Why Did My GFC-500 Do That?

[RoundTwo]

I apologize for the clickbait title but I think you’ll understand.

First off, I’m new to the world of flying automation and I’m learning as fast as I can. When I hung up flying in 1991, my Cherokee 140 had no cool whiz bang goodies like I now have, so I’ve been on a whirlwind journey to say the least these last few months flying behind glass.

Now that it appears the gremlin gods have been temporarily appeased, I’ve been flying quite a bit locally and have gotten in a few nice trips. However, I keep getting surprised by the GPS/Autopilot when I try to do something and it has a different execution than I had expected it to have. My standard response is “Why did it do that?”. I always assume it’s me and not the autopilot doing something wrong, but I’m still left wondering. So far, what I’ve been doing is let the AP fly and I’ll see what it had in mind.

I’m just getting started on my instrument training and am wondering where does one go for the official “rules of engagement” that the AP programmers would use when designing the AP software. I have to assume these things are pretty straightforward and I just need to learn the rules.

[GeeBee]

First rule of automation. "It is dumb and it is dutiful". It will do what you asked it to do, relentlessly.

Second rule of automation "If it is not doing what you want, disconnect and hand fly". So always monitor performance and be ready to fly at any second.

Third rule of automation "Know the modes cold". Know where the mode is annunciated and always verify what you pushed is what you got, You have to study the manual to know what the modes do. Keep an extra copy by the toilet.

[RoundTwo]

17 minutes ago, GeeBee said:

First rule of automation. "It is dumb and it is dutiful". It will do what you asked it to do, relentlessly.

Second rule of automation "If it is not doing what you want, disconnect and hand fly". So always monitor performance and be ready to fly at any second.

Third rule of automation "Know the modes cold". Know where the mode is annunciated and always verify what you pushed is what you got, You have to study the manual to know what the modes do. Keep an extra copy by the toilet.

I’ve mastered disconnect and hand fly for sure. I need to learn the proper “when and where”. For example, 15 miles out from KDNN, perpendicular to the runway, isn’t the right time to “Load and Activate”. The results are less than impressive.  I was just out of KCZL and experimenting.

[LANCECASPER]

One thing I've had to learn this last year and a half behind the G1000 is that if you are already on the approach don't hit Activate or it will want to take you back to the Initial Approach Fix, not a good thing in IMC.

[Ragsf15e]

To reinforce what was just said, know the difference between hitting “activate approach” on your gps and hitting “approach” mode on your autopilot mode.  They are different things and need to be used at the correct times.

[Pinecone]

Garmin has online courses to teach your autopilot and nav.  If you bought the gear and had it installed, there should have been a card with a code for free access to the training.

Some of their training is on You Tube for free.
 

They also have in person classes, that cost money.

[PT20J]

The source for the “Rules of Engagement” is the AFMS.

[Vance Harral]

PT20J has it right above: read the AFMS.  That said, one key to the Garmin Autopilots is the "scoreboard" displayed at the top of the G5/G3X/whatever.  Left side is lateral mode, right side is vertical mode.  Green means active.  White means armed (one mode can be active while another is armed).  Blinking means transitioning from armed to active.  I'm deliberately not providing more details, you can get those from the AFMS.  But in my instructing, most people I fly with who are surprised by autopilot actions are not correlating the control inputs they're surprised by with the mode in which the autopilot is operating.

[dzeleski]

Start with this:

Generally when an AP does something you didnt want it to and the question is: "Why did it do that?" The answer is because you told it to do it, even if it was on accident.

AFMS: https://static.garmin.com/pumac/190-02291-20_02.pdf

When ATC says expect ILS 24 I will ONLY load the approach, once I am cleared for the approach I will then activate it based on where on the approach you are you might need to modify the flight plan but I never activate before I am cleared. But as said above that button on your GPS and the button on your AP do different things, the manual explains it.

 

Edit: Before you enable the AP ALWAYS verify the command bars, does it look like/match what you are asking for? You have to do this every single time you touch that AP. If Yes continue with enable, if no you need to scan that panel and look for the incorrect annunciations. Were you on HDG rather then NAV? etc.

[PT20J]

4 minutes ago, Vance Harral said:

PT20J has it right above: read the AFMS.  That said, one key to the Garmin Autopilots is the "scoreboard" displayed at the top of the G5/G3X/whatever.  Left side is lateral mode, right side is vertical mode.  Green means active.  White means armed (one mode can be active while another is armed).  Blinking means transitioning from armed to active.  I'm deliberately not providing more details, you can get those from the AFMS.  But in my instructing, most people I fly with who are surprised by autopilot actions are not correlating the control inputs they're surprised by with the mode in which the autopilot is operating.

Excellent point. I see people often just press a button without verifying that it is the correct button and that it actually took effect. The pros figure out what they want the automation to do, then double check that they have selected the correct button, and then verify that it has taken effect. 

[Ragsf15e]

15 minutes ago, dzeleski said:

Start with this:

Generally when an AP does something you didnt want it to and the question is: "Why did it do that?" The answer is because you told it to do it, even if it was on accident.

AFMS: https://static.garmin.com/pumac/190-02291-20_02.pdf

When ATC says expect ILS 24 I will ONLY load the approach, once I am cleared for the approach I will then activate it based on where on the approach you are you might need to modify the flight plan but I never activate before I am cleared. But as said above that button on your GPS and the button on your AP do different things, the manual explains it.

 

Edit: Before you enable the AP ALWAYS verify the command bars, does it look like/match what you are asking for? You have to do this every single time you touch that AP. If Yes continue with enable, if no you need to scan that panel and look for the incorrect annunciations. Were you on HDG rather then NAV? etc.

One thought with “never activate the approach before cleared”… what about vectors to final?

I agree that you might not want to activate before cleared the approach sometimes (like when using gps for steering), but when you’re receiving vectors, it’s good to activate it ahead of time so you can verify everything is set up correctly and there’s less button pushing on that final turn to the faf.  If you’re getting vectors, you’re in heading mode, so you can get the gps or ils set up ahead of time and check it.  That’s exactly what “activate the approach”does on the gps.
 

Another time I can think of is when you’re cleared to the iaf but not cleared the approach.  Activating the approach will give you steering to the iaf (you could also select direct).  It will not descend you below the selected altitude.
 

in both these cases, the ap will not fly the vertical part of the approach because you haven’t selected “Approach mode” which will give you gp or gs and allow it to descend below your selected altitude on glidepath.

I will not select “Approach” mode on the autopilot before being cleared for the approach because it will descend below your selected altitude on glidepath.  But activating the approach is simply setting up your gps for the approach.  In a g1000, they seem so similar because it’s all integrated, but with separate boxes it’s a little easier to see the difference.

[RoundTwo]

I guess I’m failing in my explanation. I’m not saying I’m having a problem pushing the proper button and getting an improper response.

What I’ve run into problems is not knowing where I need to be in the sky, and headed in what direction, that it is correct and appropriate to push the button. It’s pretty easy to know when to do it if I’m at cruise altitude and enable a VNAV descent for an approach, but less clear if I’m just flying around locally, going from airport to airport, and learning how everything works. It’s an issue of being relatively close to the airport, and nowhere near the published approach course. I’m assuming there are “rules” that the AP is operating on that govern when it would turn me towards the IAF and when it wouldn’t. This level of detail is not anything I would expect Garmin to publish as part of the AFMS. I’m guessing this information would be more relevant to “Flying Approaches with an Autopilot for Dummies.”

[dzeleski]

1 hour ago, Ragsf15e said:

One thought with “never activate the approach before cleared”… what about vectors to final?

I agree that you might not want to activate before cleared the approach sometimes (like when using gps for steering), but when you’re receiving vectors, it’s good to activate it ahead of time so you can verify everything is set up correctly and there’s less button pushing on that final turn to the faf.  If you’re getting vectors, you’re in heading mode, so you can get the gps or ils set up ahead of time and check it.  That’s exactly what “activate the approach”does on the gps.
 

Another time I can think of is when you’re cleared to the iaf but not cleared the approach.  Activating the approach will give you steering to the iaf (you could also select direct).  It will not descend you below the selected altitude.
 

in both these cases, the ap will not fly the vertical part of the approach because you haven’t selected “Approach mode” which will give you gp or gs and allow it to descend below your selected altitude on glidepath.

I will not select “Approach” mode on the autopilot before being cleared for the approach because it will descend below your selected altitude on glidepath.  But activating the approach is simply setting up your gps for the approach.  In a g1000, they seem so similar because it’s all integrated, but with separate boxes it’s a little easier to see the difference.

I never use vectors to final mode. It removes all of the fixes of the approach and if anything about that approach changes you need to reload it. So in short I never use it even when being vectored. Ill set one of the fixes/legs off of a loaded approach if im being vectored. If you are being vectored and then ATC say direct to XXX or maintain NNNNft until XXX now you need to reload the entire approach.

 

[dzeleski]

1 hour ago, RoundTwo said:

This level of detail is not anything I would expect Garmin to publish as part of the AFMS. I’m guessing this information would be more relevant to “Flying Approaches with an Autopilot for Dummies.”

It actually is all in the AFMS, youll need to cross reference that with your Navigators guide as well because in NAV mode it is also commanding inputs.

[Ragsf15e]

25 minutes ago, dzeleski said:

I never use vectors to final mode. It removes all of the fixes of the approach and if anything about that approach changes you need to reload it. So in short I never use it even when being vectored. Ill set one of the fixes/legs off of a loaded approach if im being vectored. If you are being vectored and then ATC say direct to XXX or maintain NNNNft until XXX now you need to reload the entire approach.

 

Fair enough.  You can obviously do it that way.  I will also load the full approach, but will activate vtf as it does keep the waypoints that are on the straight line segment to the faf.  
 

As you are indicating, “activating the approach” isn’t even a requirement.  There’s no magic.  It just takes care of a bunch of tasks you’ll eventually need to do like setting the gps mode to vloc (depending on your system and approach) and setting steering to the iaf or faf.  If you do those things yourself, no need to activate.  Although you do have to be careful how you activate the leg into the faf as going direct to it will not work and could lead you to believe you’re established when you aren’t.

Activating the approach mode on the autopilot is a whole different beast because it selects new autopilot modes and authorizes the autopilot to steer you laterally and vertically on the approach.

[dzeleski]

2 minutes ago, Ragsf15e said:

Fair enough.  You can obviously do it that way.  I will also load the full approach, but will activate vtf as it does keep the waypoints that are on the straight line segment to the faf.  
 

As you are indicating, “activating the approach” isn’t even a requirement.  There’s no magic.  It just takes care of a bunch of tasks you’ll eventually need to do like setting the gps mode to vloc (depending on your system and approach) and setting steering to the iaf or faf.  If you do those things yourself, no need to activate.  Although you do have to be careful how you activate the leg into the faf as going direct to it will not work and could lead you to believe you’re established when you aren’t.

Activating the approach mode on the autopilot is a whole different beast because it selects new autopilot modes and authorizes the autopilot to steer you laterally and vertically on the approach.

I guess my workflow is a bit different because I run an Avidyne. I dont need to do any of those things as it will auto tune to ground based nav aids, ID them, set the inbound course, and switch to the correct mode based on the approach type. Some of those things happen prior to activating and some happen after, but its all automatic, including when I go missed.

 

As far as being careful about going direct to the FAF, yes you do need to understand that and generally I dont use direct unless im told to go direct. What I do is activate the leg, and just maintain the heading given until it captures automatically (assuming the AP is configured correctly). And because I set a leg of the approach as the active leg it assumes the approach is activated.

[rbp]

1 hour ago, dzeleski said:

I never use vectors to final mode. It removes all of the fixes of the approach and if anything about that approach changes you need to reload it. So in short I never use it even when being vectored. Ill set one of the fixes/legs off of a loaded approach if im being vectored. If you are being vectored and then ATC say direct to XXX or maintain NNNNft until XXX now you need to reload the entire approach.

Activating VTF on GTN devices does not remove the IAF and IAF waypoints

I believe (but I can't find the reference) that an update the the GTN software was made so that it no longer removes the IAF/IF waypoints when activating VTF

[dzeleski]

23 minutes ago, rbp said:

Activating VTF on GTN devices does not remove the IAF and IAF waypoints

I believe (but I can't find the reference) that an update the the GTN software was made so that it no longer removes the IAF/IF waypoints when activating VTF

I havent used a GTN in several years now but if they did update it to work like that is a great change.

[Vance Harral]

1 hour ago, rbp said:

Activating VTF on GTN devices does not remove the IAF and IAF waypoints

I believe (but I can't find the reference) that an update the the GTN software was made so that it no longer removes the IAF/IF waypoints when activating VTF

This change was made in GTN software revision 6.11, released in 2016.  But it is incorrect to say the GTN "no longer removes the IAF/IF waypoints" after this change.  Activating VTF does still remove waypoints, sometimes including the IAF.  It just doesn't remove any waypoints that are along the final approach course.  So for example in the GPS 29 approach to KLMO below, if you load the approach from the FIPPS IAF, then later activate VTF, FIPPS will be removed from the flight plan, but FIMUR and MELVN and RW29 will remain.  Prior to revision 6.11, only MELVN (the FAF) and RW29 (the MAP) would remain.

 

[rbp]

19 minutes ago, Vance Harral said:

This change was made in GTN software revision 6.11, released in 2016.  But it is incorrect to say the GTN "no longer removes the IAF/IF waypoints" after this change.  Activating VTF does still remove waypoints, sometimes including the IAF.  It just doesn't remove any waypoints that are along the final approach course.  So for example in the GPS 29 approach to KLMO below, if you load the approach from the FIPPS IAF, then later activate VTF, FIPPS will be removed from the flight plan, but FIMUR and MELVN and RW29 will remain.  Prior to revision 6.11, only MELVN (the FAF) and RW29 (the MAP) would remain.

 

good catch. how does it decide which ones to remove and which remain? 

 

[Vance Harral]

51 minutes ago, rbp said:

good catch. how does it decide which ones to remove and which remain? 

 

Only waypoints along the final approach course are retained.  Any waypoints not aligned with the final approach course are removed.

[PT20J]

9 hours ago, RoundTwo said:

I guess I’m failing in my explanation. I’m not saying I’m having a problem pushing the proper button and getting an improper response.

What I’ve run into problems is not knowing where I need to be in the sky, and headed in what direction, that it is correct and appropriate to push the button. It’s pretty easy to know when to do it if I’m at cruise altitude and enable a VNAV descent for an approach, but less clear if I’m just flying around locally, going from airport to airport, and learning how everything works. It’s an issue of being relatively close to the airport, and nowhere near the published approach course. I’m assuming there are “rules” that the AP is operating on that govern when it would turn me towards the IAF and when it wouldn’t. This level of detail is not anything I would expect Garmin to publish as part of the AFMS. I’m guessing this information would be more relevant to “Flying Approaches with an Autopilot for Dummies.”

The questions you are asking are more about basic use of navigation systems and autopilots than anything specific to the GFC 500. If I were you, I would buy a couple of hours of dual with an instructor and it should all become clear.

[Fly Boomer]

On 5/9/2023 at 6:24 AM, RoundTwo said:

Now that it appears the gremlin gods have been temporarily appeased, I’ve been flying quite a bit locally and have gotten in a few nice trips. However, I keep getting surprised by the GPS/Autopilot when I try to do something and it has a different execution than I had expected it to have. My standard response is “Why did it do that?”. I always assume it’s me and not the autopilot doing something wrong, but I’m still left wondering. So far, what I’ve been doing is let the AP fly and I’ll see what it had in mind.

It’s a computer.  Either 1) it’s broken, or 2) it’s doing exactly what it was told to do.  Either instance can be quite frustrating.

[M20F]

On 5/9/2023 at 7:44 AM, GeeBee said:

First rule of automation. "It is dumb and it is dutiful". It will do what you asked it to do, relentlessly.

 

This is why I will never have a 3 axis autopilot.   

[RoundTwo]

15 hours ago, PT20J said:

The questions you are asking are more about basic use of navigation systems and autopilots than anything specific to the GFC 500. If I were you, I would buy a couple of hours of dual with an instructor and it should all become clear.

Yes, basic questions and that’s why I’m assuming there must be reference texts or books that explain it.