WX Blues

Posted on 13. Feb, 2007 by in Featured

Weather blues most definitely. My trip today was cancelled for weather, specifically snow and ice here in St. Louis. You might think, “hey stop whinning you get the day off”; true, but no fly no pay. It brings up the question of why do we cancel? Good question, the answer is simple but does need a little explanation. Ice accumulates on the aircrafts wings, tail, and fuselage. It also can accumulate in the engine or chunks can go down the engine causing problems. Modern aircraft are equiped with anti-icing systems, in normal icing conditions (rime icing) the systems can keep the wings/tail and engine free of ice so that normal operation is assured. Freezing rain however is bad stuff and can glaze an aircraft like a donut, so the airlines choose to avoid it. Why is it so bad?

The reason is not weight, the weight of the ice obviously does effect the aircraft, but it is ice’s effect on the air flow that is the biggest concern. Angle of Attack (AOA) determines when a wing stops producing lift and thus stalls. Many think it is only speed that determines whether a wing will continue to produce lift. In straight level 1 g flight it does seem true. However you can stall a wing at 200, 300, or even 400 knots by simply pulling too hard. Been there, done that! Generally the faster you are going the wilder the ride, especially if you were trying to turn. Let go of the stick and eventually you will start to fly again. Why? Because you exceeded the AOA of the wing and when you let go the aircraft pointed itself back into the wind and bingo you are flying again. Seems complicated, not really. How does a wing work? Easy; the top is curved, the bottom flat. When two molecules of air show up at the leading edge of a wing simultaneously; with one going over the top and one the bottom, by the rules of physics they will arrive at the trailing edge of the wing at the same time. To do that the molecule over the top has to go faster, that acceleration creates a lower pressure and lift results. 3 primary forces effect flight: lift, weight and thrust. Lift and thrust combine to overcome weight. Thrust gets the aircraft going fast enuff that the wing produces enuff lift to get the airplane into the air. AOA: put your hand flat on the desk top, thumb toward the key board. The desk top is the relative wind, your hand the wing, with your hand flat the AOA is low. Now raise the thumb edge of your hand only. The angular difference between the table top and your hand is angle of attack. So if you slow too much the angle of your wing will get higher, eventually if it gets too high the air can’t flow over the top of the wing smoothly and it becomes turbulent (buffet), then disrupts completely and the wing stops producing lift and stalls. Same thing happens if you pull too hard too fast, air flow breaks up and the wing stops flying. Ice can coat a wing and cause the flow to break up at much higher speeds, freezing rain is the worst and that is why we cancelled today. Once it changes to snow you can de-ice your aircraft and be on your way. Remember rule one in Aviation is keep the airplane flying, doesn’t matter if you are dodging SAMs over Hanoi in an F-8 Crusader or landing your Cub on a grass strip in Kansas. chip

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