NicoN Posted March 20, 2020 Report Share Posted March 20, 2020 Yesterday I found the Elevator Trim ON/OFF-switch in OFF-position for the second time. It was always discovered after take-off (shame on me), because the electrical trim was not working After switching to ON anything was fine. But, I am thinking why it was found in OFF-position. Normally, this switch is never used and always in ON AFAIK, this switch is also a C/B, so, it might be trapped rather than swapped to OFF by a human being. AFAIK, the electrical elevator trim has lots of items to switch it off: - the TRIM interrupt-button on the yoke - a C/B in the panel as part of the rocker switch assembly directly next to the yoke column - why this in addition to the rocker-switch? - a separate rocker switch (with C/B-function?) to switch electrical trim on or off My first idea is/was: I was flying with to people only in the front seats. So, I supposedly used maximum nose-up trim for the landing. Maybe, the trim motor ran into mechanical stop and caused the switch-type C/B to trap ? Aren't there limit switches on the trim motor gear ? I don't have a maintenance manual with me right now, so just an idea. It is a Mooney M20K-231 from 1980 Quote Link to comment Share on other sites More sharing options...
PT20J Posted March 20, 2020 Report Share Posted March 20, 2020 Yes, the switch is also a breaker. I assume you have a King autopilot in which case the manual electric trim is part of the autopilot system. There are no limit switches as the trim servo has a friction clutch that slips when the trim reaches the stop. The problem could be something with the servo, or the switch getting weak, or the autopilot. My KAP 150 autopilot had an intermittent problem that would trip the trim switch/breaker. If you frequently run the trim to the stops, you might want to make sure you the stepped stop nuts to prevent trim jamming. It’s also a good idea to periodically manually run the trim wheel from stop to stop to check the trim system for friction. Skip Quote Link to comment Share on other sites More sharing options...
ArtVandelay Posted March 20, 2020 Report Share Posted March 20, 2020 1 hour ago, PT20J said: ...you might want to make sure you the stepped stop nuts to prevent trim jamming. Can you elaborate by what do you mean by “stepped stop nuts”? Quote Link to comment Share on other sites More sharing options...
PT20J Posted March 20, 2020 Report Share Posted March 20, 2020 7 minutes ago, ArtVandelay said: Can you elaborate by what do you mean by “stepped stop nuts”? The original design had jam nuts for the trim stops that can -er, jam -- if you run the trim hard to the stops. Some have reported that the electric trim servo can have enough torque to jam the system such that manual operation of the trim wheel to break the jam is difficult or impossible. The "stepped" nuts stop against the step rather than by jamming the threads. 1 Quote Link to comment Share on other sites More sharing options...
NicoN Posted March 21, 2020 Author Report Share Posted March 21, 2020 Our M20K has a Century 21 A/P - so no auto-trim. But we have electrical trim - as mentioned with the combined switch on the yoke, a rocker switch and a separate c/B Quote Link to comment Share on other sites More sharing options...
PT20J Posted March 21, 2020 Report Share Posted March 21, 2020 In that case, I would try running the electric trim to the stops and see if that trips the switch. You will need to reproduce the problem before you can troubleshoot it to isolate the cause. You might also consider paying for a bit of troubleshooting teleconsulting with @Bob Weber. Skip 1 Quote Link to comment Share on other sites More sharing options...
N201MKTurbo Posted March 21, 2020 Report Share Posted March 21, 2020 I have found when these switches mysteriously turn off, I have bumped them with my knee. Quote Link to comment Share on other sites More sharing options...
carusoam Posted March 21, 2020 Report Share Posted March 21, 2020 There is Something about CBs and CB switches... They really are mechanical devices... They really are electrical devices as well... They wear... They are temperature sensitive... How you use it today... may not fully reflect how it has been used in the past... A new CB switch has a distinct Feelable click going in either direction... A worn one has a softer click to it, until no click is felt at all. As the click goes away... so does it’s over-centering safety device... The overcentering requires an amount of force to allow the switch to flip... A more worn switch has the ability to flip without much force. As current runs through the CB, it warms up... the click gets softer... If the inside air temp warms up... the click gets softer... If something lightly bumps the worn switch... it turns off... My M20C taught me this lesson... the ancient landing light CB couldn’t stay on. It used a lot of current, it was hot out, and the drooping panel above supplied the little force it needed... Compare the clicks to the other CB switches... see if you can tell the difference in the clicks... Is there anything heating the CB switches? Nearby radios or heater output...? Are they warm to the touch? Most importantly... if the amperage going to the CB is close to its rating.... it may get warm and shut off... this is the usual mechanism of our mechanolectrical Switches... PP thoughts only, not a mechanic... Best regards, -a- 1 Quote Link to comment Share on other sites More sharing options...
Bob Weber Posted March 22, 2020 Report Share Posted March 22, 2020 On 3/20/2020 at 2:02 PM, ArtVandelay said: Can you elaborate by what do you mean by “stepped stop nuts”? Castellated nuts, designed primarily to have a cotter pin installed to lock it. https://www.albanycountyfasteners.com/Brass-Castle-Nuts-p/129000-002.htm 1 Quote Link to comment Share on other sites More sharing options...
Bob Weber Posted March 22, 2020 Report Share Posted March 22, 2020 20 hours ago, carusoam said: There is Something about CBs and CB switches... They really are mechanical devices... They really are electrical devices as well... They wear... They are temperature sensitive... How you use it today... may not fully reflect how it has been used in the past... A new CB switch has a distinct Feelable click going in either direction... A worn one has a softer click to it, until no click is felt at all. As the click goes away... so does it’s over-centering safety device... The overcentering requires an amount of force to allow the switch to flip... A more worn switch has the ability to flip without much force. As current runs through the CB, it warms up... the click gets softer... If the inside temp warms up... the click gets softer... If something lightly bumps the worn switch... it turns off... My M20C taught me this lesson... the ancient landing light CB couldn’t stay on. It used a lot of current, it was hot out, and the drooping panel above supplied the little force it needed... Compare the clicks to the other CB switches... see if you can tell the difference in the clicks... Is there anything heating the CB switches? Nearby radios or heater output...? Are they warm to the touch? Most importantly... if the amperage going to the CB is close to its rating.... it may get warm and shut off... this is the usual mechanism of our mechanolectrical Switches... PP thoughts only, not a mechanic... Best regards, -a- There is an alloyed metal "spring" sized and alloyed to heat and release, over the decades this spring will lose tension. The biggest culprit I have seen over the years to cause this was corrosion and arc slag causing a resistance across the contacts which then becomes a little heating element. Under normal conditions, for a surprising length of time, the system will function correctly until the long term Annealing process reduces the over center friction as stated above. Not only is this working against us, but it also reduces the temp needed to trip the breaker causing it to trip at a lower current.. I'm working on an article for the Mooney Flyer trying to cover things like this, watch for it "The challenges and responsibilities of maintaining an aged aircraft". The most neglected component The one "component" of an aging aircraft, and arguably the most critical, is the wiring harness, it acts as the central nervous system of your aircraft. Our "new and improved problems" reach new levels with this component. There are multiple fronts of battle with maintaining and troubleshooting it as well. Fly Safe Bob Weber webairconsulting.com 1 Quote Link to comment Share on other sites More sharing options...
PT20J Posted March 22, 2020 Report Share Posted March 22, 2020 3 hours ago, Bob Weber said: Castellated nuts, designed primarily to have a cotter pin installed to lock it. https://www.albanycountyfasteners.com/Brass-Castle-Nuts-p/129000-002.htm Not in this case. These are special Mooney parts that are threaded on the front trim shaft screw (beneath the trim wheel) and act as trim stops. The stepped nuts have a machined step that forms the stop instead of the nut jamming at the end of travel. Skip 1 Quote Link to comment Share on other sites More sharing options...
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