We've been looking at what raises the stall speed — Page 235, Lesson 288

Let me walk you through the final part of this section on factors affecting stall speed, and then we'll touch on airframe aging before we move into the next chapter. We've been looking at what raises the stall speed. Heavy rain is one of those factors. When you take off or land in heavy rain, the water on the wing surface disrupts the smooth airflow, and the result is a higher stall speed. Now, the key point here is operational: adjustments to your operating speed should be made in accordance with the recommendations of the aircraft manufacturer or the aircraft operator when taking off and landing in heavy rain. So you don't just guess — you follow the published guidance. Now let's look at airframe aging. Over a period of years, the condition of the airframe will deteriorate. This happens because of small scratches, minor damage, repairs, and the general accumulation of dirt and grease. These are all gradual, cumulative effects. The overall effect of this deterioration is to increase the drag of the aircraft, and mainly it's the skin friction drag that goes up. Skin friction drag is the drag caused by the air rubbing against the surface of the aircraft — and a rougher, dirtier surface creates more of it. The consequence of that increased drag is a consequent increase in fuel consumption. So the cost of operating the aircraft will increase with the age of the airframe. Now, here's an important point for you as a pilot: the normal deterioration of the airframe is allowed for in the performance charts of the aeroplane. That means the performance data you use already accounts for the expected aging of the airframe. So you don't need to add your own correction for normal wear — it's built into the charts. Let me also address a misconception about airframe icing, because there are some statements that are false and you need to know which is which. First, the pilot can visually identify build-up on the wings, tailplane, or flight controls by looking through the flight deck windows, and at night by using the ice detection lights. That's true — that's a real capability. But here's the critical point: visual evidence of the accumulation of airframe icing may not exist. In other words, you cannot always see the ice. And the statement that due to the high speed of modern aircraft, significant airframe contamination with frost, ice, or snow will not occur — that is false. High speed does not protect you from icing. You must not rely on that assumption. So to summarise what we've covered: heavy rain raises stall speed and you follow manufacturer or operator recommendations for speed adjustments. Airframe aging increases skin friction drag, which increases fuel consumption and operating cost, but the normal deterioration is already allowed for in the performance charts. And on icing — you can sometimes see it from the cockpit, including at night with ice detection lights, but you cannot always see it, and high speed does not prevent contamination. That brings us to the end of this section. The next chapter is Stability and Control, which is a big one — it covers static stability, the aeroplane reference axes, static longitudinal stability, the neutral point, static margin, trim and controllability, and much more.