Effects of wind

 

During the recent storms, the winds at Santa Ynez Airport were being reported by the wind measuring equipment at 25 knots (29 mph) with gusts to 37 knots (44 mph). Winds of these speeds pose significant challenges to pilots during both takeoff and landing phases of flight and while flying enroute.

Enroute winds pose challenges in several different ways to the pilot of an aircraft. A pilot must make computations prior to a flight relative to the performance of the aircraft.  The pilot has a limited amount of fuel, and if the flight is planned for a significant trip he must plan to land with a minimum of 30 minutes of fuel onboard during daylight visual conditions. Should a headwind be encountered, or if headwinds are stronger than forecast, the fuel computations must be made during flight to ensure that the destination can be reached with the required reserve. A pilot must be prepared to divert to an alternate airport for a fuel stop if the trip cannot be completed with the reserve. 

 

The headwind components of prevailing winds are not the only computation the pilot must take into consideration.

The crosswind component is significant, in that the heading to be flown must be computed to adjust for the upwind or downwind effect.

As an example, if a pilot were to plan a flight to a destination directly to the north of a departure airport, and a wind from the west of 40 knots is forecast for the route, the pilot must adjust the aircraft heading left of course and apply a wind correction angle to remain on the planned course.

If no wind correction angle is applied the pilot will be blown 20 miles to the east of the planned course for each 30 minutes of flight, and he easily could become lost.

 

Another effect of winds enroute is the creation of turbulence. Turbulence may be created by air currents moving in the vertical (updrafts and downdrafts), or by changes in direction or velocity at different altitudes (wind shear).

Turbulence also is created by the interaction of winds with the surface terrain.

 

Just as water in a stream or river running over rocks causes disturbances on the surface, so does the interaction between moving air masses over mountains cause disturbances in the air mass. Pilots flying near the Santa Ynez range of mountains are very familiar with the severe turbulence that is created on the lee side (Santa Barbara side) during times of strong north winds.

Cloud formations are good indicators of severe turbulence.

Rotor clouds on the downwind side of mountains just below the summits are indicators of turbulence that pilots should avoid.

Another obvious indicator of high winds is the formation of lenticular clouds over mountain ridges. These clouds, sometimes shaped like flying saucers, form over mountains and do not move with the air mass. Sometimes known as “standing lenticular,” they maintain position above peaks and are a certain sign of high level winds.

 

Airlines sometimes take advantage of high level winds by making use of accurate forecasting of jet streams. These high winds are predictable and can be mapped with some accuracy by meteorologists. By using forecasts of jet streams that mostly move from the west to the east, jet aircraft flying at altitudes of greater than 30,000 feet may take advantage of these 100- to 200-mile-per-hour winds to shave time off flights from the West Coast to the East Coast. 

During the takeoff/landing phase of flight, winds can have a significant effect on aircraft performance. Low level wind shear and turbulence while the aircraft is near the ground may cause such a divergence in the desired course that the aircraft may come in contact with the ground. 

 

Wind shear affects a landing aircraft in two ways. An aircraft approaching a runway may have a 40-knot headwind 500 feet above the runway. Due to friction with the ground wind, speed may decrease by 20 knots within a few feet above the runway.

This sudden loss of headwind, if immediately reflected on the aircraft airspeed indicator, can result in a sudden loss of altitude.

A sudden loss of airspeed and altitude may not be recoverable, and contact with the ground short of the runway may result. 

 

The second way wind shear affects an aircraft near the ground is the possible change in direction of the wind, which can cause the aircraft to be displaced from the runway centerline. Any sudden movement away from the runway that cannot be compensated by applying the proper crosswind correction must then result in a rejected landing, or “go-around,” by the pilot. 

During the landing phase, a pilot must be able to determine the effects of wind. During training a pilot is taught to recognize those effects by observation of smoke and dust on the surface; the ripples on ponds and lakes; the lighter underside of leaves that show on the upwind side of trees; and wind speed and direction indicators such as wind socks at the airport.