King autopilot and GPS approaches

I went and did some LNAV/VNAV approaches yesterday with a King KFC 200
autopilot. Looking for some tips with these autopilots on GPS approaches.

1. In NAV or APP mode the commanded rate of turn appears slower than in
HEADING mode. Spinning the HSI to the desired heading (often 90 degree
turns) caused the plane to try to go in the wrong direction, or the rate of
turn (slightly less than standard) caused an overshoot and didn't capture
the new course.

2. If flying in APP mode, and then making a step down in altitude, the AP
would not capture the glideslope (I've noticed this with ILS approaches as
well, even if intercepting the GS from below)

3. Of course, roll steering would be ideal, but barring this, is there a
better way to utilized the autopilot to fly these approaches?

4. Would it be better to fly in heading mode, and then engage approach mode
just prior to the FAF?

"Viperdoc" wrote in message
...
I went and did some LNAV/VNAV approaches yesterday with a King KFC 200
autopilot. Looking for some tips with these autopilots on GPS approaches.

1. In NAV or APP mode the commanded rate of turn appears slower than in
HEADING mode. Spinning the HSI to the desired heading (often 90 degree
turns) caused the plane to try to go in the wrong direction, or the rate
of turn (slightly less than standard) caused an overshoot and didn't
capture the new course.

One problem that's endemic to an analog autopilot (IMOHO)


2. If flying in APP mode, and then making a step down in altitude, the AP
would not capture the glideslope (I've noticed this with ILS approaches as
well, even if intercepting the GS from below)

Intercepting from above or below?


3. Of course, roll steering would be ideal, but barring this, is there a
better way to utilized the autopilot to fly these approaches?

4. Would it be better to fly in heading mode, and then engage approach
mode just prior to the FAF?

If you want a significantly increased workload, but no, it's not better.

Have you run all the self tests? Sounds like the AP controller is not
working at 100%.

What the KFC225 won't do is

a) intercept a GS from above - I was given a dodgy final vector
yesterday which led to that

Isn't that generally a bad idea anyway? I was taught that false lobes
could lead you to grief and a GS should always be intercepted from below.

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.

Jose wrote:
What the KFC225 won't do is

a) intercept a GS from above - I was given a dodgy final vector
yesterday which led to that


Isn't that generally a bad idea anyway? I was taught that false lobes
could lead you to grief and a GS should always be intercepted from below.

Jose

That's right - due to inevitable ground reflections, there
are always false glideslopes above the correct one. The
ICAO planning instructions require the procedure to be
planned so thet the GS is captured from below.

In the same way, the localizer capture planning rules
require an intercept angle below 45 degrees, preferably
30 degrees.

--

Tauno Voipio (avionics engineer, CPL(A))
tauno voipio (at) iki fi

"Tauno Voipio" wrote in message
...
Jose wrote:
That's right - due to inevitable ground reflections, there
are always false glideslopes above the correct one.

Would you be so kind as to explain that one? False glidescopes?

Matt Barrow wrote:
"Tauno Voipio" wrote in message
...

Jose wrote:
That's right - due to inevitable ground reflections, there
are always false glideslopes above the correct one.


Would you be so kind as to explain that one? False glidescopes?

An ILS signal is finally created at the receiver antenna
as a combination of three signals which are sent separately.

(For the perfectionists: This is a simplified description).

A localizer antenna group is built of three sections: left,
center and right. There are several antennas per section,
often 4 to 8. The center section radiates a signal called CSB
(carrier and sidebands) which alone will create a centerline
display at the receiver. The side sections radiate a signal
called SBO (sidebands only) which contains a kind of difference
signal between the centerline signal and a side indication.
The left and right side signals are radiated in opposite phase,
so the right signal is positive when the left signal is negative.
When the side antennas are at equal distances from the aircraft
antenna, the side signals cancel at the receiver. When there is
a distance difference (as it is when the aircraft is off-centerline)
the side signals combine producing the corresponding side signal.

The glideslope is produced in the same way, but there is a problem:
We cannot install a proper bottom antenna, as it should be some
tens of feet below ground. Here, we have luck: If an antenna is
put above a conductive flat surface, the field pattern is similar
as the pattern with an opposite-phase antenna at the same distance
at the other side of the surface, and that is just what is needed
here. We'll use ground surface as the reflector to create the illusion
of a bottom antenna.

The pattern must then be tilted upward by the glideslope angle.
This creates an asymmetry in the pattern, and we get false
glideslopes above the correct one. For a 3 degree glideslope,
the first false glideslope is usually around 10 degrees.

HTH (hope this helps)

--

Tauno Voipio
tauno voipio (at) iki fi

Would you be so kind as to explain that one? False glidescopes?

The glide slope signal gets reflected off the ground, and your receiver
hears the echo. Under some circumstances, it may interpret the echo as
a glide slope, and falsely display a fly up or fly down indication. If
you follow that, you will probably descend too steeply, and short of the
runway.

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.

On Apr 1, 6:31 am, "Viperdoc" wrote:
I went and did some LNAV/VNAV approaches yesterday with a King KFC 200
autopilot. Looking for some tips with these autopilots on GPS approaches.

1. In NAV or APP mode the commanded rate of turn appears slower than in
HEADING mode. Spinning the HSI to the desired heading (often 90 degree
turns) caused the plane to try to go in the wrong direction, or the rate of
turn (slightly less than standard) caused an overshoot and didn't capture
the new course.

It's true. The rate of turn is somewhat reduced. Shouldn't turn the
wrong
direction unless turning 180 degrees. If you start the turn as
commanded by
the GPS, it will make it. If it goes the wrong way on a 90, there is
something
wrong with it.

2. If flying in APP mode, and then making a step down in altitude, the AP
would not capture the glideslope (I've noticed this with ILS approaches as
well, even if intercepting the GS from below)

If it's working right, it should intercept the gs from either attitude
mode or
altitude mode. The gs must pass thru center in either case and it may
be
slightly misadjusted. Descending onto it from above requires a
serious rate
of descent and it could be argued as unsafe.

3. Of course, roll steering would be ideal, but barring this, is there a
better way to utilized the autopilot to fly these approaches?

Most of the dozens of KFC200s I've run into at BPPP clinics track just
fine
by turning the course arrow as commanded by the GPS. If you have a
sandel,
this is done automatically for you. Note that in strong winds, after
a 90
degree turn your ap will have to re-psych the wind; if you use APR
mode this will
be fairly rapid.

4. Would it be better to fly in heading mode, and then engage approach mode
just prior to the FAF?

That would work. There are a lot of features in the coupling modes of
the 200.
In some cases if something isn't working right it would be really hard
to detect.
It's all analog switches driven by combinational logic. I'm sure that
some I've
seen aren't working correctly because the vast majority of them do the
problem
correctly.

Bill Hale BPPP instructor

On Apr 3, 8:18 am, Jose wrote:
Would you be so kind as to explain that one? False glidescopes?

The glide slope signal gets reflected off the ground, and your receiver
hears the echo. Under some circumstances, it may interpret the echo as
a glide slope, and falsely display a fly up or fly down indication. If
you follow that, you will probably descend too steeply, and short of the
runway.

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.

Worth noting: Even without grounds, the lobes on the antennas would
cause false paths. To wit: The LOC has false nulls. Note how it
looks
as you fly around the place--swithing back and forth in a seemingly
random
way.

A place where it really matters: Jackson Hole WY. There you need to
position yourself on the loc using other navaids. Otherwise you might
run into something descending on a false LOC path.

Bill Hale BPPP instructor

My understanding is that at least the KAP 200 and 225 will not track a
glideslope (either from an LNAV+V, an LPV, or an ILS) approach when
intercepting from above, which is also my experience.

I also observed that adjusting the altitude with the autopilot up/down
switch will negate the glideslope intercept even from below while in
approach mode. Have you seen this behavior as well?