Ah, the mag vs true wind raises its ugly head again

[Dave S.]

“ If you read it it’s true, if you hear it it’s magnetic”. Remember that? Add the different sources to the mix and no wonder students get mixed up.

ASOS is usually configured to true. The ASOS usually gets transmitted to the system directly and the METAR (you read a METAR) reflects the true wind. When a controller makes an ATIS he usually reads the weather off the ASOS screen. This results in...Uh oh!...true wind on the ATIS which you hear thus violating the rule of thumb.

But wait the FAA had a clever solution years ago and is still in place today. When the controller gets to the wind when making the ATIS from the ASOS screen, the direction from the ATO is to use the direct wind indicators such as the SAWS (either the 10 or 120 second reading) or wind instruments either of which are magnetic. So the ATIS has some info from the ASOS and some from the direct wind instruments.

So you can rely on that rule to ensure that what you hear on the ATIS is truly magnetic, or magnetically true as it were.

The question becomes did the controller follow that directive or did he just read it off the ASOS screen.

My new philosophy is that a hundred years from now none of this will matter.

tex

 

[Dr. O]

It won’t take a hundred years to not matter if you are depending on others to tell you how to fly. They go home for supper, you get shoveled into a basket.
So, the nice man says wind is 290 At five and you are cleared to land on runway two-seven.
So why are you crabbing hard to the left to keep the needle centered?

 

[Salty]

I wanna know where someone flies where the wind readings are more accurate than the error between mag and true.

 

[texasclouds]

Is the sock true or mag???

 

[Kelvin]

Orange...

Is the sock true or mag???

 

[William Pete Hodges]

I didn't realize that there was a difference, so I am glad I read this. I guess I never noticed much because most of the places I fly the variance is only 11 degrees. It would make a big difference if the variance is 30 or more degrees.

ASOS AWOS and ATIS are all supposed to be magnetic, according to what I read.

 

[Capt. Geoffrey Thorpe]

DURG DALGT HRS TAILWINDS MAY EXIST OVER APCH ENDS RWY 07 & 25 SIMULTANEOUSLY.

Winds quoted are typically within 180 degrees of what you will find when you touch down.

 

[Salty]

I didn't realize that there was a difference, so I am glad I read this. I guess I never noticed much because most of the places I fly the variance is only 11 degrees. It would make a big difference if the variance is 30 or more degrees.

ASOS AWOS and ATIS are all supposed to be magnetic, according to what I read.

Most of use will never see variances over 11ish

 

[Dana]

Most of use will never see variances over 11ish

The 14° line runs right across the New England states...

 

[RussR]

Most of use will never see variances over 11ish

The 14° line runs right across the New England states...

The whole west coast is over 11, starting at 11.5 in San Diego and going up to 15.5 by Seattle.
And the 11W line runs through Baltimore, meaning everything north and east of there (Philadelphia, New York, Boston) is greater than 11, up to as @Dana says, 14, or even 16 in Maine.

That's a big chunk of the population.

 

[Larry in TN]

I've landed at an airport where the variation was 56W.

 

[sarangan]

“ If you read it it’s true, if you hear it it’s magnetic”. Remember that? Add the different sources to the mix and no wonder students get mixed up.

ASOS is usually configured to true. The ASOS usually gets transmitted to the system directly and the METAR (you read a METAR) reflects the true wind. When a controller makes an ATIS he usually reads the weather off the ASOS screen. This results in...Uh oh!...true wind on the ATIS which you hear thus violating the rule of thumb.

But wait the FAA had a clever solution years ago and is still in place today. When the controller gets to the wind when making the ATIS from the ASOS screen, the direction from the ATO is to use the direct wind indicators such as the SAWS (either the 10 or 120 second reading) or wind instruments either of which are magnetic. So the ATIS has some info from the ASOS and some from the direct wind instruments.

So you can rely on that rule to ensure that what you hear on the ATIS is truly magnetic, or magnetically true as it were.

The question becomes did the controller follow that directive or did he just read it off the ASOS screen.

My new philosophy is that a hundred years from now none of this will matter.

tex

I don't think it will take a hundred years. I can't remember the last time I used the whiskey compass. We just need a reliable reference. Whether that is the magnetic pole, geographic pole, or something else in outer space makes little difference.

 

[Palmpilot]

Most of use will never see variances over 11ish

The highest I've found in the U.S. is 22, in the northeast corner of Alaska.

https://skyvector.com/?ll=69.70080363660658,-141.52770995860624&chart=301&zoom=4

The highest I've found in the contiguous U.S. is 16, in the northwest corner of Washington state.

https://skyvector.com/?ll=48.01182200497676,-123.72848510396734&chart=301&zoom=4

 

[Dan Thomas]

So, the nice man says wind is 290 At five and you are cleared to land on runway two-seven.
So why are you crabbing hard to the left to keep the needle centered?

Hard to the left in a 5-knot wind??

 

[PaulS]

Doesn't matter, I take all wind reports with a grain of salt, the only wind that really matters to me is the one I'm experiencing a foot above the runway during the flare. And it only really concerns me if I run out of rudder on the side slip, then I have to go around.

 

[jimhorner]

Most of use will never see variances over 11ish

There’s a whole lot of pilots in the SF Bay area. 13° to 13.3° for us.

 

[Dan Thomas]

I learned to fly in 1973 in an area that had 23°E variation.

Historical interactive map: https://maps.ngdc.noaa.gov/viewers/historical_declination/

 

[Lindberg]

It won’t take a hundred years to not matter if you are depending on others to tell you how to fly. They go home for supper, you get shoveled into a basket.
So, the nice man says wind is 290 At five and you are cleared to land on runway two-seven.
So why are you crabbing hard to the left to keep the needle centered?

As my friend and instructor would say, "Who cares what he said, just do the thing."

 

[Huckster79]

Wind reports are a tool in the toolbox of a pilot- at the end of the day- fly the airplane.

If winds are different than reported by 15 degrees will you intellectually be able to calculate the amount of rudder needed or will you have to rely on your stick and rudder skills?

 

[Capt. Geoffrey Thorpe]

So, the nice man says wind is 290 At five and you are cleared to land on runway two-seven.
So why are you crabbing hard to the left to keep the needle centered?

Because the wind where you are at is not the same as the wind at the wind measuring thing?

I recall one early morning AWOS winds were nothing to get excited about. Climbing out, I noted I reached pattern altitude before I got to the end of the runway. Strange. Heading south west over a park at about 1400 AGL, I noticed that I was getting passed by a golf cart. OOPS. So, the wind there had to be on the order of 40+ knots vs. a handfull on the ground. I gave up on the idea of flying any further that morning and made the proverbial 180. Didn't take long to get back to the airport... Landing was uneventful due to not much wind on the ground.

 

[Salty]

Because the wind where you are at is not the same as the wind at the wind measuring thing?

I recall one early morning AWOS winds were nothing to get excited about. Climbing out, I noted I reached pattern altitude before I got to the end of the runway. Strange. Heading south west over a park at about 1400 AGL, I noticed that I was getting passed by a golf cart. OOPS. So, the wind there had to be on the order of 40+ knots vs. a handfull on the ground. I gave up on the idea of flying any further that morning and made the proverbial 180. Didn't take long to get back to the airport... Landing was uneventful due to not much wind on the ground.

I've seen the socks on opposite ends of the field pointing at each other, and standing out nicely, not just lazily swinging around.

 

[Capt. Geoffrey Thorpe]

I've seen the socks on opposite ends of the field pointing at each other, and standing out nicely, not just lazily swinging around.

Opposite magnetic or true?

 

[Salty]

Opposite magnetic or true?

Yes

 

[denverpilot]

Because the wind where you are at is not the same as the wind at the wind measuring thing?

I recall one early morning AWOS winds were nothing to get excited about. Climbing out, I noted I reached pattern altitude before I got to the end of the runway. Strange. Heading south west over a park at about 1400 AGL, I noticed that I was getting passed by a golf cart. OOPS. So, the wind there had to be on the order of 40+ knots vs. a handfull on the ground. I gave up on the idea of flying any further that morning and made the proverbial 180. Didn't take long to get back to the airport... Landing was uneventful due to not much wind on the ground.

Inversion over the Denver bubble of hot air and winds whipping out of the west is pretty much an everyday occurrence here in winter. Ground winds calm inside the bubble. Wind blowing like hell at my house out East outside the bubble.

 

[JVerb]

Here I am spoiled in Houston with only 3 degree difference.

 

[ateamer]

Wind reports are a tool in the toolbox of a pilot- at the end of the day- fly the airplane.

If winds are different than reported by 15 degrees will you intellectually be able to calculate the amount of rudder needed or will you have to rely on your stick and rudder skills?

The reported wind and the sock are great aids to determine which runway to set up for, but after that just fly the centerline and do what ya gotta do to land on it or change the plan.

 

[brcase]

“ If you read it it’s true, if you hear it it’s magnetic”. Remember that? Add the different sources to the mix and no wonder students get mixed up.

ASOS is usually configured to true.

What source makes you think ASOS winds are based on True North?

the "“ If you read it it’s true, if you hear it it’s magnetic”" memory aid is getting to be outdated, since you can read nearly all weather now. You have to consider the original source, ASOS is an automatic Stations that is transmitting live observations, it is then sent to create a Metar which is a surface observation. the original source was heard.

A better memory aid is Surface observations/forecasts are Magnetic, Area and altitude observations are true. This makes sense since the only thing you should be using surface observations for is to determine which runway (magnetic heading) to use (or not use). enroute you should be using Winds aloft observation/forecasts, which are true.


Brian
CFIIG/ASEL

 

[IK04]

There’s a whole lot of pilots in the SF Bay area. 13° to 13.3° for us.

I did all my training and flying in the Bay area up until I left to find a job.

My first two weeks of instrument training included a whole lot of compass turns and it is still burned into my memory how to lead and lag...

 

[ARFlyer]

I’ve never noticed as I’m basically on the agonic line. So everything around here is +/- 1° if that.

 

[BrianNC]

There’s a whole lot of pilots in the SF Bay area. 13° to 13.3° for us.

I would consider that 11ish. 2° is insignificant.

 

[MauleSkinner]

I’ve never noticed as I’m basically on the agonic line. .

So you’re not sure whether variation exists or not?

 

[Dave S.]

What source makes you think ASOS winds are based on True North?


Brian
CFIIG/ASEL

FAA order JO 7300.5E paragraph 7.13

“NOTE: Local displays of wind direction are always in reference to magnetic north. Automated weather observing systems also show direction with respect to magnetic north locally (when the AUX/WX page is displayed), but adjust wind direction to "true" for transmission.“

and

ASOS user manual...

“ Wind direction is reported relative to true north in the METAR/SPECI message, in the daily/monthly sum- maries, and on all video displays. Wind direction is re- ported relative to magnetic north in the computer-generated voice messages, and on the OID “AUX” data display screen.”


the ASOS transmits true and displays to the controller magnetic.

the problem comes when the displayed magnetic wind is manually transmitted by the certified controller ( such as when transmit function is OTS) without converting to true. In such a case the manually transmitted magnetic wind will be reported in the METAR as true. It doesn’t happen that often but the point is that humans won’t always make the conversion. At manually observed stations where there is no ASOS the observer must always convert to true but I don’t think that can always be counted on.

tex