KAP 200 Autopilot Trim Interrupt / Autopilot Disconnect Switch

[Lima Whiskey]

I wanted to share somethings that I learned this past week about the yoke mounted Trim Interrupt switch on our club's 1979 M20J.

The red button on the trim interrupt switch was broken off accidentally by one of our club members.  It immediately disabled the electric trim and as a result the autopilot.  I was under the impression that the Trim Interrupt switch not only was used to stop a runaway trim but also to disconnect the autopilot, however we never really used the button.  We always disconnected the autopilot with the on/off switch on the unit.

When our mechanic pulled out the switch it was stamped with part number C2003R - which turns out you can get from Textron for $99.95.  C2003R has four contacts.  As I understand it, pins 1 and 2 are normally closed and pins 3 and 4 are normally open.  Pins 1 and 2 are used for the Trim Interrupt and pins 3 and 4 are used for the autopilot disconnect.  So when you press the button it takes power away from the trim and also sends a signal to the KAP 200 to disconnect.

However, our switch only had wires to pins 1 and 2 - the Trim Disconnect.

When the mechanic installed the switch the electric trim began to work and we could use the autopilot.  

If anyone can tell me the thought process behind the trim interrupt switch, I would appreciate it.  It only stops a runaway trim when it is depressed.  As soon as you release the button the runaway trim would continue.  It seems to me that the only way to really stop the runaway trim is to turn off the trim switch on the panel.  So the yoke mounted trim interrupt switch seems kind of useless.  Maybe the idea is that you depress the yoke mounted switch and then you immediately switch off the panel mounted trim switch.

I share all of this because we ended up spending $600 on the repair.  Textron had the part on backorder until the middle of August (and I have an 18 day East Coast to West Coast and back trip planned for this coming Friday) so we had to get the switch from another supplier.  At the end of the day we paid $360 for the switch and expedited shipping and then the labor.

Hopefully this helps someone in the future.

I would be interested if anyone has any knowledge on how to make the switch actually disconnect the autopilot.  I would imagine that two wires need to be run from the autopilot control unit to the yoke and soldered to pins 3 and 4 of the C2003R switch, but I don't have any kind of wiring diagram for the connectors on the back of the KAP 200 control unit.

This is my first post... so go easy on me :-)

 

 

 

[PT20J]

Welcome aboard. We’re here to help each other.

The AP Disconnect/Trim Interrupt button should disconnect the autopilot when pressed and released. It will disconnect the trim servo while held down. This is to allow you to manually control the aircraft until you can remove power from the autopilot in the event of a runaway trim failure.

Any avionics shop familiar with the KAP 200 should be able to connect the switch. Perhaps @Jake@BevanAviation or @Bob Weber can help.

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[hubcap]

On my 1982 M20K with KAP 200, the Disconnect/Trim Interrupt button disconnects the autopilot when pressed. I have no idea how its wired, just what it does when you press it. I would certainly agree with @PT20J that your avionics shop should be able to connect the switch correctly.

[Jake@BevanAviation]

There are 3 different versions of a IM for the M20J with a King 200 system.  Late in the production (24-1419 and later) it looks like they changed the function of the trim interrupt switch assembly.  The earlier version of the switch assembly functioned as a means to interrupt the trim power, which would remove power to the drive transistors stopping a runaway auto-trim condition as long as the switch was activated.  The early version used a disconnect relay that would cause the computer to disconnect if using the MET switch as you can't have auto-trim and MET trim at the same time so the computer has to disconnect.  

The pic below shows the interconnect function of the trim interrupt switch and the disconnect relay function.  In this version the computer would disconnect when the AP/disconnect relay is energized from the yoke mounted trim arm side of the split rocker switch.  When the left side of the split rocker moves up or down it will send bus voltage to the disconnect relay causing it to energize and remove the engage voltage to the control head.  At the same time it routes power to the other side of the MET switch allowing you to go up or down with MET trim.

The later revision used a different switch assembly that would remove power to the trim servo and the computer engage voltage at the same time.  The disconnect relay was removed due to the switch change.  The part that bothers me is when if you look at the parallel from the TB after the trim interrupt switch that goes back to the computer on P2951 pin k.  I think if this voltage is removed it should disconnect the system as well, it is mention in the IM that if the trim breaker is pulled the computer should disconnect.  Opening the trim interrupt switch should have the same function as pulling the breaker.

Per the IM the following conditions should cause the system to disconnect. 

• activation of the pilot's disconnect switch
• activation of the manual trim switch, either up or down
• pulling of the autopilot or pitch trim breakers
• if the heading flag comes into view when using any lateral command mode

I hope this helps

[carusoam]

Great insight Jake!

Welcome aboard LW!

Go MS!

Best regards,

-a-