compass errors

Instructional videos always advise pilots of GA aircraft in Flightsim to check their instrument heading against the magnetic compass to correct for gyro drift. As an ancient mariner I know that the magnetic compass is itself subject to errors - and they can vary with the course you are steering - and presumably the heading one is flying. One of my last tasks at sea in a small boat was spending a morning crisscrossing a transit line on different courses, checking the magnetic compass error.

So, how do pilots deal with this? In a small aircraft the magnetic compass is close to a large lump of metal (namely an engine).

Robert

In a real GA plane there will be a card under the compass showing the error +/- degrees at several points of the compass. For example it might show it's -3 degrees for a 90 degree heading. It's a small enough error that I don't think anyone ever pays attention to it.

Can only be a non-pilot talking...

Real pilots know the importance of regular calibration, and compass error and what it implies!

Calibrate against known-accurate devices. GPS is good for this.

Calibrate against known-accurate devices. GPS is good for this.

Or a compass rose. For those not familiar, a compass rose is just markings on the ground laid out against magnetic north and designed so that an aircraft can be placed in the middle and aligned with various compass headings, as needed.

Or a compass rose. For those not familiar, a compass rose is just markings on the ground laid out against magnetic north and designed so that an aircraft can be placed in the middle and aligned with various compass headings, as needed.

There are no airfields in Great Britain with that feature...

Get's in the way of Industrial units and housing developments I guess!

So, just to clarify: The magnetic compass can be checked against a reliable instrument like a GPS. So I could take my hiking GPS in the aircraft to check the compass? But then if I have my pocket GPS couldn't I use that instead of the magnetic compass in order to check for gyro drift?

In a plane with a GPS navigation system presumably it doesn't used a gyro so one can use it to check the magnetic compass - just in case the GPS system fails?

Robert

Cambridge has one, and so does Redhill. Visible on the Google Earth view of the airfield, the circle NW of the threshold of 08L. There are probably others... Blackbushe (faint though) and White Waltham for example.

Redhill did away with theirs several years ago.

Don't rely on Google `anything` for up-to-date information, it's always out-of-date.

So, just to clarify: The magnetic compass can be checked against a reliable instrument like a GPS. So I could take my hiking GPS in the aircraft to check the compass? But then if I have my pocket GPS couldn't I use that instead of the magnetic compass in order to check for gyro drift?

In a plane with a GPS navigation system presumably it doesn't used a gyro so one can use it to check the magnetic compass - just in case the GPS system fails?

 

Robert

Exactly.

Until the batteries run out in the all-singing, all-dancing GPS... Then you'll wish you had a manual alternative.

So, just to clarify: The magnetic compass can be checked against a reliable instrument like a GPS. So I could take my hiking GPS in the aircraft to check the compass? But then if I have my pocket GPS couldn't I use that instead of the magnetic compass in order to check for gyro drift?

In a plane with a GPS navigation system presumably it doesn't used a gyro so one can use it to check the magnetic compass - just in case the GPS system fails?

 

Robert

A GPS has to be moving to give you a track, it compares your current position to a position you were X number of seconds ago and calculates the track. It also calculates the distance and uses that against the time between the two positions to give you a groundspeed. If you're not moving a GPS can't even tell you which way is North so nope, the handheld GPS wouldn't work. For that matter a GPS doesn't give you a heading anyway, it gives you a course. It would only be possible to use a GPS to accurately calibrate a compass if you knew precisely the wind direction and speed (which is unlikely in practice), then you'd need to precisely calculate your wind correction angle (more precisely than you can do with an E6B) and add or subtract it from your GPS track (course) to come up with a heading.

In the US the process for "swinging a compass" is detailed in the FAA publication AC 43.13-1B and yes it involves using a compass rose painted on the ramp. If you're interested here's a link to that document in .pdf form and the "compass swing" procedure begins on page 620 of the .pdf.

https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_43.13-1B_w-chg1.pdf

Thanks all, this has been a most interesting discussion. Being somewhat versed in these matters from a maritime perspective I would certainly not be so rash as to embark on any land, sea or air venture without a properly calibrated magnetic compass _and_ some spare batteries. I presume FSX and MSFS don't model all his but provide a magnetic compass that are always correct (I'll try 'flying' over the North magnetic pole some time)

Robert

Thanks everyone, this has been an interesting conversation, from which I have learnt a lot. As someone well versed in these issues from a maritime perspective I would never set out on a land, sea or air venture without carefully checking my magnetic compass (and taking spare batteries for anything else!)

When walking in remote territory I usually write the time of meridian passage in my notebook so that if the Sun in shining I can check my sighting compass. I've never actually needed to do this - but better safe than sorry.

I don't know how much of all this is modeled by flightsim, but I will 'fly' over the magnetic north pole just to see.

Robert