Stec 40 A/P issue

rbalisterrbalister
in General Discussion
I recently purchased a 79 Archer and am still working through learning the airplane. This plane has an Stec 40 and this is my first experience with an A/P. It seems that the wing leveler and heading modes work fine. However, when I try to get it to track a VOR it will drift off to the left of the needle. Since this is new, to me, does anyone have a suggestion as to what may be the problem? An adjustment or a serious issue?

Comments

  • Bob HBob H
    Hi Rbalister,
    How is the aircraft set-up? What nav-coms do you have and is there a switch for nav-1, nav 2?
    The VOR needle is centered when you activate tracking? Once activated, if you turn the obs knob and move the needle manually, does the autopilot chase the needle? Let's start there?
    Bob Hart APG Eastern Avionics rhart@avionix.com Visit our Website? www.avionix.com
  • rbalisterrbalister
    Bob,
    Thanks for the response. I have two KX170Bs and a GX55 that replaced a LORAN long before I got the plane. There's a rotary switch to select between the three nav inputs. It's labled NAV1, NAV2 and LORAN. I'm not sure if the LORAN position connects the GPS or not. I'll leave that for another time. I tried to do some nav testing on the ground, but the only nav signal that I can get is the localizer and that's not proving to be too helpful (full left deflection in the hangar). I'll try to fly it again this weekend and test using some VORs.

    When I tried the heading bug on the ground, it seemed that the long term trend was to go into a turn one way or another. I figured this would be the expected behavior. I'm guessing that they've implemented the roll servo loop as a PID controller of some sort and the constant, uncorrectable error input, however small, from being on the ground would cause the integrator portion to wind up one direction or the other resulting in a roll command.

    Raymond Balister
  • Bob HBob H
    Hi Ray, Sorry, I've been so busy with active customers, it's been cutting into my forum time! The GX-55 was likely a direct slide in for the Loran so it should track the GPS if the Loran was connected previously (likely). It does not sound like you have reliable link between any of your Nav sources. The rotary switch is not an STEC item but something the installer added at the time of installation. That could be suspect? Initially, you said you thought heading was OK but in your last hanger check your reporting something else. In heading mode, the heading bug at 360 degrees should result in ailerons level and no movement. It should stay that was until you move the heading bug left or right and the ailerons should respond with a turn toward the bug ... simple as that. Is the autopilot doing something on it's own with no inputs from you? It should not. If the autopilot is drifting in heading mode, it may be drifting an any mode?

    Send me an email directly so I can send you back some info? rhart@avionix.com Thanks! Bob Hart APG Eastern Avionics
  • rbalisterrbalister
    For those that might be interested, I was finally able to solve the problem. With more testing in-flight I found that it was drifting to the left in all modes. This was confirmed on the ground as well. After reading through several S-Tec manuals, including a shop service manual that someone had put on the Internet, it appears that the problem was simply an adjustment on the computer. Behind the top left screw, above the READY light, there is a trimpot. This is used to compensate for a gyro bias left or right. The direction from S-Tec (in said service manual) is to adjust the pot CW or CCW as necessary to center the yoke while on the ground. Now the AP tracks the nav sources (including the GX55) as expected.
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