Do you use your magnetic compass?

A recent compass swing on our plane has turned up some opinions about
magnetic compasses that are surprising to me..

A club member has asked me why we spent money to have a 14 degree error
removed from our compass since it is just a back up instrument if everything
else quits. He just sets the DG to the runway heading on takeoff and uses
that. A couple of 360 in our 172 to look at something on the ground will
put our DG 15 -20 degrees off and it drifts about that much each hour. That
doesn't seem to concern him.

An A&P I asked in another forum said he hopes his customers don't expect him
to get the compass closer than about 10 degrees. Our shop says 10 degrees
is what is allowed.

I used to do a lot of sailing and a degree or two in a compass is a big deal
to me. Even though I can do direct to with the Loran or GPS, I like to be
able to start out in the right direction. If I'm looking for an airport or
landmark, knowing pretty accurately where the aircraft is pointed helps. If
everything else quits, I'd really like to know where the plane is pointed
while I try to find a place to land.

I agree that the compass is pretty fuzzy in an airplane. By the time you
get it to settle down, set the DG, and add in the difficulty of figuring out
exactly where the axis of the airplane is, 10 degrees may be the best you
can do. However, my experience with both navigation and engineering tells
me that it's still worth being precise where you can. If you accept a 10
degree error in the compass itself and then add the 10 degrees of other
factors, you could be up to 20 degrees. That seems like a lot to me.

Am I being overly compulsive about this? I know that everything the
magnetic compass tells you has to be verified with all other available
information but it is still the primary source of direction information in a
simple aircraft like our 172.

Are these casual attitudes towards compass accuracy I'm encountering common?


--
Roger Long

VFR I don't see the big deal about having a working compass and stable
DG. Most of us have a GPS these days, and in the Midwest we can
navigate by the 'grid' road system.

IFR a working compass and DG is a lot more important. If the DG
precesses, it can make long vectors a real pain. Since an accurate
compass is the only easy way to get magnetic heading aloft, it is
necessary to figure the winds aloft, which is fairly important while
beginning an approach.

I'm assuming your club members fly both IFR and VFR, so you should
maintain the plane to what makes sense for the IFR members.

-Nathan


On Sat, 15 May 2004 12:33:25 GMT, "Roger Long"
om wrote:

A recent compass swing on our plane has turned up some opinions about
magnetic compasses that are surprising to me..

A club member has asked me why we spent money to have a 14 degree error
removed from our compass since it is just a back up instrument if everything
else quits. He just sets the DG to the runway heading on takeoff and uses
that. A couple of 360 in our 172 to look at something on the ground will
put our DG 15 -20 degrees off and it drifts about that much each hour. That
doesn't seem to concern him.

An A&P I asked in another forum said he hopes his customers don't expect him
to get the compass closer than about 10 degrees. Our shop says 10 degrees
is what is allowed.

I used to do a lot of sailing and a degree or two in a compass is a big deal
to me. Even though I can do direct to with the Loran or GPS, I like to be
able to start out in the right direction. If I'm looking for an airport or
landmark, knowing pretty accurately where the aircraft is pointed helps. If
everything else quits, I'd really like to know where the plane is pointed
while I try to find a place to land.

I agree that the compass is pretty fuzzy in an airplane. By the time you
get it to settle down, set the DG, and add in the difficulty of figuring out
exactly where the axis of the airplane is, 10 degrees may be the best you
can do. However, my experience with both navigation and engineering tells
me that it's still worth being precise where you can. If you accept a 10
degree error in the compass itself and then add the 10 degrees of other
factors, you could be up to 20 degrees. That seems like a lot to me.

Am I being overly compulsive about this? I know that everything the
magnetic compass tells you has to be verified with all other available
information but it is still the primary source of direction information in a
simple aircraft like our 172.

Are these casual attitudes towards compass accuracy I'm encountering common?

"Roger Long" wrote:
Am I being overly compulsive about this?

Not at all, IMO.

I know that everything the magnetic compass tells you has
to be verified with all other available information

It's the other way around, isn't it? Once you're airborn, the compass
is the only thing you have that directly displays your heading. The HI
is not much use if you have no way to remove its precession.

Are these casual attitudes towards compass accuracy I'm encountering
common?

I don't know, but to me the compass is as important as any other flight
instrument. I definitely want to know which way the nose is pointed.
--
Dan
C172RG at BFM

Am I being overly compulsive about this?

Maybe. Although I still religiously set the DG to the compass in flight,
it's more from habit than from need.

I've got two moving map GPSs, two VORs, DME, and the road grids to navigate
with -- any one of which is far more accurate than my whisky compass. And
the odds of losing all of those tools (including battery back-ups) are
almost nil.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Roger Long wrote:

Are these casual attitudes towards compass accuracy I'm encountering common?

I've found that the CFIs who have flown with me tend to assume that the compass is
very accurate. If I turn 90 degrees and the compass and DG don't agree, they assume a
problem in the vacuum system. Mechanics, on the other hand, seem to have the idea
that 15 or 20 degrees of error is ok. None of them want the job of swinging the
compass properly on a taildragger.

Now, I haven't asked very many mechanics to do this. After striking out a couple of
times, I just started resetting the DG to the ground track on my LORAN on long trips.

By the way, my compass inaccuracies are caused by adding all the avionics without
re-swinging the compass.

George Patterson
I childproofed my house, but they *still* get in.

In article ,
Roger Long om wrote:
A club member has asked me why we spent money to have a 14 degree error
removed from our compass since it is just a back up instrument if everything
else quits.

Not for an NDB approach! Oh, wait, an NDB approach is a backup if
everything else quits. ;-)

A couple of 360 in our 172 to look at something on the ground will
put our DG 15 -20 degrees off and it drifts about that much each hour.

I've flown behind DGs that bad, but most hold heading better than that.

An A&P I asked in another forum said he hopes his customers don't expect him
to get the compass closer than about 10 degrees. Our shop says 10 degrees
is what is allowed.

I asked about this on rec.aviation.ifr and not many people said they
used the correction card. I've only used it on the ground to assure
myself that it was mostly small 1-2 degree errors so I could ignore it
in flight.

--
Ben Jackson

http://www.ben.com/

On any airplane I fly, if the mag compass is not within 5 degrees of
accuracy, I don't fly the plane, it's that important!

The mag compass is the primary navigation device in my Champ, so if the
mag compass is off, navigation can be a real bitch.

I did, last Wednesday

Not long after leaving the airport northbound, the Garmin III+ popped
up a message: Battery Low. Then it went off, and after one more rep it
stayed off. Happily there was not much wind, and the position of the
compass (there is no gyro in the Cub) was well-remembered as between N
and 3 outbound and between S and 16 on the return trip, which enabled
me to go and return with reasonable accuracy. (Viz was about 10 miles
so I couldn't steer by the mountains as I prefer to do.)

It made me realize that it had been a very long time since I'd checked
my progress on the chart.

all the best -- Dan Ford
email: (put Cubdriver in subject line)

The Warbird's Forum www.warbirdforum.com
The Piper Cub Forum www.pipercubforum.com
Viva Bush! blog www.vivabush.org

On Sat, 15 May 2004 12:33:25 GMT, "Roger Long"
om wrote:

A recent compass swing on our plane has turned up some opinions about
magnetic compasses that are surprising to me..

A club member has asked me why we spent money to have a 14 degree error
removed from our compass since it is just a back up instrument if everything
else quits. He just sets the DG to the runway heading on takeoff and uses
that. A couple of 360 in our 172 to look at something on the ground will
put our DG 15 -20 degrees off and it drifts about that much each hour. That
doesn't seem to concern him.

An A&P I asked in another forum said he hopes his customers don't expect him
to get the compass closer than about 10 degrees. Our shop says 10 degrees
is what is allowed.


the australian standard is 5 degrees maximum error with an annual
compass swing.
most compasses will be well within that.

most common problem is a leaking diaphragm followed by some wear on
the spindle. both are easily repaired by someone knowledgable.

yes it does matter. when you need it most is a situation where you
cant recognise a damn thing below and will need to fly a known bearing
until something that you do recognise comes into view.

they are pretty reliable in a pinch since the earth's magnetic field
doesnt have an off switch :-)

Stealth Pilot

Actually, according to the discovery channel, the earth's poles are
weakening, and will ultimately reverse polarity...

Pilots everywhere are going to become very confused very quickly.

Most, though, will be most horrified when their toilet bowl flushes the wrong
way.


Stealth Pilot wrote in
:

they are pretty reliable in a pinch since the earth's magnetic field
doesnt have an off switch :-)

Stealth Pilot