GFC 500 Trim CB

[PT20J]

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

[carusoam]

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-

 

[Ragsf15e]

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?

[Brent]

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. 

[PT20J]

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.

[PT20J]

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

[Brent]

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.

[WaynePierce]

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...

[Fly Boomer]

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!

[PT20J]

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.

[WaynePierce]

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.

[WaynePierce]

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...

 

[65MooneyPilot]

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.

[jetdriven]

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. 

[jamesm]

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

[65MooneyPilot]

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.

[ArtVandelay]

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.

[Jerry 5TJ]

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.   

[201Steve]

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.