Unexpected compass errors. . .

C

cre8flyer

New Member
Does a magnetic compass ever get old and inaccurate?

I had 2 students get "lost" on their first dual X-C flights. Reviewing their headings in flight I couldn't find any errors, so I checked compass headings vs. very straight roads that show up on the charts and can be measured exactly.

I found errors of around 20 degrees (i.e. road on the chart runs directly East / West true, correcting this for the local variation should give a magnetic heading for a windless day. The compass deviation card has only a 2 degree correction for Easterly headings). Overall, the tiny WCA and deviation corrections could never explain a 20 degree difference between the chart and the compass.

The compass deviation card is dated 1999.

What else could cause this large error?
 
Have there been any new avionics installed since the last compass swing?
 
I was just going to amend my post to include that all of the electrical equipment was working correctly.

Your question about whether any new equipment was installed is excellent. I'll have to check. However, I doubt that any new equipment was installed because this airplane has only 1 old Nav/Com radio, a transponder, and no other bells or whistles.
 
Compasses just do that after a while. There are a couple of screws under the compass card sticker and the technicians can reset the correct heading. I've been fighting to get our planes to the avionics shop to have a Compass Swing done on all of them.
 
In what part of the country was that? I don't see you mentioning anything about magnetic variation, which is around 18-19 degrees in the NW USA.
 
Try turning on and off different equipment while on the headings that have the most errors. Depending on the wire routing, some items can have a large effect on the wet compass and it may not have been taken into account during the original compass swing.
 
Torqued said:
Try turning on and off different equipment while on the headings that have the most errors. Depending on the wire routing, some items can have a large effect on the wet compass and it may not have been taken into account during the original compass swing.
Click to expand...

The other day I was doing instrument approaches with my student and I noticed that whenever he held runway heading, his nose would be pointed like 30 degrees away from the runway. At first I thought he had just forgot to set the DG to the compass, but I checked and the compass and DG were both the same.

When we landed on runway 31, the magnetic compass was reading almost north. It was night and no one else was around, so we just stopped on the runway to see what could have been causing the interference. The first thing I tried was the alternator, because that seems to be the biggest source of interference for the radio. Sure enough, when I flipped off the alternator switch, you could see the magnetic compass swing 40 degrees back to runway heading. Since it was night, we couldn't just keep the alternator off, so we just continued the flight but with me doing my best to keep the DG set, according to my judgment.

So if you ever notice the magnetic compass being way wrong, flip off the alternator. Additionally, if you can't hear the radio because theres a high pitched wine on all frequencies, try turning off the alternator. Those things can cause a lot of electromagnetic interference.
 
It seems a compass can simply get old and inaccurate. . . the mechanics adjusted (which I didn't know was possible) it and it is now working fine again.
 
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