Hi: Does anybody know what the airspeed limit for a coupled ILS is?
Mine flies the approach like a trooper, but as I slow below 120 mph at about 500 ft to start putting in flaps the
autopilot slips below the glide path and maintains the minimum speed of 120 mph. If I increase speed back above 120 mph +-,
It will go back up to reintercept the glide path.

Never noticed this before as I always disconnect the autopilot at the outer marker and hand fly the approach. A concerned passenger (my wife) wanted
me to show my non IFR co-pilot (son) how to land the plane in event of incapacitation of pilot (me).
It would be nice if this minimum speed could be reduced to 100 mph so that he would be able to fly the approach with full flaps from the outer marker
and down to minimums where he could just disconnect the autopilot and land the plane, fully in trim and on speed.

Thanks,
Gary

My altimtic III B manual lists no limiting speeds... however, it does stress that "The airspeed is controlled with the throttle while the Glide Slope Coupler controls the angle of descent."

It seems to me that I hear that it cannot be >140 MPH... but I can't find that in writing. My Altimatic shocked me yesterday... the Glide Slope Coupler armed and took control while being vectored to final... I was in the "Heading" mode and no ILS frequency was loaded in the NAV... h'mmmmmmmmm?
Jim

Jim. Your Gliseslope computer needs the radio coupler to be in the loc norm position to power the glides slope computer via
a ground connection. The Glide slope computer should be in all respects "dead" if in the heading mode. It does have a positive 14 volts at all times, but
without the ground from the radio coupler it shouldn't work.

Secondly, the G/S computer needs the glide slope to be at least 60% up deflection for at least 20 secs to enable arming as the needle centers. If your indicator was connected to the gps and
it was giving a vertical path then the computer would be happy with following the gps glideslope as you intercepted the path.

Third requirement is the autopilot must be in the altitude hold mode...which it probably was when being vectored.

Soooo... you most likely have a bad radio coupler in that it is providing a ground to the G/S coupler when the switch is in the heading position... or ( and this is a good place to look
if you have had recent panel work) the ground wire from the coupler to the G/S computer has been pinched and is grounded to the aircraft chassis.

If you are so inclined...there are 3 leads from the radio coupler find the one thats says cd34 and take connector off radio coupler ... Now I can still read fine print in good light but I need 2+ lenses to read the small letters printed on the connectors so bring a magnifying glass! Actually Its not that hard, the pin letter is "K" and it is located in the middle of all the male pins coming out of the back of the radio coupler cd 34 connector. ( You will need to remove the radio coupler from the panel to do this)
With the radio coupler out of the panel and cd34 connector taken off and no power on the airplane, using a multimeter touch one probe to the unpainted chassis of the radio coupler and one probe to the male pin "K" of the CD34 connector coming out of the radio coupler.

You should not get continuity when the coupler is in the heading mode...if you do then something is amiss in the coupler and you have found your problem. If there is no continuity in heading position, then when you rotate the switch to loc norm you should then get continuity, if you do, then the radio coupler is not the problem.

If your radio coupler checks out... then it most likely is your wiring from the coupler to the G/S slope computer... To check, disconnect the large medal wire bundle connector attached to the G/S computer and put a multimeter lead on the number "4" pin then put other lead on to the female pin "k" from the connector cable CD34 you took off the radio coupler.
(the large medal connector has 24 pins 12 on each side only the pin 1, 12, 13, 24 are numbered so you have to count across to find number 4.

If the wiring checks out.... then you have something amiss in your G/S computer that needs to be taken care of.. or if you don't need the coupled feature then disable it by disconnecting the power wire to the computer. ( unwanted descents at low altitude is not good)

I am not sure of your technical abilities, but this will help your mechanic to trouble shoot if you are not up to checking this yourself.

As an aside, I heard it somewhere in the forum, many problems with the "old" autopilots stem from "mucking" around with the grey cables that attach various parts of the autopilot. These cables being 40 years old are moved around stretched and retied into new avionics bundles during many upgrades over the years and sometimes give to the strain and break down. I have
had to resolder a few connectors on my cable connectors as the old solder joints let go. I have seen ground shields hanging by a few threads ready to cause problems when the last thread lets go... so a good check on these cables whenevr they are exposed is always a good thing.

Good luck,

Gary

Dear Gary,

Thanks for the in-depth descriptoin of trouble shooting the glide slope coupler and its relation to the radio coupler. In reference to your initial question though, I have to wonder how the autopilot would have any idea what your airspeed is. Is the autopilot linked to a GPS that shuts off the glide slope output for the autopilot under a certain speed? I didn't know that any raw airspeed data was sent to the autopilot, ie; from the airspeed indicator or pitot air supply. I would guess that if you find any airspeed input to the autopilot, then you will also find the reason that it tends to disengage the altitude hold under a certain speed. I would be interested to hear your findings.

Sincerely,
Skip Dykema

Skip:
The altimatic III has a hose connection from the pitot line. This is plumbed into the Altitude hold/selector box. This box is located behind the right side instrument panel.
There is a rubber tube that tees into the pitot tube line and a rubber hose that tees into the static line. This is also the box that the altitude preselect flex cable leading from the console
goes into.

This is the box that limits the climb speed to 135 +- in the altitude preselect mode and 120+- in the pitch mode, as well as the descent speed limits.
The console has pots behind the faceplate to adjust these speeds if necessary.
But unfortunately there is no such adjustment for the minimum speed in altitude hold mode. (as far as I know)

Looking at this box upside down, bent like a pretzel, with flashlight in hand, I can't see any field adjustments on it. I was just curious if anybody has come across this before and if there is any
adjustments on the other side of the box that I can't see.

Funny thing is that the operating manual doesn't mention anywhere of a min speed for altitude hold mode. Perhaps this is a question for century.

It could be that a part of this box is bad...

Anyway could be a common problem in the fleet, but as myself, if you rarely use the autopilot for coupled approaches, then you would not be the wiser.

As I said before, all other aspects of this autopilot are top notch and flies the A/C like a trooper!

For now I will fire off this question to the techs at Century, they should know.

When I get an answer, I will post it.

Gary

Dear Gary,

Interesting........I had no idea that the autopilot was plumbed into the pitot system. Although a normal static check doesn't necessarily check the continuity of the pitot system, and I know some of the more thorough shops do look at the pitot system, IMHO it is important to know that any potential leak could be traced back to the autopilot. Just another potential leakage area on what used to be a fairly simple system.

Getting back to your original question (again), maybe there is some leakage in the autopilot portion of the pitot system. Have you checked the integrity of that system. Although I'm not sure of the supply of pressurized air going into the system and if a leak at one point of the system would cause the rest of the system to also indicate low. After verifying that there is no leakage in that area, maybe you could isolate that portion of the system and temporarily tie in an airspeed indicator or some such pressure indicator and fool the autopilot into thinking it is flying so that some experimentation can be performed to see exactly what speed cuts out the glide-slope function and if you can find anything else that affects this feature. Some GPS's have a test function where you can control the glide slope output for testing purposes.

Just food for thought.

Sincerely,
Skip Dykema

Skip:

Thanks for your thoughts on this. I was concerned about all the rubber hoses behind the panel during the panel upgrade and had them
all replaced including the vacuum hoses. Before the final "smoke test" of the new panel all connections where double checked for
security.
My thoughts are since the box does hold the climb and descent speeds at the correct airspeeds, the leak would be ruled out.
I am going to test the G/S coupler again next time I go flying and just make sure it wasn't an intermittent fault.

Gary

I have been researching Altimatics lately as my twinkie has an Alti III (no B or C shown) Can someone tell me what the differences are? I'm having trouble finding that info. My 430 WAAS upgrade manual states that there is no vertical coupling on WAAS or ILS

Cliff

The altimatic 3 came with a glideslope coupler option. This coupler takes the information from the Nav box, be it Narco, King , or Garmin.
The coupler is an analogue computer that converts the up/down signal from the Nav receiver into a pitch signal that is sent to the altimatic console/computer.
So yes you can couple the 430 to your altimatic for nav mode and if you have the Glideslope option installed you can use the Vnav or full ILS coupling

The glideslope computer uses the same signal that is sent to the CDI instrument. In the case of the 430 this signal comes directly from the 430.
The 430 also sends a lateral deviation signal to the CDI and the Altimatic taps into this information for lat deviation when using nav, or loc mode.

Hope this helps,

Gary

Thanks MD11
I guess mine didn't come with the coupler as I have no verticle option, only lateral guidance. Rats!

Cliff, Yeah the tell tale sign you have the coupler is if you have the green G/S capture light in your panel.
Now if you really wanted one you could get a used one and wire it in. The avionics shop would have the authority
to install it but I think since the Comanche service manuals have the part numbers you could order the wiring harness from
piper (or maybe not, not sure what Piper caries anymore) and then it is just plug and play.

Also the old route of purchasing a used one complete with harness, have it recertified and install.

Fully coupled would be nice but personally the Altimatic does a nice job in pitch mode and with a little tweeking of the pitch wheel you could
easily follow the G/S manually to your personal limits before autopilt disconnect.

my 2 cents,

Gary

That's kinda my thought process. Would be nice to have but I can get along without it. All the Boeings I flew had million dollar autopilots and now I'm flying the lower end so to speak. Can I use the thumb wheel Yes, Can I hand fly it sure. Do I want to go to all the trouble to install one for a few approaches a year? Probably not. Thanks for the input

Gary,
There are upper and lower end limits on airspeed w/ the Altimatic IIIB.
What they are, I'm not sure. I'll see if i can find a book at home tonight.
It should do the same in the altitude preselect, the climb has airspeed limits, supposedly.
steve

I've been reading through the old threads and came across this one. Doesn't look like it was ever answered, so thought I'd do so in case someone else comes along.

The airspeed table for the Altimatic III is in the Altimatic III Service Manual. You can find it for free through google. Page 5-46 shows table V-II and says for the PA-30, the climb airspeed should be 135 +/-5 and descent airspeed should be Cruise +12 +/-3. Maximum approved speed is 225. This is adjustable via a screw on the box itself (top of the box, maybe?). As I read it, the autopilot will only adjust pitch to maintain these limits and glideslope; therefore, the pilot must adjust power to allow the autopilot the pitch authority to maintain desired glideslope.