How can a compass suddenly go out by 20 degrees?

Sounds like something steel was introduced and has now become magnetized...
denny
"Roy Smith" wrote in message
...
In article ,
"Bob Chilcoat" wrote:

Recently one of my partners reported that the compass (standard whiskey
compass, not a vertical card) on our Archer was reading 20-25 degrees
off of
runway heading at departure. Today a new partner was up with an
instructor
for his sign off and reported the same thing. I hadn't noticed, but we
seem
to have a definite problem. Nothing has changed in the plane for a long
time. We recently replaced the old Garmin GPS with a Lowrance AirMap
300
(which has its antenna on the top of the glare shield near the compass),
but
the first incident was before that substitution was made. Any idea how
something like this can happen?

--
Bob (Chief Pilot, White Knuckle Airways)




There are two basic possibilities:

1) Something is wrong with the compass. Things that can go wrong with
compasses include leaking fluid, worn or damaged bearings, or somebody
twiddling with the conmpensation magnets.

2) Something changed in the environment the compass operates in. The
aircraft has its own magnetic field. What the compass senses is the
(vector) sum of the Earth's field and the aircraft's field (this is what
the compensating magnets compensate for). If you've had equipment
installed or removed, or wires re-routed, this could cause the problem.

Note that "installed" doesn't mean it's got to be bolted in. If it's in
the plane, it'll affect things. Maybe your partner is carrying
something in his flight bag that might affect things?

Bob Chilcoat wrote:
: Recently one of my partners reported that the compass (standard whiskey
: compass, not a vertical card) on our Archer was reading 20-25 degrees off of
: runway heading at departure. Today a new partner was up with an instructor
: for his sign off and reported the same thing. I hadn't noticed, but we seem
: to have a definite problem. Nothing has changed in the plane for a long
: time. We recently replaced the old Garmin GPS with a Lowrance AirMap 300
: (which has its antenna on the top of the glare shield near the compass), but
: the first incident was before that substitution was made. Any idea how
: something like this can happen?

: --
: Bob (Chief Pilot, White Knuckle Airways)

I had a compass problem that ended up coming from the coaxial cable going to the
GPS antenna. The RG-142B coax has a copper-clad steel center conductors and running it
up the center pillar in my Cherokee caused the compass to have an error. If you swapped
your GPS's, this could have happened.

It doesn't have to be a magnet in the area to cause problems... just some magnetic
material (like unmagnetized iron/sheet metal/etc). Might also check any bracketry or
screws that may have been replaced. I know the stock compass mounting screws are usually
brass for that reason.

Basically, if it's not just a broken compass but is truly magnetic
interference, you need to find out *absolutely everything* that changed between when it
worked an when it was off. Even stupid little things can cause the error.

-Cory




--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

I used to own a 1969 Mooney M-20C Ranger. I know your's isn't a
Mooney, but I can tell you of similar problems and how I was able to
fix them. Perhaps you might benefit from my experience. Mooneys have
a steel cage with non-stressed aluminum over the cockpit forward, and
use monocoque aluminum construction aft of the cabin. The wet
compass, an Airpath, worked just fine for at least the first year that
I owned the airplane (1996-1997). After a time, it would exhibit up
to 40 degree errors, and this was an intermittent problem.
I researched the problem and found that one way to fix the
problem was to degauss the steel cage. Seems that there are a lot of
ground loops in the plane, as most systems just use the aluminum as
ground. The currents can eventually cause the steel cage to be
magnetized. Or so it is said. I got lots of advice on how to do it.
One of the better Mooney shops can rent out a de-gausser.
Alternately, I heard that they are available at a local TV repair shop
(although I doubt that many folks get TVs repaired any more). The
device, as I understand it, wraps around the steel tube. The operator
presses a button, holds it down, then removes the device from the
aircraft before letting go of the button. I heard that it was
sufficient to wrap it around the post in the center of the windshield.
Now for the confusing part. I had advice all over the map. One
shop said that the procedure described above was all I had to do.
Another said that I had to remove all electrical equipment from the
panel before doing the process. That is, take out all radios, and all
instruments. Even the ammeter and fuel gauges, etc. For anyone who
has worked on a Mooney panel, this is a very daunting task. Since I
couldn't get consensus, and didn't know if the procedure would even
work, I continued investigation.
Since the problem was intermittent, I asked the folks flying the
plane what the conditions were. Here in Colorado, we have a pretty
dry climate. It seemed that the problem was worse if there was
precipitation, even if just misty or foggy. That led me to suspect
P-static. I drove all around my home field and looked at the Mooneys
taht were tied down. None of them had static wicks, and neither did
mine. All the other brands of aircraft had at least a few.
I decided to spend about ~$300 on a static wick kit. This
included 15 wicks (three each for ailerons, elevators and rudder), and
the threaded attachments that rivet to the control surfaces. It also
included all of the STC paperwork. The wicks themselves were the
solid jet style and were about 3" long. After installing them, the
compass problem went away.
Just my experience. It may not apply to your situation, but you
may have a look at the static wicks on your plane to see if they are
in OK condition. I also have owned a Cessna single, a T207. It had
about 3-5 static wicks, the flexible rope style. I now have a 310
with a few of the rope style. I haven't seen any effects of P-static
on either of these planes, and have flown a lot more in instrument and
skuddy conditions.

Hope this is helpful,
Brian Cox

I've got a stupid question. Why would somebody use silver-plated copper coated
steel coax with teflon dielectric at about a buck a foot RG-142 when any decent
active antenna will drive 50' of nickel a foot and no magnetic errors RG-174?

Jim



shared these priceless pearls of wisdom:

- I had a compass problem that ended up coming from the coaxial cable
going to the
-GPS antenna. The RG-142B coax has a copper-clad steel center conductors and
running it
-up the center pillar in my Cherokee caused the compass to have an error.



Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

Jim Weir wrote:

I've got a stupid question. Why would somebody use silver-plated copper coated
steel coax with teflon dielectric at about a buck a foot RG-142 when any decent
active antenna will drive 50' of nickel a foot and no magnetic errors RG-174?

Jim, dontcha know? If it costs more, it's gotta be better. :-)

George Patterson
A diplomat is a person who can tell you to go to hell in such a way that
you look forward to the trip.

Two words:

Installation manual.


That's the last time I'll follow one of those. Oh wait... DOH!

-Cory

Jim Weir
wrote:
: I've got a stupid question. Why would somebody use silver-plated copper coated
: steel coax with teflon dielectric at about a buck a foot RG-142 when any decent
: active antenna will drive 50' of nickel a foot and no magnetic errors RG-174?

: Jim


:
: shared these priceless pearls of wisdom:

: - I had a compass problem that ended up coming from the coaxial cable
: going to the
: -GPS antenna. The RG-142B coax has a copper-clad steel center conductors and
: running it
: -up the center pillar in my Cherokee caused the compass to have an error.



: Jim Weir (A&P/IA, CFI, & other good alphabet soup)
: VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
: http://www.rst-engr.com

--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

Jim Weir writes:

I've got a stupid question. Why would somebody use silver-plated copper coated
steel coax with teflon dielectric at about a buck a foot RG-142 when any decent
active antenna will drive 50' of nickel a foot and no magnetic errors RG-174?

Same reason people will buy this:

http://www.stereotimes.com/cables071003.shtm

-jav

In article ,
Javier Henderson wrote:

Jim Weir writes:

I've got a stupid question. Why would somebody use silver-plated copper
coated
steel coax with teflon dielectric at about a buck a foot RG-142 when any
decent
active antenna will drive 50' of nickel a foot and no magnetic errors
RG-174?

Same reason people will buy this:

http://www.stereotimes.com/cables071003.shtm

-jav

That's hilarious. I assume this is for real? People really do buy
these things? I was waiting for the part where they said they're
lovingly assembled by virgins and all the welds are x-rayed.

uhhh, are any of those virgins looking for a new challenge?
denny

"Roy Smith" wrote in message I was waiting for the part
where they said they're
lovingly assembled by virgins and all the welds are x-rayed.

On 16 Feb 2004 17:26:24 -0800, Javier Henderson wrote:

Jim Weir writes:

I've got a stupid question. Why would somebody use silver-plated copper coated
steel coax with teflon dielectric at about a buck a foot RG-142 when any decent
active antenna will drive 50' of nickel a foot and no magnetic errors RG-174?

Same reason people will buy this:

http://www.stereotimes.com/cables071003.shtm

This article is reminiscent of the "blue windshield corner lights cut
headlight glare," or "new gizmo increases gas mileage 500% -- for only
$19.95," and similar automotive nonsense addvertisements )



-jav