Setting DG from compass question

LongRoadBob

LongRoadBob

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Since the ground school and exam stress exact answers, a student gets the idea that is important. It may well be, but it also may be that since winds aloft, etc. are not nec. precise, etc. that this is also just a "in the ballpark" thing but...

The compass has a deviation chart. So maybe N is right on the money where dev = 0, and you don't need to adjust but maybe W has you using the compass to heading 273 deg.

Up until now, pre taxi, I set the DG to the compass heading. Also we check just before takeoff that DG, compass, and runway all agree (I like this idea, which would help keep me from lining up on the wrong runway...though it wouldn't help when there is a L and R).

Ok...But if I happen to set it while the plane is pointed West, which value do you set? 270 or 276? thinking you would have to add since the deviation is for going the other way, where you desire a heading and have to subtract, here we use the compass to find what really is the heading.

But either way one does it, because of precession one is supposed to reset the DG every 10-15 minutes, and that would be from the compass while flying straight and level, not accelerating or decelerating. But then again, adjust for compass deviation?

No matter which way, it's always going to be off from the compass as once the DG is set, it is going to adjust relative to it evenly, where the compass has no set deviation.

I can't find it mentioned in ground school.
 
The compass correction card tells you what to steer to achieve the appropriate heading. So if your card indicates 275 to steer 270 (W) then if your compass says 275 you set 270 on the DG. The card is calibrated every 30 degrees, so you may have to interpolate and estimate for nearby headings. For example, if your compass reads 270, set 265 on the DG. (Still close to 275.) If heading between two corrections on the compass card, split the difference.
 
When you are on a parallel taxiway or on the runway, set the DG to runway heading.

Done.

It is likely a long time since the deviation card was made, so it is also likely not accurate anymore.

-Skip
 
Ok, I get how it works, but you guys both answered my real question, thanks!
So I have been setting it wrong, just setting what I read off the compass without checking the dev. chart.

SO that makes a lot of sense, since the DG will stay accurate until procession hits, and then I just adjust according to the chart after reading the compass. Thanks guys!
 
Skip Miller said:
When you are on a parallel taxiway or on the runway, set the DG to runway heading.

Done.

-Skip
Click to expand...

But aren't runways rounded up or down? Also as the mag. north changes, it can take time to rename a runway if needed?
 
LongRoadBob said:
But aren't runways rounded up or down? Also as the mag. north changes, it can take time to rename a runway if needed?
Click to expand...
All true. Unless you can fly more accurately than I can, and can determine the winds aloft with great precision, the degree of accuracy you aspire to as a student is not relevant.

-Skip
 
On the ground, use your GPS (track)to set your DG. Then it is set as exact as possible.
 
LongRoadBob said:
But aren't runways rounded up or down? Also as the mag. north changes, it can take time to rename a runway if needed?
Click to expand...
If there's an airport diagram then the runway heading to the nearest tenth of a degree will be on there. Airnav and Skyvector also list runway headings even for airfields without an airport diagram.
 
Skip Miller said:
All true. Unless you can fly more accurately than I can, and can determine the winds aloft with great precision, the degree of accuracy you aspire to as a student is not relevant.

-Skip
Click to expand...

I'm sure I can't but think like the old saying "measure with a micrometer, mark with chalk, cut with an axe" or something like that.
 
kgruber said:
On the ground, use your GPS (track)to set your DG. Then it is set as exact as possible.
Click to expand...

Actually, the ground is the place to set to the magnetic compass. Least interference there. In the air, ATC is looking at your ground track and tend to be very poor at wind correction. Better to just stick with the GPS then.
 
Considering most the correction cards are so old and faded, you can hardly read the values, and were never corrected after new avionics and other EQ were installed, their accuracy is questionable.
 
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In the west you can set your DG using section lines, making an informed guess on the wind drift angle.
 
Clip4 said:
Considering most the correction cards are so old and faded, you can hardly read the values, and were never corrected after new avionics and other EQ were installed, their accuracy is questionable.
Click to expand...

Isn't it in the regs that it has to be redone during service if any changes, or barring that even over a certain time since last?
 
I disagree with most people here. Set your DG to the compass. First off, its only going to be off by a few degrees at most, and i see that as better than the risk of adding 5 degrees when you should have subtracted and now you are off by 10 which is more noticeable. Also, if you are thinking of VFR cross countries for training, you likely already account for the deviation in your flight plan
 
The DG should be set to magnetic HEADING. That is, you correct for DEVIATION (errors in your mag compass indication) but not VARIATION (the difference between true and magnetic north). Most people don't have deviation errors of more than a few degrees (and hard to set the DG that fine anyway) so it doesn't much matter if you look at the correction card or not.
 
kgruber said:
In the west you can set your DG using section lines, making an informed guess on the wind drift angle.
Click to expand...
Section lines often aren't either true or magnetic north. Though the way most people fly, it's within the error margin of their ability to track a heading anyhow.
On the ground, use your GPS (track)to set your DG. Then it is set as exact as possible.
Click to expand...
Presumes that you are taxiing straight. Again, beyond the skill of many pilots.

;)
 
LongRoadBob said:
Isn't it in the regs that it has to be redone during service if any changes, or barring that even over a certain time since last?
Click to expand...

If you change the compass, but not if you change other equipment. There is also no requirement to periodically recheck.
 
Michael A said:
better than the risk of adding 5 degrees when you should have subtracted and now you are off by 10 which is more noticeable
Click to expand...

It's confusing probably because the method of notating the calibration card (i.e. with "For" and "Steer") seems to have been designed in a pre-DG era to make it easy to fly a heading with the compass.

Instead of
For 360 Steer 004
something like
CH 004 MH 360
or better yet
CH 360 MH 296
would be easier.
 
I always just set it to the whatever the compass said and called it good. I might adjust it in the air once or twice on long flights. Short flights I never touch it. Doing it this way has never gotten me so lost I couldn't find the ground.
 
Clip4 said:
If you change the compass, but not if you change other equipment. There is also no requirement to periodically recheck.
Click to expand...

Wow, that seems strange. I mean, I’ve learned that deviation is due to many magnetic factors, magnetic fields, electro magnetic, etc,
Seems certain that if you change out other electrical equipment it might affect it, wouldn’t it?
 
LongRoadBob said:
Wow, that seems strange. I mean, I’ve learned that deviation is due to many magnetic factors, magnetic fields, electro magnetic, etc,
Seems certain that if you change out other electrical equipment it might affect it, wouldn’t it?
Click to expand...

Yes, it also used to be common the have correction cards listing deviation for both lights/radios on and lights/radios off.
 
What ever you set it at it will need to be updated every so often. Just ball park it.
 
Precession is real. Check at least hourly en route. Otherwise, you’ll be on radar vectors near your destination and be asked to “say winds” because, for some reason, your track over ground is noticeably different than other planes flying the same vector at the same airspeed and altitude.
 
N1120A said:
Actually, the ground is the place to set to the magnetic compass. Least interference there. In the air, ATC is looking at your ground track and tend to be very poor at wind correction. Better to just stick with the GPS then.
Click to expand...

While ATC may not be good at wind correction, they assign you a heading and you steer it. That's a heading, not a track. If corrections are needed due to winds, they'll issue them. As far as sticking with the GPS goes, take a look at the YELM 2 (or later?) departure from OLM. It tells you to use a magnetic heading of 060 degrees. That's a magnetic heading, not a GPS track. And there can be a difference. If your plane isn't GPS equipped (and one of our club planes is still just steam gauges) you really don't have a choice.
 
Can't remember where I heard the story: The navigator was trying as hard as he could to be as exact as possible. He tells the captain, "Turn left 2 degrees." The captain said, "2 degrees? I can't make a correction that small!" The navigator says, "Turn left 15 degrees". Captain says, "That's more like it!" The navigator says, "Turn right 13 degrees." The captain says, "Yeah, I can do that!"
 
Ghery said:
While ATC may not be good at wind correction, they assign you a heading and you steer it. That's a heading, not a track. If corrections are needed due to winds, they'll issue them. As far as sticking with the GPS goes, take a look at the YELM 2 (or later?) departure from OLM. It tells you to use a magnetic heading of 060 degrees. That's a magnetic heading, not a GPS track. And there can be a difference. If your plane isn't GPS equipped (and one of our club planes is still just steam gauges) you really don't have a choice.
Click to expand...

Naturally, flying without a GPS would necessitate using the magnetic compass. That doesn't mean that the practicalities of how ATC are looking at you in the sky make using the GPS more practical - especially when you are on an approach and there isn't time for correction.
 
kgruber said:
Section lines are ALWAYS due true N/S/E/W...........with the emphasis on ALWAYS!
Click to expand...
And the emphasis is WRONG. While the intent is to align the sections with true north, the implementation is far from perfect for a number of reasons. Typically, only one end of the state has the grid aligned with true north, the rest deviate because the squares are, well, um, square. To keep all the section lines pointed at true north, they'd need to curve in as they go north.
 
flyingron said:
And the emphasis is WRONG. While the intent is to align the sections with true north, the implementation is far from perfect for a number of reasons. Typically, only one end of the state has the grid aligned with true north, the rest deviate because the squares are, well, um, square. To keep all the section lines pointed at true north, they'd need to curve in as they go north.
Click to expand...
I'm no cartographer, but I believe the geography printed on the chart is "distorted" (conformed may be a better word) in order to keep the squares square and accurate. Such corrections have to be made due to the lack of curvature of the flat chart as well.
 
Pilots shouldn’t think too much. Set DG to compass, periodically check for precession. Your ASA flight log has provision to account for compass card deviations. Fly the heading calculated on your ASA flight log aka Compass Heading.

Then get your private license and start following that magic magenta line.

725BB9F3-E19F-4BDE-90A6-17D0BED5305D.jpeg
 
Salty said:
I'm no cartographer, but I believe the geography printed on the chart is "distorted" (conformed may be a better word) in order to keep the squares square and accurate. Such corrections have to be made due to the lack of curvature of the flat chart as well.
Click to expand...


Alaskans can certainly chime in here, but lambert charts, sectionals here curve in as they move north.
That is, the distance between longitudinal lines get closer together as they move north, noticeably so.
I know I overthink, but the lines are not squared.

Probably in North America is isn’t that noticeable, but here it is.
But I bet if you measure a sectionals longitudinal lines even there at the most souther point and then most northern they northern will be less distance between.

I don’t know how they are in the US, but here they have three sectionals that cover all the length of Norway. Which means the one for southern Norway (including Oslo where I live) unfolded is about the size of a twin bed. I toyed with the idea of asking for “folding tips” but realized I’d just get hit with a lot of “use an iPad” responses. But man..no way one could unfold and go to a new section when flying. I haven’t checked (again, overthinking, it’s what I do) but hope that any two sections on one sectional would be where I would have to refuel anyway, so I could land, spend a half hour refolding the sectional and be on my merry way.

There is not room enough in the cabin to open up the sectional and refold it. I can barely do it in my living room!

When I was a kid, my dad (a pilot) for some reason I found strange once sat me down to teach me how to effectively read the old style newspapers like the Chicago Tribune. He folded it in a special way, where he could just refold one little section to get to the next. Hard to describe but I was really puzzled why he was showing me this. Now I think he worked it out first with sectionals, then realized it would also work with newspapers.

A lot of things he taught me I think stemmed from flying.
 
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