Landing/Takeoff Considerations, Part II: Wind

“The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails.”

--William Arthur Ward—

Last time, we talked about some of the challenges involved in dealing with sloped runways. We looked at some of the dynamics of how slope affects landing and takeoff performance and we discussed some “Rules of Thumb” that can help a pilot face the challenges of sloped runways with a reasonable expectation of success. Today, I’d like to take a look at the factor of wind and how it affects our landing and takeoff performance.

An aircraft flies on airspeed. It takes off and lands at the edge of the air where, as they say, the rubber meets the road and airspeed and ground speed interact.

A basic concept which presents some challenges to new students learning to fly—and some more experienced aviators as well—is that, once an aircraft is airborne, once its wheels or floats or skis have given up contact with the surface of the earth, it does not experience wind. It will respond to changes in pressure—that is how control surfaces provide their functions—and it does respond to sudden changes in airflow—we generally refer to those changes as gusts—but it has no experience of wind itself.

Think of a goldfish swimming peacefully in its bowl. Now, place the bowl in the back seat of your car and drive down the highway at, say, 100 kph. The goldfish is, indeed, traveling over the ground at 100 kph, but its experience is of peacefully swimming in calm water. It experiences no motion. Or, toss a stick into a flowing river. The stick travels over the ground at the speed of the water flow but it experiences no “water speed”. The stick’s experience is one of sitting in still water.

Ground speed and track are affected by wind.  Airspeed is not.

We can think of ground speed as air speed plus or minus wind speed relative to the direction the aircraft is traveling. If, for example, we are flying at a TAS of 100 knots in no wind conditions, we will have a ground speed of 100 knots and our track over the ground will be the same as our heading through the air. If we are facing a 20 knot direct headwind, our ground speed will be 100 knots minus the wind speed, 80 knots; if we make a 180 degree turn and fly with that same wind directly behind us, our ground speed will now be 100 knots plus 20 knots, 120 knots. In both cases, our track over the ground will be the same as our heading.

If the wind is not directly ahead of or behind us, we can do the calculations and compute the relative headwind or tailwind components of the breeze and its horizontal or crosswind component. That’s what that delightful little piece of equipment, your E6B Flight Computer, is for. In this case, our ground track will be affected by the horizontal component of the wind, the percentage of the total wind that carries the aircraft sideways in relation to its heading. Our heading and our ground track will no longer be the same.

In terms of taking off and landing, our preference is to takeoff and land into the wind. Taking off or landing into the wind will shorten our ground roll and reduce our ground speed in proportion to the strength of the wind.  

If our normal lift-off speed is, for example, 60 knots, a headwind of 10 knots will allow us to lift off at 50 knots of ground speed. The aircraft, when facing into the wind, has an air speed of 10 knots just sitting still on the runway surface. If our normal approach speed is 60 knots, that same 10 knot headwind will give us a ground speed of 50 knots, reducing the speed at which we make contact with the ground and the amount of kinetic energy we must dissipate bringing the aircraft to a stop.

Taking off or landing with a tailwind will have the reverse effect. Our ground speed will be increased in proportion to the wind speed and our takeoff and landing rolls will also be increased.

The basic Rules of Thumb for taking off or landing with a light aircraft in wind are:

A direct headwind will shorten our ground roll on both takeoff and landing by approximately 1% for each knot of breeze. For example, if we are taking off into a 10 knot headwind and our normal takeoff roll is 800’ in no wind conditions, our ground roll will be reduced to just over 700’ (800’ x 0.9 = 720’).

A direct tailwind will lengthen our ground rolls on both takeoff and landing by approximately 5% for each knot of breeze. For example, if our normal landing roll with no wind is 500’, a 5 knot tailwind—no great breeze—will increase that distance to over 600’ (500’ x 1.25 = 625’).

Clearly, the disadvantage of working with a tailwind is significant. A tailwind takeoff or landing with more than 10 knots is not recommended for light aircraft under any normal circumstances.

A headwind, in effect, increases the performance length, the effective length, of our landing surface in proportion to the speed of the wind. A 10 knot headwind effectively increases runway length by 10%. A 1500’ runway with 10 knots of headwind effectively becomes a 1600’ runway (1500’ x 1.1 = 1650’).

As a general Rule of Thumb, a 10% increase in groundspeed results in a 20% increase in ground roll. This is a good argument against being casual about approach speed on landing and against landing or taking off with a tailwind.

If our touchdown speed is, for example, 50 knots and we require 500’ to execute our landing roll in no wind conditions, landing with a 5 knot tailwind will increase our landing distance by approximately 100’ (5/50 = 0.1 or 10%; 1.2 x 500’ = 600’).

On takeoff with the same 5 knot tail wind, our ground roll will be increased approximately the same 20%, perhaps slightly more.

A slightly more complex and perhaps slightly more accurate Rule of Thumb for calculating the effect of a tailwind up to five knots on takeoff is: the takeoff distance is equal to 110% of the computed takeoff distance for the existing density altitude plus the value of the tailwind component divided by the rotation speed of the aircraft. Yikes. What does that look like if we apply some numbers?

Let’s take a C-172 at gross weight under standard pressure and temperature conditions at sea level with a 10 knot tailwind and a rotation speed of, say, 55 knots. Our normal takeoff distance is approximately 900’ in no wind conditions. 900’ x 1.10 = 990’; 900’ x 10/55 (tailwind component divided by rotation speed) = 180’; total distance = 990’ + 180’ = 1170’ or approximately a 20% increase in takeoff distance.

A basic Rule of Thumb for all flying is “If you’re not SURE it’s safe, don’t do it.” Consult METARs, TAFs and PIREPs prior to flight for the best available weather information. Think about how best to approach your destination airport and calculate the required runway length under existing conditions for a safe landing based on the best available information.  Keep in mind, however, things change.  Having a plan “B” in your back pocket is always a good idea.

If your destination airport or aerodrome is uncontrolled or no weather information is available, complete a good inspection before deciding on which runway to put your valuable wheels or whether a safe landing is even practicable. Knowledge is our key to success.

Wind can provide some interesting challenges to flight. It is an excellent plan to know its effects on your aircraft’s performance particularly when operating at the edges of the air where we interact with the surface of our lovely planet. Trusting to luck is not a great plan.  Better to know and make decisions based on knowledge rather than to simply carry on and “hope” for the best.