Sudden Compass Error

J

jbrrapa

Pre-Flight
PoA Supporter
Joined
Jan 16, 2010
Messages
42
Display Name
Display name:
JBR
My pedestal mounted compass suddenly began to be off by about 10-12 degrees in every direction. Nothing new has been added to the aircraft or on the glareshield. I did notice that the fluid level is slightly below the top, although it may have always been that way, There are no obvious signs of a leak.

Any thoughts?
 
Suddenly magnetic personality?
New phone?
New tablet?
 
Perhaps a first indication of an electrical or some radio issue emerging?

Although, if it’s off by the same amount on every direction.....is it possible the mount is loose and someone or something bumped it?

Carrying anything new in your flight bag?
 
jbrrapa said:
My pedestal mounted compass suddenly began to be off by about 10-12 degrees in every direction.
Click to expand...
Define suddenly. Sounds more like something was placed near it that is made of iron or is electrical to be "sudden."
 
Last edited:
If I remember correctly compasses in airplanes and boats have adjustment screws to compensate for the magnetic signature of the vessel. Maybe one of them slipped? You can go to a compass rose and readjust it if necessary. There is a procedure on how to do it and to fill out the compass correction card.
 
Sounds like it needs to be removed and serviced. While out it can be checked for accuracy away from the airframe.
 
Bell206 said:
Define suddenly. Sounds more like something was placed near it that is made of iron or is electrical to be "sudden."
Click to expand...
By suddenly, I mean it happened out of the blue two flights ago. The only significant occurrence prior to the first flight was removal, inspection and replacement of a magneto. The aircraft is a Seneca II. The magneto is far from the compass.
 
jbrrapa said:
By suddenly, I mean it happened out of the blue two flights ago.
Click to expand...
So there is nothing "new" in the vicinity of the compass? it doesn't take much to throw it off. Simply installing a steel screw or bolt nearby can cause this. Or if a power wire in the pedestal shifted closer to the compass. If nothing obvious externally may want to pull compass and check as mentioned above.
 
The best ref I’ve ever seen on this topic was a Mooney SB from ‘60s.

The steel tubing and Johnson Bar were common problem areas.

Using Jumper Cables can inadvertently magnetize steel even if just

draped over via induction.

The Hi- Fi ( audio) guys use a Gauss Meter to check for magnetic fields.

Isolating the source is usually the biggest issue.


What I’ve encountered had nothing to do with the compass.

Suggest you put a known good compass near yours to compare before

ordering and installing another.


Btw. Squids that were on Minesweepers are very knowledgeable on this.
 
Did someone leave a headset on the glareshield next to the compass for a while? Or some other device that has magnets in it? You can permanently ruin a compass by doing that. Headsets have magnets in their speakers.

Or did you change some habit while flying, like starting to use the landing lights in the daytime near the airport?
 
jbrrapa said:
My pedestal mounted compass suddenly began to be off by about 10-12 degrees in every direction. Nothing new has been added to the aircraft or on the glareshield. I did notice that the fluid level is slightly below the top, although it may have always been that way, There are no obvious signs of a leak.

Any thoughts?
Click to expand...

The fluid is to dampen oscillation, and a low level should not affect accuracy in stable flight.
 
Clip4 said:
It is worn out. Get a new one.
Click to expand...
A sudden 10-15 degree shift in all quadrants indicates a problem that needs diagnosing first. Otherwise, the new one might get trashed the same way.
 
I agree with Dan.

Thoughts. ( there may be better )

1. If you take any type of compass and note reading away from aircraft.

Now place in aircraft.

If it changes there is a magnetic field present from some source.


2. With both compasses in aircraft in aircraft note readings.

Start engine and turn on electrics.

Any change?

Normally aircraft with electric systems swing the compass and fill in card

with radios etc ON.

If you have a change outside the norm it’s likely a current path has been

changed.

It could happen by repositioning a wire or leaving a ground off.


3. You can magnetize steel by passing a current through it.

Improper connection of a Battery Charger might well be causal.



It might help if we knew the type of aircraft.
 
Dan Thomas said:
A sudden 10-15 degree shift in all quadrants indicates a problem that needs diagnosing first. Otherwise, the new one might get trashed the same way.
Click to expand...

When the plane is in the hangar or is otherwise not in use, I place a headset on each side of the glareshild in close proximity to the compass. That situation has not changed for many, many years. Is it likely that the internal magnets in the compass may finally have become permanently affected by the magnets in the headset?
 
jbrrapa said:
My pedestal mounted compass suddenly began to be off by about 10-12 degrees in every direction. Nothing new has been added to the aircraft or on the glareshield. I did notice that the fluid level is slightly below the top, although it may have always been that way, There are no obvious signs of a leak.

Any thoughts?
Click to expand...
One morning I arrived at the airport to notice for an early work flight to Toronto. Once I started taxiing, I noticed that the compass was stuck pointing north no matter which direction I turned, so I taxied back to my spot, shut down, and waited for the maintenance hangar to open. While I was waiting, I noticed the tiniest little blister of melted aluminum on the corner of one of the propeller blade tips.

Long story short, my plane had been hit by lightning while parked. The entire firewall, mount, and engine block was magnetised -- if you walked anywhere within 10 ft of the plane with a mag compass, it would swing around and point at my cowling. The insurance company paid to tear down and overhaul the engine, and we sent it and the mount to a naval facility in Halifax for degaussing. We couldn't send the firewall, so the shop had to spend several days with a handheld degausser, chasing down spots until it was demagnetised as well. The plane was down for a few months, but I got an overhaul with nearly all new parts (cylinders, crankcase, crankshaft, etc etc) mostly paid for by the insurance company.

I doubt this is what happened to you, but it might be worth bringing a handheld compass into your plane and moving it around to see if there's anywhere the needle suddenly swings.
 
jbrrapa said:
When the plane is in the hangar or is otherwise not in use, I place a headset on each side of the glareshild in close proximity to the compass. That situation has not changed for many, many years. Is it likely that the internal magnets in the compass may finally have become permanently affected by the magnets in the headset?
Click to expand...
Probably not the magnets in the compass, but it’s possible to magnetize something else...my Maule had one of the tubes in the windshield magnetized, probably from headset storage, and we had to get that resolved before the compass would swing.
 
MauleSkinner said:
Probably not the magnets in the compass, but it’s possible to magnetize something else...my Maule had one of the tubes in the windshield magnetized, probably from headset storage, and we had to get that resolved before the compass would swing.
Click to expand...

The compass swings freely. The problem is doesn't swing to the direction to which it is pointing.
 
Did one of the adjustment screws fall out?
 
Put a GoPro on it then get out of the airplane to see if it's you.
 
Headsets anywhere on the glareshields were forbidden when I was DoM at the flight school. It's not just the compass that gets damaged; headsets scratch the windshield, too.

The sudden change could happen if one of the tiny compensating magnets fell out of its shaft. There are two compensators in there; one is in a fore/aft shaft, the other in a left/right shaft. Rotating the shafts affects the orientation and therefore the fields of those little magnets to neutralize the fields in the airplane that cause magnetic deviation.

Or did someone monkey with the compensating screws?
 
An XM antenna could cause this if it is one of those with a magnet in the base.
 
Type Aircraft?

Some models have unusual quirks.

ie The 1966 C150 had a magnet mounted to the underside of the Glare Shield

which directly above the tube radio of that era.
 
JAWS said:
Sounds like it needs to be removed and serviced. While out it can be checked for accuracy away from the airframe.
Click to expand...
Whatever do do NOT even add fluid. The FAA considers this a capital offense. Only instrument shops and compass fairies are allowed to do such a critical life and death operation.
 
Snowmass said:
The FAA considers this a capital offense.
Click to expand...
I guess it depends on how you define an instrument. I prefer the Part 1 definition. Regardless, the local ASIs around here never considered "servicing" as a repair either. But not saying others might have a different look on the subject. So I think it falls to the person to make that decision whether they service or not.;)
 
Snowmass said:
Whatever do do NOT even add fluid. The FAA considers this a capital offense. Only instrument shops and compass fairies are allowed to do such a critical life and death operation.
Click to expand...

Which is why I said "servicing", and not "repair".

Plus, in Canada, we can monkey with a compass as long as we have the manual for it. Not exactly rocket science to replace the diaphram and service it with fluid.

EDIT: For clarity ...
Instrument
  • 5 Maintenance of instruments, other than display devices whose operation is integrated with an appliance to which another category of specialized maintenance applies, if the work is beyond the limits recommended in the manufacturer’s maintenance manual or service instructions for service in the field, is instrument specialized maintenance.
    (CAR571, Schedule II)
 
Last edited:
OP - A clarification pls:

Does the compass always read 10 or 15 degrees too high, or too low? Or are there directions that the error cancels out?

If it reads the same amount and polarity of error in all directions, that would suggest that the internal magnet has slipped (or been remagnetized) with respect to the display. I dont know how that would happen, but that's the only thing that would appear that way.

If it reads randomly up to say plus and minus 10 or 15 degrees of true, the compensating magnets need tweeking.

A compass has no simple zero offset capability that I know of.
 
nrpetersen said:
OP - A clarification pls:

Does the compass always read 10 or 15 degrees too high, or too low? Or are there directions that the error cancels out?

If it reads the same amount and polarity of error in all directions, that would suggest that the internal magnet has slipped (or been remagnetized) with respect to the display. I dont know how that would happen, but that's the only thing that would appear that way.

If it reads randomly up to say plus and minus 10 or 15 degrees of true, the compensating magnets need tweeking.

A compass has no simple zero offset capability that I know of.
Click to expand...
nrpetersen said:
OP - A clarification pls:

The compass has an error of 10 to 15 degrees in all directions, I don't remember if it is high or low.
Click to expand...
 
You must log in or register to reply here.
Back
Top