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True or Magnetic?

Posted: Fri Feb 07, 2020 12:26 pm
by WB714PR
I am going through my ground school books and such for my Sport Pilot, and I am confused as to when one uses True North or Magnetic North. Is there some easy way to keep track of what to use when without riding around with the FARAIM in your lap? It just seems rather odd.
Example (that everyone reading this already knows): METARs wind direction are True, yet the runway headings are Magnetic? But aren't you supposed to chose your runway direction based on the wind direction?

Who thought this up?

:shock:

Any guidance would be appreciated.

WB

Re: True or Magnetic?

Posted: Fri Feb 07, 2020 2:50 pm
by drseti
Yes, runway headings are magnetic. And yes, on the runway, you line up into the wind. This works because surface winds are given in magnetic in METARs, TAFs, and weather reporting systems such as ASOS, AWOS, ATIS, and DIGIWX.

Winds aloft, on the other hand, are reported and forecast reference to true North. This is because, when you plot a course line on a sectional aeronautical chart, your plotter gives you true course (that is, it references the top of the chart).

That is convenient, except when you try to navigate by VOR radials, GPS, or a compass. Those navaids all reference magnetic North, so for enroute navigation, you have to do a conversion. But fortunately, you do this on the ground (during preflight planning), not in the air when your hands are full.

To make it easy, figure that in the traffic pattern, everything is done by magnetic directions. During cross-country flights, planning involves true directions and winds, which you have to convert to magnetic to use your compass or electronic navaids.

Re: True or Magnetic?

Posted: Fri Feb 07, 2020 9:37 pm
by Warmi
WB714PR wrote:I am going through my ground school books and such for my Sport Pilot, and I am confused as to when one uses True North or Magnetic North. Is there some easy way to keep track of what to use when without riding around with the FARAIM in your lap? It just seems rather odd.
Example (that everyone reading this already knows): METARs wind direction are True, yet the runway headings are Magnetic? But aren't you supposed to chose your runway direction based on the wind direction?

Who thought this up?

:shock:

Any guidance would be appreciated.

WB



On a more positive note, once you get your ticket all of that will be akin to using an abacus to do your taxes .... you just gonna use GPS for navigation and not worry about any of that.

Re: True or Magnetic?

Posted: Mon Feb 10, 2020 1:34 pm
by WB714PR
drseti wrote:Yes, runway headings are magnetic. And yes, on the runway, you line up into the wind. This works because surface winds are given in magnetic in METARs, TAFs, and weather reporting systems such as ASOS, AWOS, ATIS, and DIGIWX.

I hate to disagree*, but Figure 7-1-7 on pg 955 and figure 7-1-21 pg 982 of the AIM indicate that METAR, TAFs, ASOS, AWOS, etc are reported in True Wind headings (I assume that these are surface winds???). So you can see why I am confused.

WB

*disagreeing with my Engineering Profs was never a highly successful endeavor for me.

Re: True or Magnetic?

Posted: Mon Feb 10, 2020 1:44 pm
by drseti
I checked with my adjunct CFI, who is a retired FAA air traffic controller and FSS weather briefer. He says that when they recorded ATIS tapes or read the surface winds to incoming and departing traffic, they had to convert the METAR numbers (which are indeed True) to Magnetic. So, technically, METAR and ATIS winds shouldn't agree. (No wonder you're confused!)

Re: True or Magnetic?

Posted: Mon Feb 10, 2020 2:02 pm
by drseti
As a further check, I just called a friend who was tower chief (now retired) at KIPT. He says when they read out winds on ground or tower frequencies, they always corrected to magnetic. But METAR numbers are indeed in True (I misspoke on that one). And what you get from your electronic flight bag app comes from the METAR, so it's also True. So, METAR and ATIS winds won't necessarily agree! (Best bet is to just look at the wind sock when choosing a runway).

Re: True or Magnetic?

Posted: Mon Feb 10, 2020 2:39 pm
by drseti
For the final answer, I went to Controllers of America:

All printed wx (METARs, TAFs, and EFB apps) give winds in True.

All wx broadcasts (ATIS, ASOS, AWOS, DIGIWX, or controller broadcasts) give winds in Magnetic.

So:
If you read it, it's true.
If you hear it, it's magnetic.

Class dismissed.

Re: True or Magnetic?

Posted: Mon Feb 10, 2020 2:58 pm
by WB714PR
Yay!
I can roll with that. Thanks!!

Re: True or Magnetic?

Posted: Mon Feb 10, 2020 6:31 pm
by Wm.Ince
Thanks for the leg work and clarification, Paul.

Re: True or Magnetic?

Posted: Tue Feb 11, 2020 2:18 am
by RTC'83
If you read is true
If you hear it magnetic

I feel like I understand now!

Thank you, Dr.

Re: True or Magnetic?

Posted: Tue Feb 11, 2020 10:36 am
by FastEddieB
drseti wrote:So:
If you read it, it's true.
If you hear it, it's magnetic.

Class dismissed.


That sounds like an elegant solution.

Back in the day, I was taking aviation classes at Miami Dade Community College. Two of our instructors were crusty old* “99’s”. Fran Sargent and Katie Strehle. I think it was Fran who stood up in front of the class and shouted: “Winds are true north! True North!! TRUE NORTH!!!”. She then dropped the major exception on us, “...unless being given in relation to landing”.

Not sure if it was 100% true then - or now - but it did stick.



*Thinking back, those “crusty old” instructors were very likely younger than many of us are now!

Re: True or Magnetic?

Posted: Wed Feb 26, 2020 5:13 pm
by WDD
Want another headache?

VOR's are Magnetic. So if you're on the VOR north radial, it's magnetic north.

Magnetic North as determined when the VOR was put into service. Might not have been / probably has never been changed over the years. So if its 30 years old and given the magnetic north has shifted, you now have True North, Magnetic North on your compass, and the VOR North (whatever the magnetic north was when when the VOR became active).

Re: True or Magnetic?

Posted: Wed Feb 26, 2020 8:34 pm
by drseti
WDD wrote:Magnetic North as determined when the VOR was put into service.


Well, that's mostly true. Yes, the VOR ground station was calibrated for magnetic North when it was first installed. But, the system gets flight-tested and recalibrated by the FAA periodically. So, if magnetic declination has changed over time, that will be picked up and corrected for at the next periodic calibration.

Re: True or Magnetic?

Posted: Wed Feb 26, 2020 11:01 pm
by Otto
AWOS is True, ASOS magnetic. Different gov't agencies operate the systems. Should be close enough to pick the right runway and then it doesn't matter what the meter says it matters how you handle the wind where your aircraft is.

Re: True or Magnetic?

Posted: Fri Feb 28, 2020 1:33 pm
by HAPPYDAN
Paul summed it up - If you read it, it's True. If you hear it, it's Magnetic. Except runway markings and test questions! But do allow me to diverge slightly. Remember those Squawk codes for the transponder? Sure, 1200 is VFR. But what about 7500, 7600, 7700? Confusing. But here's the easy answer:
7500 "Hi JacK!"
7600 "Can't talk now!"
7700 "Got an emergency!"