Winds – Can’t live without them, can’t live with them

So, as I write this, I am sitting at a campground in south central Kansas under a beautiful blue sky with just a touch of a breeze to make the air temperature feel perfect. But I am thinking that winds are not always so kind. About twelve hours ago, my wife and I, as well as many other campers, were scurrying around, lowering tents, raising awnings, stashing loose odds and ends, and otherwise trying to adjust to winds that we could not control. And I thought, “Isn’t that the same as flying? We spend a lot of time trying to find winds that are favorable, and then trying to minimize the effects of those that are not favorable.”

Actually, in the big scheme of things, winds are really a very good thing. Think about it. Wind is really the effect of two phenomena that we cannot live without – air, and the earth’s rotation. Remember back to your private pilot ground school days, and you will recall that the sun warms the earth and sets up a rising column of air at the equator (and plus or minus about twenty degrees latitude, depending on the season.) The air then drifts toward the poles and falls back down as it cools. And without the earth’s rotation, it would just wander back again to the south. But with the rotation, it moves generally from west to east.

But more to the point for us pilots, the wind sets up a number of situations (challenges?) that have bedeviled pilots since the beginning. Don’t believe it? Why did Orville and Wilbur leave their warm cozy nest in Dayton and travel to the North Carolina Outer Banks? Privacy, was one reason, but also because they could get a consistent sea breeze with a downhill run. And all of pilots today love to have a downslope and a headwind for takeoffs.

Pilots are affected by winds in three different phases of flight, and our response to each phase is a bit different. In cruise, we are slowed by headwinds, boosted by tailwinds, and shoved off course by crosswinds. And here are two True/False questions to test your “enroute winds” knowledge. First - If a pilot flies from one point to another and returns to the point of departure, the overall time will be the same regardless of wind velocity as long as it is the same for both legs and is a direct headwind / tailwind. And second - A direct west crosswind (that is a wind with no headwind or tailwind component) has no affect on our overall flight time.

I hope you found the first statement “false”. Many of us have a flown a long cross-country with a significant headwind, and told ourselves, “At least I will make that up on the way back.” But it never seems to work out that way, and it never will. Disregard all of the time spent in the climb, and taxiing on the ground, and in maneuvering for landing. Just look at the cruise portion, and let’s take an easy example.

Assume a cruise distance of 100 nautical miles, and true air speed of 100 knots and no wind. Time across the ground? One hour each way, two hours total. Now let’s assume a headwind of twenty-five knots on the first leg and the same as a tailwind on the way back. Now, we have ground speeds of 75 and 125 for the two legs. This gives us time-enroute times of 1.3 hours and .8 hours respectively. And that gets worse as the winds increase. Try a fifty knot wind. Two hours out, and forty minutes back. And of course, with a wind equal to our true airspeed, we can never reach our destination, but time back home will be cut in half.

So what about a crosswind? Is that benign? Actually, no. In order to fly a straight leg relative to the ground, we have actually flown a longer leg through the air. And in order to cover the same distance over the ground, our leg through the air needs to be longer.

So what about landings and takeoffs? Well math gets to us again. Look at the takeoff and landing charts for any aircraft, and you will see that we improve (shorten) our rolls a bit with a headwind, but we increase our takeoff and landing rolls dramatically with a tailwind. For example, looking at the notes on the takeoff chart of a newer Cessna 172, I see a ground roll of about 900 feet at sea level on a 20 day. Liftoff speed is 48 kias, according to the chart. But there is a note on the bottom of the chart stating, “decrease distances by 10% for every 9 knots of headwind; increase distance by 10% for every 2 knots of tail wind.” And this same note appears on the landing chart. A two knot tailwind adds the same distance as a 9 knot headwind subtracts.

What about final approach? Why do we always seem to need more power with a headwind? Assume that we roll out from base to final at about 400’ agl, and a half-mile from the runway. With a given power setting, we will descend 400 feet in the same amount of time regardless of the wind. But while we may be able to glide to the runway in that amount of time with no wind, a headwind slows our travel time over the ground. And so, we will find ourselves somewhat short of the runway when the time runs out.

And then there are those pesky cross-winds on final. If we just flew in a crab to the runway, we would be in the same position as we were when we flew a cross-country with a cross-wind. That is, our actual distance through the air would be a bit longer than our distance over the ground. And since we are descending at the same rate regardless of the distance, we will contact the ground before we reach the runway unless we make a change in our rate of descent.

But many pilots do not fly in a crab to the runway. Many pilots like to use the “wing low” system, in which the upwind wing is lowered into the wind to negate the drift, and then opposite rudder is imposed to preclude the aircraft from turning into the wind. This use of aileron in one direction and rudder in the other direction is a “slip” and always adds aerodynamic drag. Which is great if we are looking for some means of slowing down. But can be a “drag” if we just want to neutralize the crosswind on final. As every budding student pilot figures out eventually, the extra drag always requires more power if the same glide slope is to be maintained.

And parenthetically, I really dislike the terms “forward slip” and “side slip”. A slip is a slip. In a slip, the airplane will always move to the outside of the coordinated turn. This is the definition of “slip”. And as long as we are flying aircraft with fixed wings, the plane will always fly forward relative to the wind, regardless of the pilot’s turning coordination. So, by definition, all slips are both “forward” and “side”.

Wishing you tailwinds at altitude, headwinds on takeoff, and just enough crosswind on final to keep life interesting.

Don’t just practice until you get it right. Practice until you don’t get it wrong

  

Chris Hope has taught fledgling and experienced pilots for nearly 40 years, mostly in the Kansas City area. Chris holds flight instructor certificates for single engine land and sea airplanes and multi-engine land planes, as well as for instrument training. He holds ground instructor certificates for advanced and instrument training. Chris is an FAA Gold Seal Instructor and a Master Certified Flight Instructor. Chris serves as a member of the FAASTeam in the Kansas City area. His website is www.ChrisHopeFAAFlightInstructor.com

 

 

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Comment by Lee N. Robinson on October 11, 2014 at 10:04am

Hi Chris.   Excellent piece, but the terms "side" and "forward" are definitions of "purpose".   Side" slip to me has been used to correct the ground track alignment.  Both can be used for controlled altitude loss in a direct line on the approach, with basically controlling the aircraft centerline angle to the runway centerline.  Both will generate higher descent speed.  The bug in the ointment is, of course, our old friend "crosswind" !!

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