Aviation Safety Corner

GREETINGS! For this month's topic, I would like to discuss, "Wind at Your Back" The record shows that pilots should not discount the effects of a tailwind on takeoff and landing performance. This is Part I of IV.
Now let's start our discussion. One of my old how-to-fly books defines a tailwind as "any wind which approaches the aircraft from a rearward direction and acts in conjunction with its course."

A "direct" tailwind is directly aligned with the aircraft's longitudinal axis. However, in most situations in which the wind is coming from somewhere behind the wing tips, it has both a tailwind and a crosswind component.
A tailwind always acts to increase the aircraft's ground speed. A tailwind component varies directly with the relative angle of the wind to the flight path of the aircraft. The closer to the flight path, the more significant the tailwind component. The crosswind component further compounds the situation for a pilot, but the crosswind correction is essentially the same as that for a crosswind from the front.

Now, let's discuss, "Computing Components." Tailwind and crosswind components are easily figured with an E6B or electronic flight computer, or by consulting the crosswind component chart found in most Aircraft Flight Manuals (AFMs). An even simpler way is to use a hand-held calculator with trigonometric functions. To figure the crosswind and tailwind components for a wind that is not along the flight path take the velocity of the wind in knots and find the angle between the wind vector, and the flight path centerline. Multiply the sine of that angle the wind velocity, and you have the crosswind component. Multiply the cosine of the same angle, and you have the tailwind component.

For example, if your aircraft is approaching Runway 3 and the winds are 10 knots from 250 degrees, this gives you a 40-degree, quartering left tailwind. Multiply the sine of 40 by 10 knots, and you find that you have about a 6.5-knot left crosswind component. Multiply the cosine of 40 by 10 knots, and you find a tailwind component of about 7.5 knots.

Then just plug the numbers into the performance charts for the aircraft. Whatever technique you use to figure your tailwind and crosswind components, the most important thing is to remember to do it.

Now, let's discuss, "Ground Speed." The effects that tailwinds have on an aircraft depend on what the aircraft is doing. During cruise flight, tailwinds increase our ground speed, so we get where we are going faster. On takeoff/landing, however, they act against us. A take-off with a tailwind will result in a longer-than-normal ground roll since it takes longer to reach a specific airspeed.

Aerodynamically, how fast a wing moves through the air is what is important. The relative wind is the airflow parallel and opposite to the flight path of the aircraft. The relative wind is produced by the movement of an aircraft and its airfoil surfaces at an airspeed which, when combined with the angle of attack, produces a given amount of lift.

It is critical to a basic understanding of aerodynamics to realize that airspeed is independent of wind, and wind (with the exception of vertical gusts) has no real effect on airspeed. The effects of wind on the aircraft's ground speed are very important, however.

A headwind tends to hurt us at cruise, but helps during takeoff and landing by decreasing distances. With a tailwind, we need a greater ground speed to achieve the proper airspeed for takeoff. The result is a longer ground roll. When landing with a tailwind, our ground speed is higher than our airspeed, and the rollout takes longer.
Now, let's discuss, "Raising the Roll." A glance at the performance charts in your AFM will show that the effects of a tailwind on takeoff and landing performance are a lot more significant than the effects of a headwind.

For example, the manual for the Grumman AA-1C (a two-seater called the "T-Cat" or "Lynx") notes that for a headwind, we should decrease takeoff distance by five percent for every five knots of headwind component. A 10-knot headwind, therefore, decreases the takeoff roll by 10 percent. However, the notes for the chart also indicate that for tailwinds up to 10 knots, the correction is to add 10 percent for every 2.5 knots of tailwind component. Therefore, if a 10-knot wind were blowing right down the runway, we would reduce the ground roll by 10 percent if we took off into the wind. But if we decided to takeoff downwind, we would have to add 40 percent to the value.

Now, in conclusion, remember that there is nothing in the AIM, and most training manuals do not specifically state the effect that a tailwind has on takeoff or landing performance. But one of the newer training books states that a pilot should "takeoff upwind if possible" and that "taking off into the wind is GOOD AIRMANSHIP."
Don't miss next month's Aviation Safety Corner, when we'll discuss, "Raising the Mark", and "Shaky Subject" in Part II of IV.

Larry G. Harmon
FAA AVIATION SAFETY COUNSELOR

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