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YokoTsuno
Topic Author
Posts: 364
Joined: Tue Feb 01, 2011 1:21 pm

### How Is Tailwind And Headwind Measured?

How is tailwind and headwind actually measured? You can't possibly derive that from the forward pointing pitot tubes alone, can you?

boeingfixer
Posts: 573
Joined: Fri Jul 29, 2005 2:02 am

### RE: How Is Tailwind And Headwind Measured?

 Quoting YokoTsuno (Thread starter):How is tailwind and headwind actually measured? You can't possibly derive that from the forward pointing pitot tubes alone, can you?

It's done with Inertial Reference Units(IRU's). This is the heart of most modern airliners and provides attitude and direction information to the pilot and flight management computers. In simple terms it knows the aircrafts position, speed, heading, drift, course etc. and can extrapolate wind information from this data.

Cheers,

John
Cheers, John YYC

BE77
Posts: 360
Joined: Fri Nov 02, 2007 2:15 am

### RE: How Is Tailwind And Headwind Measured?

 Quoting boeingfixer (Reply 1):t's done with Inertial Reference Units(IRU's).

..or on the windspeed / wind component page of most Aviation GPS machines, especially if they are tied into the airspeed and external air temp data.

Old school is by taking the groundspeed calculated by time between two points or between navigation beacons (VORs, NDB;s etc), calculating true airspeed from airspeed, temp, and altitude data, then adding in the correction for drift required to maintain the desired track, then plotting all that on the wind side of the whiz wheel, and voila, it's done. Pre GPS some of us could actually do that fast enough for the answer to be relevant to the current flight. Of course, I also remember using a slide rule, rotary telephones, leaded car gas, and a whole truckload of other stuff I DO NOT MISS!
Tower, Affirmitive, gear is down and welded

Mir
Posts: 19491
Joined: Mon Jan 05, 2004 3:55 am

### RE: How Is Tailwind And Headwind Measured?

Pure headwind or tailwind is just groundspeed minus true airspeed. If you get a positive number it's a tailwind, a negative number is a headwind. So if you're flying at 450 knots and your groundspeed is 480 knots, you've got a 30 knot tailwind.

But since the wind is rarely directly on the nose or the tail, other factors like heading and track start to come into play. The formula for headwind and tailwind still works, but the 30 knot tailwind would now just be a component of the total wind.

-Mir
7 billion, one nation, imagination...it's a beautiful day

B747FE
Posts: 217
Joined: Mon Jun 21, 2004 11:09 pm

### RE: How Is Tailwind And Headwind Measured?

 Quoting YokoTsuno (Thread starter):You can't possibly derive that from the forward pointing pitot tubes alone, can you?

No, you can't. boeingfixer got it right...the IRS/GPS or INS in old birds take care of that.
However when in long flights in the middle of nowhere and boredom kicks in, nothing like cross-checking your instruments with a good old CR3.
Wind triangle, anyone?

Regards,
B747FE.
"Flying is more than a sport and more than a job; flying is pure passion and desire, which fill a lifetime"

Starlionblue
Posts: 20752
Joined: Fri Feb 27, 2004 9:54 pm

### RE: How Is Tailwind And Headwind Measured?

 Quoting YokoTsuno (Thread starter):You can't possibly derive that from the forward pointing pitot tubes alone, can you?

Quite. That only gives you indicated airspeed. Aerodynamically, the aircraft doesn't care if it is in in a tailwind or a headwind. It just cares about airspeed.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

YokoTsuno
Topic Author
Posts: 364
Joined: Tue Feb 01, 2011 1:21 pm

### RE: How Is Tailwind And Headwind Measured?

Thank you very much for the anwers

Are you referring to these?

http://en.wikipedia.org/wiki/Rotary_dial

Really? So you dial the guy who does the calculation for you .

sprout5199
Posts: 1681
Joined: Sat Feb 26, 2005 8:26 am

### RE: How Is Tailwind And Headwind Measured?

 Quoting be77 (Reply 2):Of course, I also remember using a slide rule, rotary telephones, leaded car gas, and a whole truckload of other stuff I DO NOT MISS!

or having to get up and change the channel on the TV. When I was young, I asked my Dad to buy a TV with a remote, his answer "we already have one, I just have you change the channel for me"

Dan in Jupiter

BE77
Posts: 360
Joined: Fri Nov 02, 2007 2:15 am

### RE: How Is Tailwind And Headwind Measured?

 Quoting YokoTsuno (Reply 6):Really? So you dial the guy who does the calculation for you

Heh heh - yes, one of those. I even spent a summer job fixing / refurbishing them at the local phone co warehouse.
Problem with your idea of calling someone to do the calculation though is that as strange as it sounds, we didn't have cell phones either. Does anyone know if there is an Iphone app to let you dial using rotary movement, hopefully with the clicking sounds and all?

 Quoting sprout5199 (Reply 7):or having to get up and change the channel on the TV. When I was young, I asked my Dad to buy a TV with a remote, his answer "we already have one, I just have you change the channel for me

How on earth did I not remember that...of course, we only had two channels (CBC English, and CBC French), we we only had to walk over to the TV to turn it on or change the volume. (And yes, it was uphill both ways through 6 feet of snow to get to school!).
Tower, Affirmitive, gear is down and welded

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