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Flying under the influenceFlying under the influence

How wind affects your judgment
How wind affects your judgment

Barry Schiff Some years ago, a Qantas Airlines Boeing 747-400 flew nonstop from London, England, to Sydney, Australia, and broke the world distance record for commercial aircraft. The 9,720-nm, 20-hour flight was only 1,000 miles shy of flying halfway around the world.

There is no reason to believe that it won’t be long before we’ll be able to fly nonstop from any point on Earth to any other. There are, of course, an infinite number of great-circle routes connecting any two points on opposite sides of the globe, which introduces some novel flight planning. Political considerations aside, such a flight could set off in any of 360 directions and still reach its destination, depending on the wind. The preferred route, of course, would be the one with the most favorable wind.

GA pilots obviously do not have such flexibility. We simply accept whatever wind happens to exist along the shorter routes we fly. We know that a round trip flown under the influence of a tailwind one way and a headwind the other takes longer than the same trip flown in calm air. This is because more time is spent fighting the headwind when flying in one direction than taking advantage of the tailwind when flying in the other.

Flying with a direct crosswind also increases the flight time between two points. This is because a crosswind requires that the airplane be turned into the wind (crabbed) to prevent drifting off course, and crabbing results in reduced groundspeed. This explains why our perception is correct, there really is a headwind component more often than not.

So what takes longer—a round trip flown with a direct crosswind (reduced groundspeed in both directions) or a roundtrip flown with an equally strong along-course wind that increases groundspeed one way and decreases it the other?

The answer is that an along-course wind typically increases round-trip flying time twice as much as an equally strong crosswind.

A maneuver required of private pilot applicants is turning about a point. The object is to fly a constant-radius circle around a point on the ground while maintaining a constant altitude.

Assume that you are flying counterclockwise during such a maneuver while under the influence of a strong northerly wind. It is obvious that bank angle must be varied to fly a perfect circle. A favorite question of some FAA-designated examiners at such a time is, “Where in the circle must bank angle be greatest?” It is a favorite question because the examiner can almost always count on an incorrect answer.

Many pilots think that bank angle is steepest when directly south of the pylon. This, they reason, is to offset the drift created by the crosswind while heading eastbound around the bottom of the circle.

Although seemingly logical, this is wrong. The steepest bank angle is required on the west side of the circle while heading south, with the wind on the aircraft’s tail. This is because the tailwind increases groundspeed and the aircraft has less time in which to turn a given number of degrees. In other words, turn rate must be increased while flying downwind, and the only way to do this is to increase bank angle.

A similar problem involves flying a rectangular course around a traffic pattern. Assume, for example, that a pilot is flying a left pattern to Runway 36 at a time when the wind is strong and from the northwest. Which of the four turns requires the steepest bank angle to fly a symmetrical rectangle with approximately constant-radius turns? Which turn requires the minimum bank angle? It might be helpful to sketch this problem before trying to solve it. (Hint: the amount of crab while flying along each of the four legs is the same, and airspeed around the pattern is constant.)

For simplicity, assume the crab angle required along each leg of the pattern is 20 degrees. (I know; it’s a big wind.) The required headings, therefore, are 340 degrees (flying upwind), 290 degrees (flying crosswind), 200 degrees (when downwind), 070 degrees (on base), and 340 degrees (on final).

The steepest turn is required when turning from downwind to base because this turn requires the largest heading change (130 degrees, in this case). The shallowest bank angle is used when turning upwind to crosswind, a heading change of only 50 degrees.

Most pilots realize that an airplane—unlike a sailboat that can tack into the wind—cannot make forward progress against a headwind that is equal to or in excess of its true airspeed. But it is worse than that. In the case of a northerly wind, for example, a pilot cannot even track east or west.

Think about it.

Barry Schiff has been an active certificated flight instructor since he was 18 years old.

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