Making a new compass card

[Gone Flyin]

The compass card in my C150, the one printed on the bottom of the magnetic compass, is worn off and unreadable. I need to re do it.

There is a compass rose at an airport not far from my home base and I can go there early in the AM to use that.

Question: I see someone on Youtube doing it by taxiing the plane around and around and stopping at various points. Cannot see the rose from the angle of the shot but assume there is one.

My friend, who works for a major helicopter manufacturer, says the way they do it is to put the ship (engine off but all avionics on) on a cart and drag it around from point to point to be very accurate.

That means I'd take my nose wheel handle with me and do it that way moving it from point to point. I feel this will be way more accurate as I can place the nose tip right over the markings.

Suggestions from those who have had to do this, please.

 

[kgruber]

You can do it MUCH faster and more accurately using your GPS.

 

[Doc Holliday]

https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_43-215.pdf

 

[RussR]

You say it's unreadable now, but do you have any panel pictures with it where it is readable? If so, just copy the numbers down onto a new one. Or, if it's just "hard to read" but you can still barely make things out, same answer.

Or, sometimes mechanics when swing it will put the same information in the log for easy duplication later on.

I guess what I'm saying is, there may be several ways to re-create the data without even leaving the hangar.

 

[Jeff Oslick]

You can do it MUCH faster and more accurately using your GPS.

We just installed a new compass, and yes, swinging it airborne using GPS is better. Interestingly, the AC (also, the Airpath instructions) that allows use of GPS for swinging the compass doesn't mention that the GPS needs to be a panel-mount.

 

[RussR]

We just installed a new compass, and yes, swinging it airborne using GPS is better. Interestingly, the AC (also, the Airpath instructions) that allows use of GPS for swinging the compass doesn't mention that the GPS needs to be a panel-mount.

I have read the AC, and I'm not sure how this would work. How are the effects of wind compensated for?

The Airpath instructions don't offer anything additionally, saying "NOTE: If aircraft is equipped, GPS can be used (allow for deviation) to establish reference headings for compass compensation."

How exactly do you establish reference headings with a GPS, which can only show ground track?

Clearly I'm missing something, but I don't know what it is.

Further, paragraph 7.5.1 mentions that for an airborne calibration, the aircraft is flown on 8 headings (the cardinal and ordinal directions), so, every 45 degrees. But the compass card is filled out in 30 degree increments. How is this difference reconciled?

 

[kgruber]

We just installed a new compass, and yes, swinging it airborne using GPS is better. Interestingly, the AC (also, the Airpath instructions) that allows use of GPS for swinging the compass doesn't mention that the GPS needs to be a panel-mount.

You shouldn't be airborne. On the ground, just like doing it with a compass rose

https://www.airpathcompass.com/general-5-1

 

[kgruber]

I have read the AC, and I'm not sure how this would work. How are the effects of wind compensated for?

The Airpath instructions don't offer anything additionally, saying "NOTE: If aircraft is equipped, GPS can be used (allow for deviation) to establish reference headings for compass compensation."

How exactly do you establish reference headings with a GPS, which can only show ground track?

Clearly I'm missing something, but I don't know what it is.

Yes.............you do the GPS compenstion ON THE GROUND!!

 

[RussR]

Yes.............you do the GPS compenstion ON THE GROUND!!

Did you read the AC or the Airpath instructions? Both specifically mention airborne compensation methods, unless, again, I'm reading them wrong.

 

[dmspilot]

Did you read the AC or the Airpath instructions? Both specifically mention airborne compensation methods, unless, again, I'm reading them wrong.

Airpath says "simulated flight conditions". The airborne method in the AC uses headings determined with INS/AHRS.

 

[Jeff Oslick]

But the compass card is filled out in 30 degree increments. How is this difference reconciled?

It is possible to fly headings in 30 degree increments.

 

[dmspilot]

We just installed a new compass, and yes, swinging it airborne using GPS is better. Interestingly, the AC (also, the Airpath instructions) that allows use of GPS for swinging the compass doesn't mention that the GPS needs to be a panel-mount.

It is possible to fly headings in 30 degree increments.

How do you fly headings in the air with GPS?

 

[RussR]

Airpath says "simulated flight conditions". The airborne method in the AC uses headings determined with INS/AHRS.

From the Airpath instructions, my bolding:

"Best results can be obtained in actual flight compensation by following the procedure outlined below:"

And later on,

"As stated, in-flight compensation will achieve the best results. "

From the AC:
"7.5 Air Swing Procedure. Air swings may be accomplished on the aircraft magnetic compass by comparing readings against those obtained from an electrically calibrated and compensated system or against readings obtained from an inertial navigation system 8/7/17 AC 43-215 5 (INS), Attitude and Heading Reference System (AHRS), or a Global Positioning System (GPS)"

It is possible to fly headings in 30 degree increments.

Um, obviously. But the AC says "7.5.1. An air swing involves flying the aircraft (in smooth air) on eight headings: north, northeast, east, southeast, south, southwest, west, and northwest". These are obviously not 30 degree increments.

How do you fly headings in the air with GPS?

Of course you can't, and that's exactly my point, and why the AC and Airpath instructions don't make sense to me.

 

[Jeff Oslick]

Of course you can't, and that's exactly my point, and why the AC and Airpath instructions don't make sense to me.

Calm wind, or nearly so, such that the error is minimal. It isn't like there isn't some error in a ground swing too.

 

[Jeff Oslick]

Um, obviously. But the AC says "7.5.1. An air swing involves flying the aircraft (in smooth air) on eight headings: north, northeast, east, southeast, south, southwest, west, and northwest". These are obviously not 30 degree increments.

This is a situation where the manufacturer's instructions supersede the specific text of an AC. Occasionally, the FAA isn't offended when you let common sense prevail.

In 28 years of flying, I can't honestly think of a time when I really studied a compass correction card in flight...

 

[Doug Reid]

If airborne compensation works best, why is there a big compass rose painted on the ramp ? Like Russ said, you compensate for NS error and EW error. Then, you document the results.

 

[RussR]

This is a situation where the manufacturer's instructions supersede the specific text of an AC. Occasionally, the FAA isn't offended when you let common sense prevail.

It's actually an internal contradiction within the AC. 7.5.1 says to fly the 8 headings, but then later it tells you to document the correction in 30 degree increments.

In 28 years of flying, I can't honestly think of a time when I really studied a compass correction card in flight...

Well, yeah, there is that. No argument there.

 

[dmspilot]

"As stated, in-flight compensation will achieve the best results."

They are talking about the DG method.

"7.5 Air Swing Procedure. Air swings may be accomplished on the aircraft magnetic compass by comparing readings against those obtained from an electrically calibrated and compensated system or against readings obtained from an inertial navigation system 8/7/17 AC 43-215 5 (INS), Attitude and Heading Reference System (AHRS), or a Global Positioning System (GPS)"

If you keep reading you will see the system must display MH or be converted to MH. A GPS with an ADC input could calculate winds and therefore MH. At the beginning of the section it also says, "equipment listed after the methods below are just examples".

 

[flyingron]

One minor point. What you are talking about is making a compass CORRECTION card. When you say "compass card" it refers to the thing that spins around in the compass.

 

[dmspilot]

Calm wind, or nearly so, such that the error is minimal. It isn't like there isn't some error in a ground swing too.

Terrible idea, and does not follow the AC, which indicates the reference system must indicate aircraft "heading".

 

[Silvaire]

There are "calm wind" days but not many of them. There are also roads, canals and runways of a known direction or one that can be determined by a chart. You can set your DG to runway heading before takeoff and record the difference between it, the compass and GPS TRK. Your DG will precess after a number of turns but if you're over the airport or features of known direction you can reset it. So, taking your time and using your head you can get a pretty decent swing. It's not that easy on the ground with a rose anyway, especially with a taildragger. You really want to be in level flight with the engine and all avionics running in cruise config.

But it's really just an exercise in attempting to comply with a regulation that was written probably in the 1920's because, as mentioned, who here has (or knows anyone who has) made it back home by the skin of their teeth solely based on using the compass correction card and would have surely been lost had it not been there?

 

[Low Level Flyer]

Had to swing mine last year due to a significant avionics upgrade. Had the results included in the logbook sticker so it's easily reproducible if the one in the airplane is lost or becomes illegible.

 

[Witmo]

It's absolutely critical to get this right for the next time you attempt an transAtlantic crossing using only your whiskey compass and a wristwatch.

 

[Skip Miller]

In the nautical world, the same problem exists. I have seen one marine “avionics” electronics dealer that had a portable mechanical gyro compass. This made the job easy. Load the gyro and true it to a lubber’s line. Then turn the plane, noting the true reading and the mag reading at desired points. Easy Peasy!

-Skip

 

[Deelee]

panel pictures with it where it is readable?

Had the same situation last year when our airpath card fell off and was lost forever. Granted, it was older and more brittle than the Dead Sea Scrolls, so maybe it just dis-integrated into thin air.

Looked back and had a panel pic of it. Was able to zoom in an create a new card based on the faded numbers that were probably written in some ancient plant-based ink.