PT20J Posted February 7, 2022 Report Share Posted February 7, 2022 The Garmin GFC 500 installation manual calls for an autopilot circuit breaker and a separate trim circuit breaker — just like my old KAP 150. The original TRIM breaker was an ETA rocker breaker/switch which I retained with the GFC 500 installation figuring it was easy to reach in case of a trim runaway. Today I did a test. I turned off the TRIM breaker with the GFC in ALT and pulled the throttle back. As the airplane slowed, I expected to get a TRIM UP annunciation on the G3X as one would get with a GFC without a trim servo. But that didn’t happen. So, I tried it again and noticed that the autopilot was controlling the trim even with the trim circuit breaker turned off. So, it turns out that the TRIM breaker only affects manual electric trim. I verified that MET is inoperative with the TRIM breaker off. Bottom line is that in the event of an autopilot trim runaway, don’t bother with the trim breaker. Hold down the yoke disconnect snd pull the autopilot breaker, Skip 2 2 1 Quote Link to comment Share on other sites More sharing options...
carusoam Posted February 7, 2022 Report Share Posted February 7, 2022 That is really eye opening! The procedure probably says turn off the AP and pull the breaker… BK probably doesn’t give the why… thus leaving the pilot to hit the trim CB if he comes across it first… great details as always Skip! Best regards, -a- Quote Link to comment Share on other sites More sharing options...
Ragsf15e Posted February 7, 2022 Report Share Posted February 7, 2022 2 hours ago, PT20J said: The Garmin GFC 500 installation manual calls for an autopilot circuit breaker and a separate trim circuit breaker — just like my old KAP 150. The original TRIM breaker was an ETA rocker breaker/switch which I retained with the GFC 500 installation figuring it was easy to reach in case of a trim runaway. Today I did a test. I turned off the TRIM breaker with the GFC in ALT and pulled the throttle back. As the airplane slowed, I expected to get a TRIM UP annunciation on the G3X as one would get with a GFC without a trim servo. But that didn’t happen. So, I tried it again and noticed that the autopilot was controlling the trim even with the trim circuit breaker turned off. So, it turns out that the TRIM breaker only affects manual electric trim. I verified that MET is inoperative with the TRIM breaker off. Bottom line is that in the event of an autopilot trim runaway, don’t bother with the trim breaker. Hold down the yoke disconnect snd pull the autopilot breaker, Skip Did the shop wire it correctly? Maybe the shop put the cb between your electric trim switch and it was meant to be on the trim motor? 1 Quote Link to comment Share on other sites More sharing options...
Brent Posted February 7, 2022 Report Share Posted February 7, 2022 2 hours ago, PT20J said: The Garmin GFC 500 installation manual calls for an autopilot circuit breaker and a separate trim circuit breaker — just like my old KAP 150. The original TRIM breaker was an ETA rocker breaker/switch which I retained with the GFC 500 installation figuring it was easy to reach in case of a trim runaway. Today I did a test. I turned off the TRIM breaker with the GFC in ALT and pulled the throttle back. As the airplane slowed, I expected to get a TRIM UP annunciation on the G3X as one would get with a GFC without a trim servo. But that didn’t happen. So, I tried it again and noticed that the autopilot was controlling the trim even with the trim circuit breaker turned off. So, it turns out that the TRIM breaker only affects manual electric trim. I verified that MET is inoperative with the TRIM breaker off. Bottom line is that in the event of an autopilot trim runaway, don’t bother with the trim breaker. Hold down the yoke disconnect snd pull the autopilot breaker, Skip I don't have electric trim and can verify the visual trim annunciations come up on the G3X as you would expect when the GFC 500 is engaged. 1 Quote Link to comment Share on other sites More sharing options...
PT20J Posted February 7, 2022 Author Report Share Posted February 7, 2022 1 hour ago, Ragsf15e said: Did the shop wire it correctly? Maybe the shop put the cb between your electric trim switch and it was meant to be on the trim motor? I checked the installation manual. All the TRIM breaker does is protect the wiring (including the trim switches on the yoke) between the bus and the pitch servo. 1 Quote Link to comment Share on other sites More sharing options...
PT20J Posted February 7, 2022 Author Report Share Posted February 7, 2022 45 minutes ago, Brent said: I don't have electric trim and can verify the visual trim annunciations come up on the G3X as you would expect when the GFC 500 is engaged. I’ve flow one without a pitch trim servo also and was expecting this one to revert to that behavior when I turned off the TRIM breaker/switch and was surprised ehen it didn’t. To be fair to Garmin, I don’t believe they have any procedure that calls for pulling the trim breaker. It’s always risky to assume one piece of equipment behaves like a different one and the manuals don’t explain everything. That’s why I’m spending so much time testing this installation. Skip Quote Link to comment Share on other sites More sharing options...
Brent Posted February 7, 2022 Report Share Posted February 7, 2022 14 hours ago, PT20J said: I’ve flow one without a pitch trim servo also and was expecting this one to revert to that behavior when I turned off the TRIM breaker/switch and was surprised ehen it didn’t. To be fair to Garmin, I don’t believe they have any procedure that calls for pulling the trim breaker. It’s always risky to assume one piece of equipment behaves like a different one and the manuals don’t explain everything. That’s why I’m spending so much time testing this installation. Skip Appreciate the posts. I'm riding your coattails as I get up to speed. Quote Link to comment Share on other sites More sharing options...
WaynePierce Posted March 4, 2023 Report Share Posted March 4, 2023 Skip, were you able to keep the thumb trim switch from your KAP 150 when you installed your GFC 500? Mine is going in to the shop this month to replace my KAP 150 with the gfc 500 and I'm hoping that my trim switch will work and I don't have to have that cheap looking metal thumb switch... Quote Link to comment Share on other sites More sharing options...
Fly Boomer Posted March 4, 2023 Report Share Posted March 4, 2023 1 minute ago, WaynePierce said: Skip, were you able to keep the thumb trim switch from your KAP 150 when you installed your GFC 500? Mine is going in to the shop this month to replace my KAP 150 with the gfc 500 and I'm hoping that my trim switch will work and I don't have to have that cheap looking metal thumb switch... Certainly nothing cheap about that BK thumb switch! Quote Link to comment Share on other sites More sharing options...
PT20J Posted March 4, 2023 Author Report Share Posted March 4, 2023 14 minutes ago, WaynePierce said: Skip, were you able to keep the thumb trim switch from your KAP 150 when you installed your GFC 500? Mine is going in to the shop this month to replace my KAP 150 with the gfc 500 and I'm hoping that my trim switch will work and I don't have to have that cheap looking metal thumb switch... I did keep the KAP 150 switches as they are working fine. The one Garmin uses is a better switch, however. Cygnet CA3112-G. $395 from Spruce. Quote Link to comment Share on other sites More sharing options...
WaynePierce Posted March 4, 2023 Report Share Posted March 4, 2023 19 minutes ago, Fly Boomer said: Certainly nothing cheap about that BK thumb switch! the one I am referring to is a silver spring loaded switch which is certainly cheap looking compared to what I currently have with the KAP 150. Quote Link to comment Share on other sites More sharing options...
WaynePierce Posted March 4, 2023 Report Share Posted March 4, 2023 4 minutes ago, PT20J said: I did keep the KAP 150 switches as they are working fine. The one Garmin uses is a better switch, however. Cygnet CA3112-G. $395 from Spruce. Thank you, good news for a change... Quote Link to comment Share on other sites More sharing options...
65MooneyPilot Posted March 10, 2023 Report Share Posted March 10, 2023 I don’t have an autopilot in my Mooney, just the wing leveler. However I do have many thousands of hours in Boeings and lots of simulator training events. It does not surprise me how the GFC 500 operates. These AP take after their big brothers from the Transport category. When ever you experience a runaway trim you are directed to disconnect the AP first thinking the AP computer is causing the runaway. Then use manual electric trim to guide the trim back. If the trim is still running away you would disconnect the electric trim and take over using manual trim. Big brother’s trim motors have two sources of power to the trim motors. There are disconnect switches on the pedestal to remove the power to the trim motors. If you ever encounter runaway trim, of course follow the checklist but as a last resort turn it all off, pull the breakers. 3 Quote Link to comment Share on other sites More sharing options...
jetdriven Posted March 11, 2023 Report Share Posted March 11, 2023 In fact the latest service letter from Garmin says this much. Under some circumstances you could have an uncommanded or unwanted autopilot induced trim runaway, and pulling the trim circuit breaker does not stop it, you have to pull the auto pilot circuit breaker. All the electric trim circuit breaker does is disable the manual electric trim. 1 Quote Link to comment Share on other sites More sharing options...
jamesm Posted March 11, 2023 Report Share Posted March 11, 2023 Does the sequence matter when pulling the CB for the GFC500 autopilot? i.e. If you were to just pull the Auto pilot CB without using the A/P disconnect switch as oppose to holding the A/P Disconnect switch then pull the Autopilot breaker ( As I recall this is what the Pilot's Guide says)? Just curious. James '67C Quote Link to comment Share on other sites More sharing options...
65MooneyPilot Posted March 11, 2023 Report Share Posted March 11, 2023 The pilot guide needs to be followed, however if your trim is running away you do what you need to do to stop the runaway. In the 737 which has a trim wheel in the pedestal the last resort is to try and grab the wheel with your hand to stop it. The newer Boeings you go for the disconnect switches as fast as you can. Bottom line is to stop the trim wheel from moving to an unrecoverable position ASAP. The same procedure can be applied in the Mooney or any other airplane you are flying. Quote Link to comment Share on other sites More sharing options...
ArtVandelay Posted March 11, 2023 Report Share Posted March 11, 2023 One problem is if the trim starts to runaway you won’t know it at first because the autopilot will compensate and then it may take a few seconds after is disconnects to realize what’s happening. I would like to have a trim annunciation when trim is activated. 2 Quote Link to comment Share on other sites More sharing options...
Jerry 5TJ Posted March 11, 2023 Report Share Posted March 11, 2023 39 minutes ago, ArtVandelay said: One problem is if the trim starts to runaway you won’t know it at first because the autopilot will compensate and then it may take a few seconds after is disconnects to realize what’s happening. I would like to have a trim annunciation when trim is activated. In my one pitch trim runaway event the nose down movement was fairly rapid and the autopilot pitch servo didn’t overpower the trend much more than a second. The KFC225 did helpfully annunciate “trim in motion” but by then I had certainly noticed that the outside view was changing rapidly. Mashing the red button stopped all servos. The fault was in the pitch servo’s force sense mechanism which jammed in the “give me nose down trim” position. The AP computer dutifully commanded the pitch trim servo to do just that. Quote Link to comment Share on other sites More sharing options...
201Steve Posted March 11, 2023 Report Share Posted March 11, 2023 Interesting thread. I was testing similar a couple weeks ago and started running the electric trim manually, pulled each breaker in different attempts. Pulling the trim breaker, there was a 2 second +\- lag before the trim actually stopped. Pulling the AP breaker it stopped immediately. Quote Link to comment Share on other sites More sharing options...
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