Finding Wind Movement at a Confined Area.

Many student helicopter pilots become frustrated when faced with the seemingly daunting task of determining wind direction at an off airport landing site. When the wind is evident but the direction not obvious there are several ways to “see” the wind.

• Wind motion on trees and water in the area will display the wind movement. Learning to read the direction of wind from these cues is a matter of practice and comparison with other signs of the wind for confirmation.


• Some pilots like the “18 in. MP / 45 mph” constant attitude technique in a turn over the area to show the wind drift. Hold the collective at a power setting of 18 inches which should result in an airspeed of 45 mph if you hold the attitude constant in the turn. The helicopter will drift downwind in a significant wind.


• If you just cannot seem to “find” the wind, look for the best two approaches from opposite directions and try each one at a high enough power setting to support the weight of the helicopter just at the point of translation speed where the lift from forward flight is beginning to have a buoyant effect. The speed or motion of the helicopter at this point will be slow and will not register on the airspeed indicator (which you shouldn’t be looking at anyway!) but will be in the range of 5 to 8 knots. The approach that has the higher power setting is the one that is downwind. An instructor will have to demonstrate this technique to you first because too often student pilots will try this technique incorrectly and get a false sense of wind direction.


• Remember that once you have found the wind direction, the best approach must be given equal billing. Too often students will find the wind direction and then approach only from this direction even if there is no approach opening on this side. The wind direction has a splay to it and as long as you are within 50° to 60° on either side you will be getting the benefit of being “into wind”. This will give you an opportunity to find the best approach into the enclosed area. Many times I have seen students attempt an approach perpendicular to a long narrow entrance to an area because it is into wind. Give wind direction and the best approach equal consideration knowing that being “into wind” has some flexibility.

Atmospheric Wind - 101

The weight of the air above the earth’s surface exerts a force or pressure on the surface which leads to the development of the winds which influence our daily weather.

The change in pressure measured across a given distance is called pressure gradient. When this force is directed from high to low pressure it is called the pressure gradient force and is responsible for triggering the initial movement of air.

As a result of the earth's rotation, air set in motion by the pressure gradient force undergoes a deflection from its path called the Coriolis force. Air moving from high to low pressure in the northern hemisphere is deflected to the right by the Coriolis force and to the left In the southern hemisphere. Slowly blowing winds will be deflected only a small amount, stronger winds will be deflected more and winds blowing closer to the poles will be deflected more than winds at the equator where the Coriolis force is zero.

Winds balanced by the Coriolis and Pressure Gradient forces are called Geostrophic winds. An air parcel at rest will begin to move from high to low pressure due to the pressure gradient force. The moving air parcel is then deflected by the Coriolis force, As the wind gains speed the deflection continues to increase until the wind blows parallel to the isobars and is then referred to as geostrophic. These winds occur at higher altitudes where they are not affected by surface friction and terrain features.

Since isobars are almost always curved and rarely evenly spaced, winds are now described as being in gradient wind balance. They still blow parallel to the isobars, but are no longer balanced by the pressure gradient and Coriolis forces, and do not have the same velocity as geostrophic winds.

Surface friction in the Boundary Layer 1 to 2 kilometers in height prevents wind from being geostrophic. Winds flow counter clockwise around the centre of a low in the northern hemisphere and with friction, the wind speed slows reducing the Coriolis force and the pressure gradient force inward becomes dominant. The rising air spiraling of into the center of a low pressure system (convergance) creates the probability of clouds, rain and storms to form.

Winds flow clockwise around a high pressure system in the northern hemisphere, but with surface friction the wind again slows down, reducing the Coriolis force and the outward pressure gradient force becomes dominant. In this case, though, the dominent pressure gradient is outward from the center of the high, the surface wind spirals away from the center (divergence) causing a sinking motion which suppresses cloud development and gives us clear skies.

35 Ways to Detect Wind Direction!

Here are 35 examples of how helicopter pilots can look around and find the direction of the wind!

Tree movement, flags, laundry, airborne drift, tall grass, waves, calm side of obstacles in the water, dust from a farmers tractor, downwind hamburger smells from Burger King, sailboat sails, low stratus moving across the ground, “cats paw” wind squalls on a lake, ground speed calculation, drift sensation in turns, smoke from a chimney, birds “hovering”, turbulence on the lee side of hills, cloud movement, boat moored a buoy, tops of coniferous trees bending downward, loose logs at one end of a lake, rotor clouds on the lee side of a mountain peak, debris moving across the ground, snow spilling over a snowdrift, the light coloured back side of leaves, wind over tide, pennants at a car dealership, dust swirls at a gravel pit, smoke from a fire, valley outflow sensation, wind sock, kites at the park, anemometer on a tower, fluorescent flagging at a logging site, tail rotor "wiggle", Pirep from another aircraft….