UAL747
Topic Author
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Joined: Mon Dec 13, 1999 5:42 am

### How Are Head/Tailwinds Calculated?

How does one calculate a headwind or tailwind in flight? Does it require the use of GPS, or can it be manually calculated somehow?

Thanks!

UAL
"Bangkok Tower, United 890 Heavy. Bangkok Tower, United 890 Heavy.....Okay, fine, we'll just turn 190 and Visual Our Way

tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

### RE: How Are Head/Tailwinds Calculated?

 Quoting UAL747 (Thread starter):How does one calculate a headwind or tailwind in flight?

Velocity over the ground minus velocity in the air = wind velocity

i.e. it's the speed difference but, since directions matter, you have to use vectors (velocities).

 Quoting UAL747 (Thread starter):Does it require the use of GPS, or can it be manually calculated somehow?

It does not require GPS but it does require some way to get velocity over the ground. GPS is one way to get that but you can also get it from taking periodic position fixes using any other navigation sources or using an inertial reference system.

Velocity in the air comes straight from the airspeed system and the heading indicator.

Tom.

KAUSpilot
Posts: 1659
Joined: Mon Jan 28, 2002 2:15 pm

### RE: How Are Head/Tailwinds Calculated?

It's calculated by Inertial Reference Units/Attitude Heading Reference Systems using trigonometry. The systems compare the aircraft's heading to its ground track and its true airspeed to its groundspeed to solve for wind direction and velocity, which is then displayed on the flight deck. The ground track and groundspeed can be determined by IRU's alone, GPS, or navigation radios receiving ground based navaids. The heading and airspeed are determined using traditional aircraft systems.

GPS can be used but something has to tell the GPS unit what the aircraft's heading and true airspeed are, so without that interface it won't be possible to display wind direction and velocity, to my knowledge. There are a few papers out there about using GPS signal reflections from the surface of the earth to calculate independent wind velocities, but I'm not aware if these methods are actually being applied.

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