FLC to avoid stalls

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smoss
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Re: FLC to avoid stalls

Post by smoss »

Dave, you are misinterpreting that. Once selected altitude is reached, there would no longer be an "Airspeed Reference" as the climb mode would disengage and change to ALT mode.
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Re: FLC to avoid stalls

Post by CFIDave »

smoss wrote: Thu May 21, 2020 6:03 pm Dave, you are misinterpreting that. Once selected altitude is reached, there would no longer be an "Airspeed Reference" as the climb mode would disengage and change to ALT mode.
I was simply pointing out that the sentence you highlighted in red did not apply when the aircraft was already climbing (as in the example I gave when climbing through convective cells); FLC won't try to keep the plane from sinking.

The sentence you highlighted in red instead describes what happens if you're already cruising in level unaccelerated flight, and then engage FLC: the plane won't climb or descend until you either change power or change the Airspeed Reference. This is because the plane is already in an unaccelerated steady state, and the Airspeed Reference will automatically assume your current IAS at the time you push the FLC button. The result is nothing happens -- the plane won't climb or descend.
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Re: FLC to avoid stalls

Post by smoss »

I see what you are saying Dave, but I can tell you with certainty this is the behavior of my plane, my friend's DA40, and a rental one I used to fly. Garmin as well confirmed for me a few years back that it was normal behavior in FLC for the flight director to never allow the plane to go opposite direction of the selected altitude (so as well if in a descent and hit an updraft, would pitch down more to prevent climbing). I can see they benefits/pitfalls of programming it both ways. In my first exposure to this, considering my relative closeness to terrain, and my distraction with other tasks, I'm glad the programming was to pitch up more and maintain as much altitude as possible, with the impending stall a side-effect which was easily preventable.
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Re: FLC to avoid stalls

Post by CFIDave »

If the GFC700 autopilot in FLC mode will never allow the plane to go opposite direction of the selected altitude despite updrafts and downdrafts, how come this behavior is not documented anywhere by Garmin? FLC isn't like Underspeed Protection (for Diamond twins equipped with Electronic Stability & Protection -- ESP) that will behave differently in preventing a stall if the aircraft is in close proximity to the ground.

It's especially hard to believe that if you're in a FLC descent to a lower altitude and hit an updraft, that the nose will pitch down (and the plane will speed up) to prevent a temporary climb. I would instead expect the autopilot to hold attitude/pitch angle (even if the plane ends up gaining altitude) in order to maintain a constant airspeed. But this is a situation that should be easy to test:

Go flying at lower altitudes on a sunny day with thermal updrafts, set FLC and reduce power for a very gradual descent of about 50-100 fpm -- and see if the plane levels to avoid a climb and speeds up (abandoning the FLC set IAS ) when it hits an updraft. Let us know what you learn. :)
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Re: FLC to avoid stalls

Post by ultraturtle »

CFIDave wrote: Thu May 21, 2020 8:06 pmGo flying at lower altitudes on a sunny day with thermal updrafts, set FLC and reduce power for a very gradual descent of about 50-100 fpm -- and see if the plane levels to avoid a climb and speeds up (abandoning the FLC set IAS ) when it hits an updraft. Let us know what you learn. :)
You don't really need updrafts to check this out. Simply set a lower altitude at a reasonably slow speed - say your normal climb speed. Press FLC, retard the throttle(s) to start down, then apply full climb power. My bet is that you will climb away from the selected altitude, maintaining the selected speed.

Now try the same experiment starting by setting a higher altitude, press FLC, apply full climb power to stabilize in a climb, then retard the power to level lower than that required to maintain level flight. My bet is that you will descend away from the selected altitude, maintaining the selected speed.

Climbing at published best rate speed puts you pretty close to the stall warning in most Diamonds. It would not take much of a gust, or increase in g loading (turbulence, or pilot induced as in say, a turn) to set off the warning at that speed. Such a scenario might well take place in the sort of unstable air of a heavy rain storm over high altitude desert that Steve experienced.
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Re: FLC to avoid stalls

Post by smoss »

Flew the experiment as suggested by Rob, in a FLC climb, reduced power so unable to maintain climb, and exactly as it always has, as soon as climb went negative, plane pitched up to continue to maintain altitude, with airspeed continually slowing. I took a video, will try to post somewhere, but feel free to try it yourself.
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Re: FLC to avoid stalls

Post by Boatguy »

smoss wrote: Sun May 24, 2020 2:25 am Flew the experiment as suggested by Rob, in a FLC climb, reduced power so unable to maintain climb, and exactly as it always has, as soon as climb went negative, plane pitched up to continue to maintain altitude, with airspeed continually slowing. I took a video, will try to post somewhere, but feel free to try it yourself.
Steve - What software version is in your G1000?

What speed was FLC set to maintain?

The video would be very useful. I can give you a place to load it to so that we can share it. Let me know if you want a link to some storage.
Last edited by Boatguy on Sun May 24, 2020 4:16 am, edited 2 times in total.
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Re: FLC to avoid stalls

Post by Boatguy »

ultraturtle wrote: Fri May 22, 2020 4:26 pm Climbing at published best rate speed puts you pretty close to the stall warning in most Diamonds. It would not take much of a gust, or increase in g loading (turbulence, or pilot induced as in say, a turn) to set off the warning at that speed. Such a scenario might well take place in the sort of unstable air of a heavy rain storm over high altitude desert that Steve experienced.
When you say "best rate" I assume you mean Vx?

I transition to cruise climb speed before engaging the AP. In my plane that's about 24kts above stall speed and 1.4x Vs0.

Do other people use AP/FLC at Vx or Vy?
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Re: FLC to avoid stalls

Post by ultraturtle »

Boatguy wrote: Sun May 24, 2020 4:12 amWhen you say "best rate" I assume you mean Vx?

I transition to cruise climb speed before engaging the AP. In my plane that's about 24kts above stall speed and 1.4x Vs0.

Do other people use AP/FLC at Vx or Vy?
By best rate, I actually mean Vy, but it does not matter because for reasons of expediency, Diamond considers Vy and Vx to be the same for the DA62. Clean at heavy weights, Vy is 89 kts.

The GFC700 Autoflight Control System incorporates both the autopilot and the flight director systems, and provides the same guidance to both, so whether or not the autopilot is engaged really does not matter. If I were climbing out of Vegas confronted with multiple threats of high density altitude of a hot summer day, heavy rain, rapidly rising terrain, and the high density traffic in the area, I would very likely consider climbing as close as reasonable to Vx, with the autopilot engaged.

I can't speak for other aircraft, but on the DA62 in level, steady state flight, the stall warning comes on about 8 kts above buffet, and 10 kts above stall. The lowest speed the GFC700 will maintain for the DA62 is 90 kts - pretty close to Vx/Vy, clean. Heavy weight stall speed, wings level is 73 kts, and 78 kts in a 30 degree bank (1.15g). That means a stall warning around 83 kts at 1g and 88 kts at 1.15g. That's only 2 to 7 kts away from the 90 kts selected for FLC.

I always hand fly using FLC on initial climbout, and typically set 90 kts when taking off opposite in direction to my destination so I can climb to pattern altitude at the best angle/rate possible so I can turn on route as quickly as possible. It is not uncommon at all for me to get a quick stall warning horn if I overbank the turn to crosswind, or hit even a slight amount of turbulence.
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Re: FLC to avoid stalls

Post by smoss »

Russ,
System software is 321.22. Had FLC set to 100, alt select 1000 above, dropped power very slowly so the flight director could easily keep up with anything, and as soon as VSI dipped negative, plane began to slow as I expected to not allow further sink, IAS dropped to 90 and stayed there at steady power setting until I added power. I didn't want to take it all the way to stall as I was with my family.
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