G1000/GFC700 AFCS Faults in Turbulence

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dmloftus
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G1000/GFC700 AFCS Faults in Turbulence

Post by dmloftus »

Flying from Atlanta to Las Vegas, I encountered moderate (sometimes borderline severe) turbulence over the remote parts of West Texas and on the leeward side of the Grand Canyon. Some probably due to mountain waves and clear air turbulence, some just high thermals from afternoon convection. The GFC700 was doing it's best to track my course, but we were being thrown around rather violently. Several times the autopilot kicked off, displaying an AFCS fault on the PFD. I would then revert to hand flying through the toughest conditions and the only way to reengage the autopilot was to pull and reset the AFCS breaker when conditions improved a bit. Of course I tried different altitudes and set down for a few hours when conditions were more severe. Once airborne and the plane began tossing violently again, the AFCS error would reappear. This happened about half a dozen times over several hours. Has anyone had similar issues? I can't think this is a normal occurrence. I also cannot find anything in the G1000 manuals that discuss conditions when an autopilot might go into an error condition if it could not track as commanded due to weather conditions. Would this be a servo overheating due to being worked too hard or a software decision when some boundary conditions were exceeded?

I know several of you are based in the west and probably face these conditions on a regular basis. Any suggestions on G1000/GFC700 modes that work the AP less? I was taught for turbulent conditions to run in heading mode and pitch hold rather than GPS/ALT to reduce workload on an AP. Maybe even just ROL and PIT modes. While we face occasional CAT in the southeast, the biggest worry is usually avoiding thunderstorms. Going home I'll try to fly exclusively in the early morning ;-)
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mhoran
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Re: G1000/GFC700 AFCS Faults in Turbulence

Post by mhoran »

I haven't experienced this myself but have experienced various autopilot faults. You can boot the G1000 into maintenance mode and check the GIA and servo fault logs. The GIA logs should tell you why the fault occurred and the servo logs provide verbose details.

You boot into maintenance mode by holding ENT before the PFD and MFD have started (or pull the breakers). Then you navigate to the diagnostics terminal via the inner FMS knob on the system page (the default, which you can get back to via the outer FMS knob.)

The interface is a bit obnoxious. If it says to "press any key to see more" you actually need to reselect the view logs action from the menu. Do this until it says end of logs.
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Re: G1000/GFC700 AFCS Faults in Turbulence

Post by Boatguy »

I had bone jarring turbulence on an approach to KSAF two years ago and slowed to Va, but the GFC700 never disengaged.

Max Trescott recommends using HDG/ROL mode rather than NAV/ALT in strong turbulence. The AP workload is reduced and the plane stays under control but may deviate altitude and course a bit. If way off, a heading adjustment or VS can bring it back before reverting to HDG/ROL.

This can be communicated to ATC as either a request for a deviation, or as a PIREP and "I can't do any better...".
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Re: G1000/GFC700 AFCS Faults in Turbulence

Post by jwx96 »

I don’t have the GFC700 AP but I think any AP can disconnect in turbulence. I heard Martha King say that in a course I watched last week. I have the KAP 140 autopilot and if I try to use HDG or NAV and ALT in moderate to severe turbulence the AP will disconnect. If I use ROL and VS it’s less work for the AP and usually I don’t have a problem. If my course or altitude change too much I can use CWS to get back where I need to be. I’ve flown the GFC700 in the past and I think these options are available but I’m not sure. I found out last week that if I turn off the automatic squelch (Garmin 530W) I can hear the servos in my headset. They work pretty hard to maintain an altitude and track a course.
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Re: G1000/GFC700 AFCS Faults in Turbulence

Post by alanhawse »

In hindsight this must of been what happened to me on Friday

I was bouncing around in the late afternoon thermals... and the AP clicked off with a warning. I was surprised and didnt understand what had happened. So, I pressed the "AP" button and turned it back on. Things were fine.

I chalked this up to accidentally pressing the red ap button on the stick and went on

But... I think that it must have turned off... I was rolling along at about 125... so kinda near the yellow...

That being said I didnt have to do anything described above to get it going again. So maybe not?

I suppose I was probably going too fast given the bumps? Maybe I should have been closer/below to Va.

The plane is brand new with new G1000NXI/GFC700 ... so I wonder if the firmware has been improved? Or maybe the reset depends on the severity of the bumps?

Alan
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Re: G1000/GFC700 AFCS Faults in Turbulence

Post by Don »

Having owned four factory new Stars, the biggest improved change we noticed was the difference between the KAP 140 and the GFC700 AP's. The servo motors for the GFC700 are much faster and can keep up with moderate to high turbulence where as our KAP 140's would disconnect in turbulence where the servo motors could not make corrections fast enough. I have never had the GFC700 disconnect in turbulence, with the exception of having accidently hitting the red disconnect button with my hand when getting jostled around in heavy turbulence.
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dmloftus
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Re: G1000/GFC700 AFCS Faults in Turbulence

Post by dmloftus »

alanhawse wrote: Sun May 15, 2022 3:27 pm I chalked this up to accidentally pressing the red ap button on the stick and went on
Yes mine was definitely disconnecting on it's own, and the only way I could think to clear the AFCS error was to pull and push the breaker. They say instructors are not supposed to pull breakers on a G1000 to simulate failures. I wonder if there is any potential for damage doing this. My annual is in a few weeks but not sure how they would test for this failure in the shop. Not looking forward to the 2000 miles home.

BTW, Alan I think we know each other from Cypress
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Re: G1000/GFC700 AFCS Faults in Turbulence

Post by alanhawse »

David Loftus... Yes... old Cypress friends... Ill send you a private note.

I can imagine reasons where pulling the fuse is a bad idea.. .and I can imagine how it is protected. But yes the Garmin people say dont do that.

I have not had it happen again... so Im not at all sure that my problem and yours is the same. In my case the AP turned back on when I pressed the AP button... so sounds different?

Alan
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Ed McDonald
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Re: G1000/GFC700 AFCS Faults in Turbulence

Post by Ed McDonald »

Don wrote: Sun May 15, 2022 4:14 pm Having owned four factory new Stars, the biggest improved change we noticed was the difference between the KAP 140 and the GFC700 AP's. The servo motors for the GFC700 are much faster and can keep up with moderate to high turbulence where as our KAP 140's would disconnect in turbulence where the servo motors could not make corrections fast enough. I have never had the GFC700 disconnect in turbulence, with the exception of having accidently hitting the red disconnect button with my hand when getting jostled around in heavy turbulence.
My first observation is that the G1000/GFC700 AFCS is amazing! Having said that, however, turbulence penetration is not one of the strong suits of this avionics package.
Having looking through the Garmin manual there is nothing in there about the AFCS and turbulence; there is lots about turbulence detection and the radar, SiriusXM but nothing about how to fly with the AFCS through turbulence. Similarly, the DA62 AFM is also silent about flying in turbulence with the oblique reference to Vo, Operating Maneuvering Speed. That is the speed you do not want to exceed when in turbulence.
Most aircraft manufacturers of big aircraft anyways, provide a recommended turbulence air penetration speed howver this is not required for aircraft under 12,500 pounds. The following article is a pretty good summary of the situation for small aircraft: https://airfactsjournal.com/2020/12/und ... ion-speed/.
In essence, one wants to fly a speed that is neither too fast (such that Vo is exceeded) or too slow where a decrease in airspeed gets you into a stall. Fortunately, with the DA62 (I can't speak for the others), that is a pretty wide band of airspeed. In the big jet world, depending upon weight, ISA and altitude, this band can be as low as 10 kts, although one would be a fool to be at an altitude with such a narrow band, often known as coffin corner (where the upper speed limit is not the structural but the high speed stall, i.e. mach buffet).
I only have 60 or so hours on the DA62 and have some turbulence experience with it now and, for what it is worth, here are my observations and techniques:
1. Slow down to give some space between the IAS and the Vo. A typical Vo for the DA62 is around 135Kts; slowing to 125 or less still provides plenty of buffer above the stall speed of about 70 kts.
2. In my short experience in turbulence with the DA62, I find the the autopilot works too hard to maintain altitude with the altitude capture active. The nose goes up to about 8 degrees in sinking air and level of a few degrees below the horizon in ascending air. I am experimenting with PIT mode only and/or setting an altitude capture different than my planned altitude (requesting a block of airspace +/- 1000 ft from the nominal cruise altitude if IFR) or just letting it go +/- 1000 ft VFR.
3. Chasing the airspeed is not recommended. Set a cruise attitude and power setting and monitor. What goes up usually comes down and vice versa.
4. I have been in moderate turbulence and perhaps a couple of jolts of severe so far (banging my head on the ceiling) and the autopilot has never disconnected. Pretty remarkable.

Modern, fly-by-wire airliners (Boeing 787 and 777), Airbus's A320 and afterwards, C-Series, etc integrate the inertial sensors with the flight control system to modulate control inputs not to make turbulence worse but rather to soften turbulence. Since the AFCS of the Garmin 1000/GFC700 is a digital system with accelerometers too, this would be a nice upgrade by Garmin to introduce a turbulence mode or at least a soft altitude hold mode into the AFCS.
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Re: G1000/GFC700 AFCS Faults in Turbulence

Post by Lou »

The only autopilot error I have experienced was is severe cold air (-25C or colder). It lasted the whole flight but never returned.

I have flown through some rough air and it has always held. Now I often disconnect the AP in rough convective air to let the plane move a little more.
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