Atmosphere And Weather

How Windsocks are used at Airports

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 “Often shalt thou confirm thine airspeed on final lest the earth rise up and smite thee.”  (Commandment number 10 of the Pilot's Ten Commandments.)

Airspeed is the velocity at which an aircraft is traveling relative to the air mass in which it is traversing. This is, of course, different from “ground speed” which is the rate at which the aircraft is traveling relative to the earth. If an aircraft is parked stationary facing into a 10 knot breeze the wing is experiencing a 10 knot airspeed.

The two most critical moments in flight are takeoffs and landings. At these times the aircraft is close to the ground at the same time its airspeed is approaching a minimum for controlled flight. For this reason a wise pilot chooses to take off or land into the prevailing wind.

In the early days of aviation many airports were simply open, grassy fields. Runways were not fixed in direction as are paved runways today. Pilots could arrange their take off run directly into the wind. If a particular aircraft required 60 knots airspeed to achieve sufficient lift for takeoff and the wind was 10 knots, the aircraft would have to reach 50 knots ground speed to begin flying. This serves to reduce the distance the aircraft has to travel across the ground gaining speed sufficient to fly. This is important because runways are not endless and they typically terminate with immovable objects such as trees, power lines and the like. The sooner and aircraft gets off the ground and gains altitude the better. Likewise, when landing, the slower the ground speed the shorter the “ground run” and the less wear and tear on the landing gear and brakes.

In order for the adventuresome pilot of early aircraft to recognize wind direction and speed he might look at the movement of surrounding trees, smoke from a chimney or open fire or any other source of such information. There is an old tale that cattle feed with their back to the wind so perhaps that is a clue to wind direction.

The windsock is not new and its inventor is unknown. It is said that it originated as a depiction of a dragon called a draco, used to determine wind direction for archers in battle. The modern windsock is a conical fabric device attached to a pole in a prominent location on the airport grounds. The early pilots used these on their grassy fields so they could “fly out of the sock”, directly into the wind.

The Federal Aviation Administration (FAA) describes the windsock within the “Pilot’s Handbook of Aeronautical Knowledge”. It is supposed to be capable of swinging freely into the wind with as little as 2 knots of prevailing wind. It is further supposed to fill completely and stand completely horizontally when the wind is 15 knots or higher. This information is important as we shall see shortly.

With the advent of fixed runways, paved or otherwise, another important factor in flying fixed wing aircraft came to be, the “cross wind takeoff or landing”. The wind is not always directly down the runway but it may be from some other angle relative to the runway. At low speeds an aircraft is a “weather vane”; it wants to turn into the wind. This would be undesirable on a paved runway because the aircraft could leave the runway, damaging the aircraft or the runway lights, upsetting the airport manager terribly. Therefore every new pilot must learn how to perform cross wind takeoffs and landings.

When an aircraft is being certified by the FAA, specifications such as takeoff and landing distances and cross wind capability are established. There will be a maximum cross wind component that the aircraft is certified to contend with. Calculating the cross wind component is part of a pilot’s flight training. The pilot needs the wind velocity and direction to make the calculation and determine if his aircraft is capable of takeoff or landing in the prevailing condition. Lacking such information from a control tower or airport Fixed Base Operator (FBO), he can use the windsock to estimate the data he needs.

When planning a departure our pilot will, among other things such as fuel load and weight and balance, determine wind direction and speed. Flight Service can give him general information about the weather conditions and he can view the windsock to confirm the local situation. Once in the air he may find “no one home” at his destination airport; this is where an understanding of the windsock is very handy.

He will approach the destination airport at a low altitude, but above the altitude maintained by landing aircraft, called the “pattern altitude”. He will fly over the airport and observe the direction and status of the windsock. The direction, of course, tells him wind direction. If it is swinging back and forth he knows the direction is variable and he will know to expect directional changes as he approaches to land. This means he may have to apply some cross wind landing control. If the windsock is straight he knows to expect a strong, at least 15 knot wind. If it is fluttering he can expect gusty conditions. This information prepares him for his final approach and landing.

Occasionally the pilot may find that conditions at the destination airport are not as predicted by Flight Service. (Pilot Commandment number 9, “Put not thy trust in weather prophets, for when the truth is not in, then they shall not accompany thee among thy ancestors.”) For this reason the prudent pilot has considered an alternate airport within his fuel supply range. Should the windsock indicate that the conditions are such that either he, or his aircraft, is not capable of successfully negotiating them, he should move on to his alternate.

One final point, one often stated by pilots of old tail wheel aircraft, “tail draggers”. Once on the ground and airplane that is graceful in flight can become an ugly duckling. An airplane with a nose wheel tends to be more stable in a cross wind on the ground, but a tail dragger will be prone to swinging with the wind resulting in a “ground loop”. It may turn into the wind violently damaging the landing gear or a wing tip. Tail dragger pilots will caution to “fly the airplane until it is tied down”. When taxiing to a parking space the windsock should still be consulted.

One final old saying: A good landing is one the pilot walks away from; a great landing is when you can fly the airplane again afterwards. Happy flying!

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