weight gives a higher stalling speed, as well as increased induced drag. Induced drag is the drag created by generating lift. The margin of thrust to drag will be decreased, reducing the ability to climb. And because you can't climb as well, increased power will be required to maintain height, resulting in increased fuel consumption. Now let's talk about the propeller. Ice formation on propeller blades can upset the balance of the propeller and cause severe vibration, particularly if pieces of ice break off from one blade. If one blade sheds ice and the others don't, the propeller becomes unbalanced. And pieces of ice shed from propellers can also cause damage to the fuselage — the main body of the aircraft. That's the propeller figure. Now, the effect on instruments. Formation of ice on static vents and pitot heads could cause errors in the readings of pressure instruments and, eventually, failure to show any reading. The pitot head measures ram air pressure, and the static vents measure ambient pressure — together they feed your airspeed indicator and altimeter. If they ice over, you lose those readings. The effect on controls is equally dangerous. Any moveable surface could become jammed by ice forming in the gaps around the control or by pieces of ice breaking off and becoming jammed in the control gaps. The controls could become difficult to operate or immovable. So your ailerons, elevator, rudder — any of them could freeze solid. Finally, water contamination. If the wings are contaminated with water due to heavy rain, the boundary layer may become turbulent further forward on the wing, particularly if the section is of the laminar flow type. Laminar flow is smooth, orderly airflow — and some wing sections are designed specifically to maintain it. Water disrupts that. This will cause increased drag and may disrupt the boundary layer resulting in a significantly higher stall speed. And the last point: adjustments to operational speed should be made in accordance with the recommendations of the aircraft manufacturer. That's the bottom line — you don't guess, you follow the book. That's the low-speed figure showing how all this plays out. So the whole picture is: contamination — whether frost, ice, rain ice, or water — degrades your wing's ability to produce lift, increases drag, adds weight, and can jam your controls and instruments. Every one of those effects pushes your stall speed up and your safety margin down.