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7 Answers

Heading Bug? Autopilot? problem

Asked by: 4536 views Aircraft Systems

The problem I am having is with the KAP140 autopilot or my heading bug on my Cessna 172. There are times when in the HDG mode the autopilot does not respond to the heading bug. Sometimes it works perfectly. The avionics people want me to problem solve some more before they take a look at it. Has anyone ever seen this problem before? Any suggestions? Thanks, Rick

7 Answers

  1. John D. Collins on Aug 29, 2011

    For this discussion, I assume you have a DG and not a HSI.
    When you are experiencing the heading bug issue, switch from HDG to ROL mode.  While in ROL mode, if you override the controls and introduce a bank, does the autopilot set the airplane back to level.  This will help determine if the roll servo is functioning.  You can also switch the autopilot back into HDG mode and then setup a course to follow on your VOR/GPS navigator where the aircraft will have to turn to align with the course. Then select NAV mode.  The autopilot should flash HDG, does it do that?  Then set the DG to the course. Does the aircraft align with the course? The DG heading bug provides the autopilot with the heading datum that is used to determine the no wind course, so if the DG heading bug is not communicating with the autopilot, it won’t be able to determine the course.
    Is your KAP140 a single axis unit or a two axis unit.  If only the roll axis is controlled, it is a single axis autopilot (ROL, HDG, NAV, APR, REV modes).  If the pitch axis is controlled, it is a two axis autopilot (ALT, GS modes).  Does it have a CWS (Control Wheel Steering) push button on the yoke?

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  2. Rick Dailey on Aug 29, 2011

    I do have a DG. I do not know if the roll mode will level the wings when this is happening. On an ILS approach, from the roll mode I push the approach button, turn heading bug to inbound course and the approach captures and turns the plane at the localizer. I have a two axis unit. I am not sure about control wheel steering. I do have a kill switch button on the yoke.There have been times when in the NAV mode the plane veers off heading. When that happens, swiching to HDG mode and using heading bug does not work to bring back on course. At times it works perfectly. There are times when the heading bug just does not work. Any suggestions? 

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  3. Brian on Aug 29, 2011

    “At times it works perfectly. There are times when the heading bug just does not work. Any suggestions? ”
    Sounds like loose wiring. I’ve personally never used this system, but typically whenever an electronic system works intermittently there is a good chance wires are loose. Does the self check always complete on initial boot up? Even on the days it fails? If so I’d still bet on the vibrations of flight causing wire connects and disconnects. 
    I tried to pull up some of the technical manuals on your system, but it seems you need login information. Maybe the guys at your maitenence shop can be of some assistence in helping you attain proper check procedures for your model? The pilot manual, at cursory glance, seems useless in the trouble shooting department. Here is a link: https://www.bendixking.com/TechPubs/servlet/TechPubsSearchServlet

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  4. Rick Dailey on Aug 29, 2011

    Never had a problem with start up checks

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  5. Brian on Aug 29, 2011

    I’d still check the wires. One partially connected, or not completely snapped in wire would give the exact intermittent response you describe. There are, of course, other possibilities. But checking if things are plugged in is certainly a simple endeavor. Good luck, sorry I can’t be of more help.

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  6. Andy Neumann on Aug 29, 2011

    Back this whole discussion up. 
    What kind of a shop says “Naw, that’s not broken until you tell us exactly how it’s broken!”   ????  I’m going to sound like someone’s father, but this problem sounds like something a qualified avionics technician should be looking at.  In other words: This is not a problem for you to problem solve in flight. 
    I would consider the autopilot inoperative (follow the appropriate regulations) until it is working 100% properly.  Tell the avionics shop exactly what it is doing and let them figure it out. 

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  7. Rick Dailey on Jan 06, 2012

    After all said and done, was the roll servo.

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