06-08-2006 09:37 AM
06-08-2006 09:37 AM
A popular cause of passenger inflight concern is the inevitable "bump in the road." Here are my thoughts on turbulence: TurbulenceÃ¢â‚¬¦. In a car, it's a bump in the road. In a boat, it's the smack of a wave on the hull. In a plane, to some people it's a white-knuckled ride on an out-of-control roller coaster. Think for a minute. Why do you suppose flying produces such a different response? Control issues? Not knowing what makes the bumps? Not really knowing what the bump's effect is on the plane? One business traveler professed to me the thought that he always envisioned the Pilots wrestling with the controls trying to keep the plane from diving out of control in moderate turbulence. I had to laugh because, at times we Pilots are worried: Will the contents of that soup spoon make it in our mouth or on our shirt? It only really gets bumpy when you try to take a sip of hot coffee. Wrestling with control? Hardly. Otto the AutoPilot is doing a fine job of flying the plane smoothly, and he does a better job in turbulence that a human can. But still, if I punched Otto off and let go of the yoke, the plane would wander on through the bumps with little change in direction. That is because the plane is designed to handle turbulence because of stability laws built into the plane by the engineers. (This Is Your Captain Speaking: Flight Training For Passengers goes into this in greater detail.) Turbulence is everywhere and all the time. Most of the time, its energy impact on the plane is barely noticeable. A little technical stuff that is critical to the understanding of turbulence. Bear with me here. This ain't Calculus. Lift is what keeps the plane up. The wonderful techies who engineered safe airplanes have learned that lift is a function of the square of the airspeed. (Don't nod off on me now!) In "Ray Simple Math," that means that if you are flying at two miles per hour (mph), the lift quotient is 2 x 2= 4. If you were going three mph it would be 3 x 3 = 9. The critical thing to realize is that, by increasing the speed here by 30 percent you get somewhere over 100 percent increase in lift. Think about that for a minute. Lift is the upward force holding the plane up. If you suddenly get more of it, the plane is going to do what? Right! It will bump up. In turbulent air (air that is unstable where masses or air are moving in different directions within a short timeframe), you are going to see rapid changes in lift on the wing. That makes for bumps. It's just natural. In the winter, jetstream winds at certain altitudes can blow at 120-160 mph. It sure isn't blowing like that when you get out of your car at the airport, so that means you'll be climbing into and out of that jetstream somewhere on your flight. Those are forceful winds!!! How can it not be bumpy? Air is like a liquid, and if you are moving fast enough, almost like a solid. That makes the ride choppier as well. Say you are moving through the air at 250 mph in an air mass that is moving up, relative to the plane at 20 mph. In a flash, the plane flies into a blob of air that is going down at 20 mph. That is a difference of 40 mph. A 40-mph gust blows your car around on the freeway, why not your plane? At least in a plane, you don't have to worry about wandering out of your lane. Remember, the HUGE surface area of your wing keeps you flying. That same HUGE surface area also catches the gusts and wind effects. (The huge tail surfaces keep the plane going straight like an arrow.) When I am riding along in the bumps, I know the plane is hitting winds from different directions because I can see the airspeed indicator bobbling around plus or minus four to six knots. When it's smooth, the airspeed hardly moves at all.That's just the way it goes. When was the last time you heard of an airliner falling apart because of bumps at cruise? I cannot remember such an event. Airliners are tested during certification. One test is the wing failure test. This test always gathers a bunch of employees because it is so dramatic a test of the wing structure. Nobody ever gets to witness a wing failure except in the test rig. One of the prototype airframes never leaves the ground. It gets installed in a hydraulic test rig that bends it to simulate thousands of hours inflight. Once the equivalent of 20 years of turbulence and bad landings is bent, jiggled, and wiggled into the fuselage, the wing is bent until it breaks. This proves beyond a shadow of a doubt that the engineers have done their job in designing a wing strong enough to take the beating that Mother Nature passes out. Most heavy wings like the 767 or 777 bend up at the tips almost 14 feet before breaking in a loud "CRACK." I have seen my airliner wing bending in upper level moderate turbulence, and it hardly moves up and down a couple of feet. That gives you some idea of the safety margins designed into the planes we fly upon. Turbulence is no big deal to me, the Pilot. My only concern is whether someone is up and about in the cabin. People are injured because they have disregarded the FASTEN SEATBELT sign and been bounced off the wall or ceiling. I have never heard of a seated/belted passenger ever being seriously injured in turb. Turbulence may be exciting, but it isn't perilous. Not by a long shot. Worry if you want to, but you are just making stuff up. The Pilots aren't worried at all as long as everyone is seated. Are you worried your car wheels are going to break off when driving down a bumpy stretch of highway? Probably not. Think of that the next time you fly. Turbulence tip: Next time you drive someplace with a friend or spouse, have them drive and fill a small cup with water (about a quarter-inch from the top). Place the cup on a doubled towel (because it will spill a little) and hold it down on the console or the glovebox door. Now, as you drive around town and out on the freeways, watch what happens to the water in the cup: It spills all over the place (hopefully on the towel). Now, when you fly, try the same routine on your tray table: Fill a cup a quarter-inch from full and let it sit there while you bounce around. I bet the cup on the plane hardly loses a drop in bumps that you thought were "terrible." The cup shows you that the airborne turbulence was less bumpy than the road turbulence experiment in your car. And you were flying six times faster through the air than a 60mph section of freeway travel. A few bumps are normal!!!
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