Compass no longer on my team

[Ned Gravel]

OK - here is the problem.  Compass started going a little wonky after my overhaul last year.  Compounded after this year's annual such that a new mount was acquired and installed.  I was told that one of the causes of having readings that are up to 30 degrees off of "nominal" was the use of high power items like pitot heat now that I fly IFR.

I have two compass cards now - one for IFR and one for VFR.  It is, however, disconcerting to not be able to trust the "non-precessing" directional instrument for reference.  Other than putting in a full functioning ADAHRs setup, I am stuck with the current DB and a whiskey compass.

So, here are the questions (prefaced by the tidbit that I have over 22 years in the Signal Corps and understand things like "induced magnetism"):

• 
  
• Anyone else ever had this problem?
  
• If so, what are the most likely culprits:
  
  • 
    
  • Some bare wire that may be laying up against the frame?
    
  • Some highpower wire that is missing some shielding at a critical point in its path?
    
  • A bad compass?
    
  • Magnetised set of steel members that need to be degaussed?
    
  
  
  

From the collective wisdom of this group, I may be able to arrive at a solution using the least amount of AMUs and causing me the least amount of concern.

Sock it to me.....

[skyking]

Hi Ned: I had the exact same problem and i needed a Degauss.

The Mooney dealer in Calgary did it for me ( Cavalier Aviation) and that ended the problem.

[skyking]

PS: I think it cost me about 200 bucks.

[takair]

As skyking mentioned there is a procedure for degaussing.  I think you technically want to remove equipment though.  The steel tube in the Mooney is prone to this.  The compass is typically mounted to this.  I have had to do a compass swing on my Mooney every few years.  It does not help when lights and pitoto heat are on.  I have gotten in the habit of checking the DG by turning these off for a few seconds.  Prior to descendign on an approach, I typically give one last check of the DG before lights and pump.  A good DG should give you at least 30 minutes before a check, plenty of time for an approach.  It seems that the vertical card compass may be slightly better in a Mooney.  You can get optional "balance balls" that are larger compensation magnets.  Seems they are a little less prone in a Mooney.

[Immelman]

You could try to go out and swing it first to see if that helps. To the disbelief of my A&P who felt all Mooney compasses were a crap shoot because of the steel frame, I got it to swing within tolerances. Tools needed: brass screwdriver, plumb bob (hang from tail to help align the plane on the rose), towbar, and a couple hours to position the airplane oh so many times on the compass rose.

[FlyDave]

Quote: Immelman

You could try to go out and swing it first to see if that helps. To the disbelief of my A&P who felt all Mooney compasses were a crap shoot because of the steel frame, I got it to swing within tolerances. Tools needed: brass screwdriver, plumb bob (hang from tail to help align the plane on the rose), towbar, and a couple hours to position the airplane oh so many times on the compass rose.

[takair]

Quote: FlyDave

Don't you want to swing the compass with the engine running and all the normally running avionics on so it's swung in "flight configuration"?

[Immelman]

Quote: FlyDave

Don't you want to swing the compass with the engine running and all the normally running avionics on so it's swung in "flight configuration"?

[richardheitzman]

OK you guys don't go out there swinging your aircraft around in circles. lol geez. First off, you need a calibrated compass rose or a calibrated compass to do the job right, and guess what, a really good avionics guy who has done it a thousand times.

I can swing a compass in about 15 minutes and you do not need to shut the engine down a dozen times. It is a two person operation, one to run the aircraft and one to line up the aircraft on the rose or using the calibrated compass. The inside guy does the paperwork and adjusts the compass and operates the aircraft.

The reason they make a compass rose it to make a place that is free from interference (power lines, metal, buildings). They certify it on a regular basis. Don't get out there on the runway trying to set your Calibrated magnetic compass with a DG that precesses or off a runway heading that you are trying to line up with.

If you think your compass is showing an error, then compare it with another compass or a GPS heading the next time you fly. If it’s more the +-4 degrees in any one heading then consider degaussing the aircraft. (max error if I remember is 2 degrees, so if you have +5 east heading error you cannot adjust it out).

There is a ton of online material to tell you how to do it, but I would suggest you review what is “authorized” for pilots to accomplish in the aircraft. Compass calibration is not one of them.

Degaussing is an art that if not done correctly can cause more problems than it solves. Make sure you have a qualified Mooney technician who has done it before. The older the technician the better J

 

[takair]

I was looking for something else and came across the procedure I mentioned in an earlier post.  Don't know if you got this resolved, but SB M20-150A applies to many older Mooneys.  There are procedures for moving some power wires, degaussing and swinging the compass.  Link is:  http://www.mooney.com/index.php?keywordsearch=m20-150a&option=com_servicepdf&modelid=&Itemid=46&sort=date&direction=DESC&model=all&categoryid=1

[Ned Gravel]

Quote: richardheitzman

...

I can swing a compass in about 15 minutes and you do not need to shut the engine down a dozen times. It is a two person operation, one to run the aircraft and one to line up the aircraft on the rose or using the calibrated compass. The inside guy does the paperwork and adjusts the compass and operates the aircraft.

....

 

[Ned Gravel]

Quote: takair

I was looking for something else and came across the procedure I mentioned in an earlier post.  Don't know if you got this resolved, but SB M20-150A applies to many older Mooneys.  There are procedures for moving some power wires, degaussing and swinging the compass.  Link is:  http://www.mooney.com/index.php?keywordsearch=m20-150a&option=com_servicepdf&modelid=&Itemid=46&sort=date&direction=DESC&model=all&categoryid=1

[takair]

I wonder if some steel hardware ended up in place of the brass or stainless.  One steel screw, installed with a magnetized driver can really mess things up.

[richardheitzman]

Interesting to get the whole story. If someone changes the housing on the compass, it might just be as simple as the hardware is not stainless steel.

Also, at the annual if the technician laid a electrical cord across the fusalage and left it there for a long time, it could have magnatized the area.

[Ned Gravel]

Found part of the problem today.

Long story short, my M12D quit on final to duPage yesterday.

Today on the way back to Toronto, I discovered it was an intermittent circuit breaker - but I discovered some other things while flying with one radio and the handheld hooked up as the backup.

The compass error was reduced to 15 degrees. Something in that portion of the avionics tray (or the breaker section) was inducing a magnetic field that the compass was reacting to. So I tried some things to see if I could reduce (or increase) the compass error.

With that breaker pulled and that radio off and the pitot heat off and the landing light (the kind that only draws 8 amps) off the compass error disappeared altogether.

When the breaker was reset, and all the goodies back on, the compass error was reduced to 10 degrees (and back in the realm of the acceptable).

So it all comes down to a 45 year old breaker.

I am buying some new Klixons this weekend.

 

[Piloto]

From your last post the problem may be related to a voltage drop when those items are turned on. Check the battery switch and CB, also check the voltage at the compass system. On a good system with the engine running the voltage should be between 13 to 14.5 volts.

José

 

[carusoam]

Ned,

Changing the klixons all at once is better than one at a time....

The solid buss bar on the back has to come off to get one or all...

Will you be upside down in the footwell or working blind from the top in front of the windshield?

Best regards,

-a-

[Ned Gravel]

End of an era:

C-FSWR came out of annual last Friday and Clarence had taken care of a whole bunch of things.

1. 
   
2. All of the breakers are new Klixons.
   
3. All of the wiring in the avionics bay now conforms to requirements.  Amazing what some people will do to take shortcuts.
   
4. I have a 406 MHz ELT in and working.  Plumbed to the panel as specified in requirements.
   
5. Heater now connected to muffler shroud.
   
6. Overhead potentiometer replaced and two LED shoulder lights installed.
   
7. GMA 340 installed to replace NAT 80 and Narco CP 135 combo.  I now have a cockpit speaker and and audio panel capable of driving it.
   
8. Strobes now plumbed in to allow the belly one to act as a beacon without turning on all the other strobes.
   
9. Davtron 701 installed the provide me with "in your face" tuning indication for my ancient Bendix T12C so that I can configure the aircraft for missed after passing the FAF without taking my eyes off the G/S and Loc.  (In Canada we are still using NDBs as holds following some missed approaches).  (I know, I know, I will get a 430 to solve all that when enough folks are headed to a new Garmin and the price of 430's comes down.).
   
10. SB M20 150A was done to reroute the 6 guage wire from the main buss to the secondary buss in the cockpit.  Neat service bulletin.  It has the wire go forward to the firewall from the main buss, then cross the steel cage at the same place twice so that the electric field generated by one direction cancels out the electric field generated by the wire coming back.  Then it follows the firewall and crosses the steel cage in exactly the same fashion to head to the secondary buss.  This is where I got to teach the right hand rule to the young AME (Y'all call them A&P) contortionist who did this.
    
11. Redone insulation in the side panels (much quieter aircraft), and finally.
    
12. Redid the cushionning in the front seats.
    

No more magnetism induced to the compass.  Better audio panel (with stereo to the front seats).  MUCH better ELT. 

Safer aircraft all round.

Thanks Clarence.