"Ross Magnaldo" wrote
You have confused me even more!
If I am flying a radial with a cross wind, should the HDG bug be on the
radial or should it be on the estimated course made good with the wind
correction?

Contrary to other answers you are getting, the correct procedure
(where it matters) is to set the heading bug to the estimated course
made good. The reason is as described - when there is a crosswind, if
you set the radial you will actually fly an offset course.

Whether it matters or not is another story.

Every autopilot is, at the core, a wing leveler. If attitude based,
it uses an attitude gyro with pickups to establish wings level. If
rate based, it uses a TC with pickups to establish zero turn/roll.
That zero is the setpoint - if you're not banking you shouldn't be
turning (if trimmed). Note that a T&S is NEVER used - the response
time is too slow, and a significant bank can be established before the
autopilot knows it. For that reason, I always find it amusing when
people tell me they prefer the old needle and ball for partial panel
flying, but that's another discussion.

Heading hold does not eliminate the basic wing leveler function - it
alters it. The heading offset is used to change the setpoint of the
wing leveler. A common setting is 1 degree of heading per degree of
roll, with a 25 degree limit. That means that if the heading bug is 5
degrees to the right of the heading being flown, then instead of
trying to maintain wings level the autopilot will try to maintain a 5
degree bank to the right. But if the heading bug is 45 degrees to the
right of the heading being flown, the autopilot will try to maintain a
25 degree bank (the limit) to the right.

If a rate based wing leveler is used, then it is usually set to treat
a standard rate turn as being equivalent to anywhere from 15 to 30
degrees of roll, and limits the turn rate to standard rate. For
general reference, the Century I and derivatives are rate-based, as
are the S-TEC's. Most everything else is attitude based. To the
user, it hardly matters (other than knowing which gyro to monitor to
ensure the autopilot isn't doing something ugly). From here on in I'm
going to assume an attitude based autopilot, but it all carries over
to rate based ones.

When you add in nav tracking, it gets interesting. Some autopilots
use the CDI to directly drive the attitude set point, just like
heading is used. What this means to you is that if the CDI is right
of center, the plane will stay in a right bank until the CDI centers,
with a bank proportional to CDI deflection (up to some limit, like 25
degrees bank or standard rate). The built-in assumption is that if
the needle is centered, you are on the correct heading, and if it's
not, you're not. Now, if I had a buck for every time I told an
instrument student that just because the needle is centered DOESN'T
mean you're on the right heading... Still, it tends to work OK
tracking a LOC because the response is so quick - because the LOC
needle is VERY sensitive. It also tends to work OK tracking a GPS,
because the needle is almost as sensitive - if you don't mind the
constant wing wobble in turbulence.

If tracking a VOR in turbulence, it just doesn't work. Therefore most
of the older autopilots (designed before everyone had GPS) don't use
the CDI output directly - they use it to modify the heading error
output. In other words, it flies like you would. For example, say
the heading is dead on the bug, the wings are level, but the CDI is a
dot left. This will generally be treated the same as if the bug was 5
degrees left of heading. The plane will bank 5 degrees left and start
to turn. As the heading deviation increases to 1 degree left, the
bank will be reduced to 4 degrees. In other words the autopilot will
make an asymptotic turn - a technique that you would be well advised
to master if you are ever going to fly partial panel in something fast
and slippery. As the CDI approaches center, the plane will turn back
to the bug - again, asymptotically.

Some autopilots (the older Century II and III come to mind) do it both
ways - depending on how you set the nav converter, they will or won't
use the heading data in tracking the nav signal. Generally, they will
not use the heading data in approach mode (because the CDI is presumed
to be sufficiently sensitive, and the response sufficiently quick) but
WILL use it for enroute nav.

OK, now how do you figure out which kind you have? Easy.

Get the plane on course and on heading, set the heading bug to the
heading you are flying, make sure the CDI is centered, engage
autopilot in nav track mode. Once you are satisfied it is tracking,
give the heading bug a big ol' twist - say 45 degrees. If the plane
tries to follow the bug initially, you know the heading data is being
used in that mode. If it doesn't twitch, it's not.

Michael

Please do not send email replies to this posting. They are checked
only sporadically, and are filtered heavily by Hotmail. If you need
to email me, the correct address is crw69dog and the domain name is
this old airplane dot com, but remove the numbers and format the
address in the usual way.