FLC to avoid stalls

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gtmize
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FLC to avoid stalls

Post by gtmize »

On several occasions on climb out the AP attempts to hold climb .. the IAS has dropped and honked my horn.

I've never used the FLC on climbout but I'm thinking I should .. it that SOP for you?

On descent I've used VNAV into the IAF, but on the flat places in the approach I'm thinking I should be using the FLC also for the same reason.

Thanks
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Re: FLC to avoid stalls

Post by dgger »

Yes, using FLC on climbout is something I always do for the exact reason you decribed. I like to think it is indeed how most everyone operate. I did not have VNV in my aicraft (being a conversion), so I normally descend in VS mode to aid planning.
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Re: FLC to avoid stalls

Post by CFIDave »

Flight Level Change is intended for climbs. On a Lycoming DA40, set FLC to 90 knots for cruise climb, and then you won't have to worry about stalling the plane despite downdrafts or (in your area) mountain waves. If the plane gets too slow, the autopilot will automatically drop the nose to maintain airspeed.

Think of FLC as really Indicated Air Speed (IAS) mode, which is how the button is more accurately labeled on some Cirrus aircraft. The term Flight Level Change originated with airliners, although it's not really the same on GA planes because most jets have autothrottles to help control climb rate.

In contrast, when descending from cruise I tend to use Vertical Speed mode: 500 fpm if flying IFR, since that's the lowest rate of descent that will satisfy an ATC descent clearance, while allowing the aircraft to stay higher (where it's faster) longer. Obviously a higher rate of VS descent is used if I need to get down more quickly, and flying instrument approaches I'll typically use APR or VNV to control descent rate.
Last edited by CFIDave on Thu May 21, 2020 1:06 pm, edited 2 times in total.
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Re: FLC to avoid stalls

Post by AndrewM »

Completely agree. I always use FLC for climbing. Never, ever use VS.

Descents always use VS 500fpm.

I could imagine a situation if you are in hard IMC, task saturated and climbing out from say from a missed approach using VS that could become a potential issue.
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Re: FLC to avoid stalls

Post by smoss »

BE AWARE! There is a situation where FLC will still stall you, and it is very scary until it happens enough times for you to quickly recognize why it's happening. By design, it is programmed to not allow a negative climb (sink). So if you are happily climbing at say FLC of 90 KIAS, then hit a zone of marked downdraft, at some point the plane will stop climbing and begin to sink, at which point the flight director will pitch up, and continue to pitch up through stalling, until the negative climb is stopped. Obviously, the solution to this is to disconnect the autopilot and maintain your max climb speed as desired, even if still sinking. This first happened to me in a patch of moderate to heavy rain (VFR), pretty close to terrain, leaving Las Vegas. I was task busy, on autopilot, not noticing changing plane pitch, when stall horn went off and plane was sinking at something like 500 FPM or more. I truly thought something catastrophic happened, and remember looking out at the wings to make sure they were still connected. Disconnected autopilot, resumed normal pitch, at which point I was out of the downdraft and thoroughly confused. Continued flight, albeit very anxious and cautious--I though FLC was supposed to prevent stalls! Upon returning home, did some reading of the G1000 manuals and came to understand what happened. Since then, this scenario has happened to me at least another 2 times, but at least I was less confused.
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Re: FLC to avoid stalls

Post by ultraturtle »

FLC is also very useful when ATC asks to expedite descent, or you otherwise need to get down quickly (as in an Emergency Descent). Just Spin, Push, Spin, Pull:

Spin altitude to whatever lower altitude you need
Push FLC
Spin speed up to the max the autopilot/flight director system allows
Pull throttles - all the way to idle if necessary

The technique protects you from overspeeding and gets you down just about as quickly as possible.
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Re: FLC to avoid stalls

Post by jb642DA »

Good one Rob!!
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Re: FLC to avoid stalls

Post by CFIDave »

smoss wrote:
Thu May 21, 2020 3:37 pm
BE AWARE! There is a situation where FLC will still stall you, and it is very scary until it happens enough times for you to quickly recognize why it's happening. By design, it is programmed to not allow a negative climb (sink).
Can you find this documented anywhere? Because I've never read this, and it's contrary to my actual flight experience:

Back when I had a DA42-VI, I took off from Sun N Fun (Lakeland, FL) into 700 foot ceilings, being vectored (HDG mode) by Tampa Approach and directed to climb to 12,000 feet. We had FLC set to 100 knots IAS with the engines set to 92% max continuous power, which in a fully-loaded DA42-VI will normally generate about a 1000 fpm climb.

However while climbing in IMC on our way up to 12,000 feet, we got vectored by Tampa Approach into some strong convective cells that we couldn't see. At one point we hit a downdraft, and were descending (sinking) at 1500 fpm while the GFC700 autopilot was perfectly holding 100 knots IAS. So the plane was "climbing" at 1000 fpm within a column of air (cloud) that was itself descending at 2500 fpm, causing a net descent (sink) rate of 1500 fpm for our plane. At no point did the GFC700 kick off, nor did it try to pitch up to counter our strong 1500 fpm sink rate -- it dutifully kept us at 100 knots IAS without even much of a pitch change. All of this was observed on the PFD, since we couldn't see anything outside the DA42 canopy in IMC.

Of course a few seconds later we hit an equally strong updraft, causing the plane to climb at over 3000 fpm -- again with the GFC700 autopilot in FLC mode holding steady at 100 knots IAS. After this wild ride, a few minutes later we popped out into smooth air and bright sunshine at 12,000 feet.
Last edited by CFIDave on Thu May 21, 2020 5:57 pm, edited 2 times in total.
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Re: FLC to avoid stalls

Post by smoss »

From G1000 Pilot Guide, v22:

//Flight Level Change Mode (FLC)
NOTE: The Selected Altitude should be set before selecting Flight Level Change Mode.
Flight Level Change Mode is selected by pressing the FLC Key. This mode acquires and maintains the
Airspeed Reference while climbing or descending to the Selected Altitude (shown above the Altimeter). When
Flight Level Change Mode is active, the flight director continuously monitors Selected Altitude, airspeed, and
altitude.
The Airspeed Reference is set to the current airspeed upon mode activation. Flight Level Change Mode is
indicated by a green ‘FLC’ annunciation beside the Airspeed Reference in the AFCS Status Box. The Airspeed
Reference is also displayed directly above the Airspeed Indicator, along with a bug corresponding to the Airspeed
Reference along the tape.
Engine power must be adjusted to allow the autopilot to fly the aircraft at a pitch attitude corresponding
to the desired flight profile (climb or descent) while maintaining the Airspeed Reference. The flight director
maintains the current altitude until either engine power or the Airspeed Reference are adjusted and does not
allow the aircraft to climb or descend away from the Selected Altitude
.//

I actually called Garmin back then to confirm I was interpreting correctly and this was normal behavior. I suppose this may vary by software version.
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Re: FLC to avoid stalls

Post by CFIDave »

smoss wrote:
Thu May 21, 2020 5:26 pm
From G1000 Pilot Guide, v22:

//Flight Level Change Mode (FLC)
The flight director maintains the current altitude until either engine power or the Airspeed Reference are adjusted and does not allow the aircraft to climb or descend away from the Selected Altitude.//
This assumes the plane has already acquired the Selected Altitude. When in a CLIMB TO a Selected Altitude not yet reached, the plane does not level off prior to reaching that altitude to prevent sink; it will instead adjust pitch as needed to hold the Airspeed Reference IAS.
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