Reading the whiskey compass

(Ben Jackson) wrote
When you're flying partial panel, to what precision do you read the
mag compass? To the nearest 5 degrees? Estimate to the nearest
degree? How long do you go between readings and rely only on timed
turns?

For that matter, do you ever try to apply values from the correction
card? In IMC or even VMC?

With a whiskey compass, it is my opinion that in smooth air +/-3
degrees is about the best you can hope for. Further, even a tiny
deviation from wings level is going to make the reading worthless.
Therefore, I ONLY read the compass when I've been maintaining level
flight for at least 3 seconds. I do not attempt compass turns - I
rely on timed turns at all times, even for small corrections.

In light chop, +/-5 degrees is about the best attainable. Anything
worse, and your guess is as good as mine. Off-field NDB approaches
with a whiskey compass in moderate turbulence are basically exercises
in dead reckoning - if you break out within a mile of the field, you
did pretty good.

With a vertical card compass with good damping, you can consistently
do a whole lot better. I find that I can effectively read heading
+/-2 degrees, at which point using the correction card becomes
somewhat worthwhile. This is not significantly degraded by light chop
because of the damping. In moderate turbulence, the solution

I'm not really sure why anyone who has a modern (flat) DG messes with
a whiskey compass anyway. With a barrel DG, it made some sense - you
wanted both instruments to read the same way to reduce confusion.
These days few pilots have even seen a barrel DG, never mind flown
instruments with one, so I really have no clue why anyone would
tolerate a whiskey compass in an IFR airplane.

Michael

"Michael" wrote:
I really have no clue why anyone would
tolerate a whiskey compass in an IFR airplane.

In my case, it's because I know of two occasions where VCC installations
failed. In each instance the hysteresis error of the compass sometimes
exceeded 10 degrees and none of the fixes proposed by the mfr. helped.

I hate whiskey compasses. If you have any insights into how to achieve
happiness with a VCC, I would be interested to read them.
--
Dan
C172RG at BFM
(remove pants to reply by email)

"Dan Luke" wrote
I really have no clue why anyone would
tolerate a whiskey compass in an IFR airplane.

In my case, it's because I know of two occasions where VCC installations
failed. In each instance the hysteresis error of the compass sometimes
exceeded 10 degrees and none of the fixes proposed by the mfr. helped.

Ouch. Was the compass on a rigid or flex mount? I've seen some of
them stick when they were hard-mounted into the panel or put on a
rigid mount, but I've yet to see one stick when it was on a rubber
flex mount. Of course the rubber part has to be replaced every few
years, but this is about $2.

Michael

"Michael" wrote:
Ouch. Was the compass on a rigid or flex mount?

IIRC, the club changed compass mounts as one of the attempted fixes, but
I don't know if they went from rigid to flex or vice versa. Apparently
the compass requires *some* vibration to work properly - don't remember
where I read that.

I still would love to have a (working) VCC. I googled up this bit of
troubleshooting experience yesterday:
http://www.avionicswest.com/articles.htm (scroll down a ways)

I'm not giving up. Others besides you (Jay Honeck, for one) have
reported good luck with their VCCs.
--
Dan
C172RG at BFM
(remove pants to reply by email)

For those of us who practice with it extensively, it's piece of mind.
fwiw
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****
"Michael" wrote in message
om...
(Ben Jackson) wrote
When you're flying partial panel, to what precision do you read the
mag compass? To the nearest 5 degrees? Estimate to the nearest
degree? How long do you go between readings and rely only on timed
turns?

For that matter, do you ever try to apply values from the correction
card? In IMC or even VMC?

With a whiskey compass, it is my opinion that in smooth air +/-3
degrees is about the best you can hope for. Further, even a tiny
deviation from wings level is going to make the reading worthless.
Therefore, I ONLY read the compass when I've been maintaining level
flight for at least 3 seconds. I do not attempt compass turns - I
rely on timed turns at all times, even for small corrections.

In light chop, +/-5 degrees is about the best attainable. Anything
worse, and your guess is as good as mine. Off-field NDB approaches
with a whiskey compass in moderate turbulence are basically exercises
in dead reckoning - if you break out within a mile of the field, you
did pretty good.

With a vertical card compass with good damping, you can consistently
do a whole lot better. I find that I can effectively read heading
+/-2 degrees, at which point using the correction card becomes
somewhat worthwhile. This is not significantly degraded by light chop
because of the damping. In moderate turbulence, the solution

I'm not really sure why anyone who has a modern (flat) DG messes with
a whiskey compass anyway. With a barrel DG, it made some sense - you
wanted both instruments to read the same way to reduce confusion.
These days few pilots have even seen a barrel DG, never mind flown
instruments with one, so I really have no clue why anyone would
tolerate a whiskey compass in an IFR airplane.

Michael