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FCS 105 Autopilot not engaged - Do you believe in witches???


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Last Friday i left the C130 where i was working, in good shape the Autopilot work ok. After me the team that would fly the aircraft in the morning did a pre flight and tested the autopilot everything ok.

In the morning, aircraft in the runway, and autopilot not engaged!!!

I changed the autopilot amplifier, the yawdumper computer, the mode coupler´s, the autopilot control box and the 2 mode selector. I have a good AC intruments voltage. The problem stays.

After that first try of getting autopilot on, the flight was aborted.

In the hangar i checked the readings in the control box of fcs 105. In the pin 34 i (yaw dumper monitor) i don´t get any voltage. I checked in other aircraft and i get 28v.

It´s like the yawdumper don´t get the right information from the elements that he need´s and doens´t get a valid information to sent to the control box in the pedestal.

Could a damage servo do this? Autopilot doesn´t engaged in yawdumper?


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A rudder servo could be inserted into the system to see if it is causing problems, without actually mounting it. A missing roll signal could also cause the computer to probably malfunction.

You will need to verify that the voltage is coming out of the yaw damper computer, pin 14. I don't know what year you are working on, but just glancing at the 78 and up models, that wire from pin 14 goes to a terminal board, then through the AUTOPILOT DISENG MANUAL TRIM NOSE DOWN RELAY, AUTOPILOT DISENG MANUAL TRIM NOSE UP RELAY, the GO AROUND switches and the DISENGAGE switches, on the way to pin 34 of the controller. On 78-82 models, it also went through contacts on the Co-pilot's AC inverter switch.

Follow the wire from pin 14 in your diagram, and go to the first terminal to see if you have voltage. If not, you could have a broken wire at the Overhead Disconnect connector.

Good luck.

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  • 6 months later...

In this case, you can also have a broken wire on a disengage switch or the G/A switch. You would need to locate the terminal board under the center console and remove the wires to check for continuity to rule out each switch. I'm sure the issue is fixed by now, but just adding info!

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