headwind

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In aerodynamics, a headwind is a wind that blows against the direction of travel of an object. A headwind reduces the object's speed and increases the time required to reach its destination. Headwinds are commonly measured in relation to the speed of vehicles — commonly air and watercraft — as well as in running events — particularly sprints. The opposite wind is a tailwind.

In aeronautics, a headwind is favourable in takeoffs and landings. As a result, aviators and air traffic controllers commonly choose to takeoff or land in the direction of a runway that will provide a headwind. In sailing, a headwind may make forward movement difficult, and necessitate tacking into the wind.

Aeronautics calculations

Pilots calculate the Headwind Component, Tailwind Component and Crosswind Component of any wind, if they do exist. Headwind and Tailwind are cosine functions of the wind while Crosswind Component is a sine function. Headwind and Tailwind do not occur together in normal conditions. Determining the ground speed of an aircraft requires the calculation of the head or tailwind.

Assume:

   A=Angle of the wind from the direction of travel
   WS=The measured total wind speed
   CW=Crosswind
   HW=Headwind

Then

   CW=Sin(A)*WS
   HW=Cos(A)*WS

For example if the wind is at 24015 that means the wind is currently from heading 240 degrees with a speed of 15 Knots and the aircraft is taking-off from runway 18; having heading of 180.

   Crosswind = Sin(240-180)*15 ≈ 13 
   Headwind  = Cos(240-180)*15 ≈ 7.5

The aircraft is said to have 13 knots of crosswind and 7.5 knots of headwind. Aircraft usually have maximum headwind and crosswind components which they cannot exceed. If the wind is at eighty degrees or above it is said to be full-cross. If the wind exceeds 100 degrees it is common practice to takeoff and land from the opposite side of the runway, it has a heading of 360 in the above mentioned example.

See also

• Tailwind
• Crosswind
• Air navigation