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Heading bug 7 degrees offset in Autopilot mode

11 Posts

17-Feb-20 13:12

Hello,

I have an old Twin Comanche with a Piper Altimatic III with Alt preselect. I had a KI 825 HSI (digital via Airinc429 only) which broke and I send it to Avionic Straubing for repair. Straubing said, only exchange no repair for aprox 13000 USD. I decided to downgrade to KI 525a, because the KI825 is an upgrade almost pin compatible to KI525a. I bought an Ki525a and discovered, that the KI825 was a new installation and not an upgrade from an existing KI525a installation and that a few cable for KI 525 are missing, which I installed (Main port at the IFD540, 15 and 5 Volt and so on). The KI525a runs now fine, the compass, GS and VOR is Ok and works as expected. But the heading bug is around 7 degrees away from the center, when I want to use the autopilot. That means, if I want to fly heading 100 via autopilot, I must turn the heading bug to 107 degrees. The same with the OBS for the VOR/LOC. Now the question, where is the adjustment for the HDG and OBS centering for the autopilot? The 6 potenziometers in the autopilot console aren´t the right one (climb/decend limit, Bank limit only). Below the panel is a KA57 with some adjustments possiblitys But in the manuel is nothing written about “centering” the HDB and OBS bug. If the GS works, I don´t know, but that will be the next step after the first one (HDG center) is OK. Many thanks in advance..

EDFC, Germany

17-Feb-20 13:23

I just saw, that I took the wrong section (Hangar talk versus Avionic) Sorry for that. I will not happen again…

EDFC, Germany

17-Feb-20 14:31

Do you positively know the used KI525a is mechanically sound and ok ?
Have you slaved and de-slaved it ?
Does it turn to a new heading well, but ends up with wrong heading ?
Could be the synchros and the wiring inside the KI 525a, or may be a problem with the remote gyro. Which I assume would be a KG102 ?
I guess Martin Scheifl could tell you what to do :)

Safe landings !

EDLN, Germany

17-Feb-20 14:33

Moved. One of my easier jobs here

AFAIK the heading bug from the KI525 is sent out as an analog voltage, positive and negative, with 0V being “straight ahead”. You don’t need much of a ground shift to move the zero point. But the course pointer works differently (X/Y/400Hz). If you set your GPS to OBS mode, it should display the HSI course pointer value then. Does it show the error?

I don’t really know any more than this.

Administrator

Shoreham EGKA, United Kingdom

17-Feb-20 15:00

Also, assuming the KI 525a works correctly, here is a copy paste of the KA57 manual from a thread in Beechtalk. Altimatic III and Century III are assumed compatible, I couldn’t find a specific one for the Altimatic III.

2.8.7 KA 52 AND KA 57
NOTE: The autopilot operation must be confirmed as correct before the following adjustments are attempted. If possible, check the autopilot components alignment on the bench.

Take off the KA 52/57 dust cover by removing the two screws on the side of the unit. The HDG and CRS potentiometers at the left rear of the printed circuit board (viewing the board from the connector end) provide the gain adjustments for the course and heading modes of operation. The following is a procedure for setting the gains for the Mitchell Century II and III autopilots. The variations in the procedure for setting the gains in other autopilots are given at the end of each section.

NOTE:
On the KA 57 R108, the HDG select gain adjust is located near the outside edge of the board. R109, the CRS datum gain adjust is located toward the center of the board. On KA 52’s, the adjustments are marked accordingly.

HDG: With the autopilot off, manually adjust the attitude gyro for +10o of offset (the attitude gyro artificial horizon rotated 10o counterclockwise from its 0o position). Turn on the autopilot and set it in the Heading mode (Radio Coupler selector switch set to Heading) and position the KI 525 Heading Select Marker for +10o of offset (Marker 10o clockwise from lubber line). Adjust the HDG potentiometer until there is no movement of the control wheel. Check the gain setting by repositioning the control wheel to the center of its range and observing that there is no movement either clockwise or counterclockwise.

Next, turn off the autopilot and reposition the attitude gyro for -10o of offset (the attitude gyro artificial horizon rotated 10o clockwise from its 0o position). Turn on the autopilot and slew the Heading Select Marker to -10o (Marker 10o counterclockwise from the lubber line).

The control wheel should stop moving when the marker reaches -10o. Center the control wheel to midrange and observe there is no movement either clockwise or counterclockwise. The absolute value of the null position clockwise and counterclockwise from the lubber line (nominally ± 10o) should be within two degrees of each other.

For Cessna Autopilots, the procedure for setting the HDG gain is the same as that described above except for the controller settings which are as follows:
Pull-Turn Control Knob – Pushed in
Intercept – TRK-HDG – HDG
CRS: Set the attitude gyro for 0o of offset, turn on the autopilot and set it for the Course
Mode (Radio Coupler Selector switch set on OMNI). Acquire a VOR station on the appropriate radio and when the KI 525/525A NAV flag disappears center the Course pointer and deviation bar under the lubber line. Next, rotate the Course pointer to +45o (clockwise from the lubber line by 45o) and adjust the CRS potentiometer until there is no movement of the control wheel. Reposition the control wheel to the center of its range and observe there is no movement of the control wheel.

Next, check the setting of the CRS gain by slewing the Course pointer to -45o (counterclockwise from the lubber line by 45o) the control wheel should stop moving when the Course pointer reaches -45o. The absolute value of the two intercepts (nominally ± 45o) should be within 5o of each other. For the Cessna Autopilots, the procedure for setting the CRS gain is the same as that described above except for the Controller Settings which are as follows:
Pull – Turn Control Knob – Pushed in
Intercept – TRK – HDG – Intercept

To check the Mitchell Century II & III Localizer operation, set the appropriate radio to a Localizer frequency and the Radio Coupler selector switch to LOC. Rotate the KI 525/525A Course pointer clockwise and note that the control wheel also rotates clockwise. Change the Radio Coupler selector switch to LOC REV, rotate the Course pointer clockwise and observe that the control wheel rotates in the opposite direction, counterclockwise. The back course approach is flown with the course pointer set to the front course to give proper deviation bar steering to the pilot.

NOTE: When the KA 57 is coupled to a Mitchell Century II or III and the system is operated incorrectly by selecting a Radio Coupler LOC mode with a VOR frequency selected on the Radio or a VOR Omni mode on the Radio Coupler with an ILS frequency on the radio, the KA 57 Course Datum output will be 180 degrees out of phase.

Last Edited by EuroFlyer at 17 Feb 15:10

Safe landings !

EDLN, Germany

17-Feb-20 15:14

Note, any of those changes should be done in Straubing anyway, as they can simulate LOC and GS signals there on the ground while connecting all the original King analyzer hardware to the plane. Pretty interesting.

Safe landings !

EDLN, Germany

17-Feb-20 15:16

I have a feeling that if the OP was content with Straubing Avionics pricing and lead times, he would not be posting here

You can simulate LOC/GS signals with an IFR4000, or some other test set, standing 10m away from the plane, but that isn’t going to exercise the heading bug / course pointer signals.

KA57 MM. Not sure if this is relevant though.

Administrator

Shoreham EGKA, United Kingdom

17-Feb-20 16:41

Yes Peter, I know Straubing and Mr Eckert and Mr Scheifle. I also know the pricing and timetable of Avionik Straubing (I don’t have so much money anymore and want the save a little and, by the way, it id interesting to understand “GA high tech”. I build myself 30 years ago a car telefon with self build 144 mhz and 27 mhz transmitter and receiver, so I am not unfamiliar with electronics and I am pretty sure, that there’s a Poti where I can center the heading bug (max be hidden in the KI525). May be the gain poti in the KA57 could that be, but for my understanding it is more for left right margin than center. But I will try it (it is below the panel with very short cables 😒😒😒). You said, the Heading bug is 0 volt in the center. The broken KI 825 worked fine, may be the KI 525 must be checked, may be it is a little out of limit. I will report…