Cessna 408 Sky Courier : FLC Overshoot

Bug Reporting Hub Aircraft & Systems
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ISSUE DESCRIPTION

Description of the issue: When in FLC mode, the AP significantly overshoots the set altitude by as much as 3000’. This is consistent across all flights and in both Free Flight and Career Modes. I believe this may be related to the G1000, as I have seen it happen in the C208 as well.

If applicable, which aircraft is experiencing this issue:
C408, C208

[PC Only] Did you remove all your community mods/add-ons? If yes, are you still experiencing the issue?

Yes

FREQUENCY OF ISSUE

How often does this occur for you (Example: Just once, every time on sim load, intermittently)?

Every flight

REPRODUCTION STEPS

Please list clear steps you took in order to help our test team reproduce the same issue:

  1. Set Altitude in AP.
  2. Turn on FLC
  3. Set FLC speed to just above Vy
  4. Climb using FLC
  5. Watch as aircraft does not slow its climb until well after passing the desired altitude.

YOUR SETTINGS

If the issue still occurs with no mods and add-ons, please continue to report your issue. If not, please move this post to the User Support Hub.

What peripherals are you using, if relevant:

Not relevant

[PC Only] Are you using Developer Mode or have you made any changes to it?

No changes made.

[PC, MSFS 2020 Only] Are you using DX11 or DX12?

DX12

[PC Only] What GPU (Graphics Card) do you use?

Nvidia GeForce RTX 4060

[END OF FIRST USER REPORT]

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Do you have the same issue if you follow the OP’s steps to reproduce it?
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Provide extra information to complete the original description of the issue:
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1 Like
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Helllooo :wave:

May I ask if you have the issue with the TBM930 ? I’ve just flown it 100% like you stated, but had no issue - hence I don’t think it’s linked to the G1000. Problem is that I never encountered this issue either with the C208, so if you’re okay, let’s make a cross try, you try on the TBM, I try on the C208 ? :smiley:

Cheers,
K.

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I’ll try it now in the TBM and see what happens…

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Ok, set the altitude for 10000. Started slowing climb at 9920 and settled out at about 10015. So the TBM does not exhibit it.

1 Like
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208 - started to slow its climb at 9950, got up to 10050, then settled back down to 10000

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408 - Started slowing climb at 9800, leveled off at 10050, then began climbing again and got up to 12350 before it leveled off again. It sat at that flight level for about a minute before it started to trim down and start descending back to 10000. It then descended at about 250-350 fpm until it got to 10000 again.

So, your surmising that it was not the G1000 is likely correct, and it is probably something in the way the FLC in the 408 is managing the trim.

1 Like
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Thanks for the tests :wink:

I did try the C208b and like you found no issue.
But the test on C408 did not go as well.

  • starting from the runway
  • flc to 10.000ft at 105kt
  • the plane climbs (a bit faster, at 110kt) to 10.000ft

And this is where the issue lies : after stabilizing at 10.000ft, the C408 will accelerate but not maintain the 10.000ft, and it climbs up. It’s like it overshoot the tolerance of leveling (something like 200ft) and then decide “it will keep the 10.000ft the next time it crosses it”. But at the 13.000ft mark, the plane stabilize at if “going too far from 10.000ft is too much”.

I think this is the same bug that has been reported here by @PreachingPigeon. The issue is that on this bug report, the OP stated that for them, they had no issue “Free Flight”, which we do.

I upvoted both bug reports and I’ll comment back in the mentionned bug so that they are aware that we have an issue in free flight.

@lsoderman, thanks for sharing this !

:warning: May I suggest a title update to reflect the current status, for example “Cessna 408 Sky Courrier : FLC Ovbershoot in Free Flight” ?

CHeers,
K.

1 Like
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Best work-around for the problem I found is to both slow the rate of climb and accelerate the aircraft by commanding a FLC speed of ~150 kn around 1000 ft before reaching the desired altitude. With that the level off usually works correctly.

Even after many, many flights in the 408 I’m still not sure what the problem really is. (Slow) Trim seems to be part of the problem, but it also appears there’s somethiing ‘deeper’ that breaks in the transition from FLC to ALTS.
For example, when you initially climb in VS, the mode works as expected and you’ll usually not find a deviation between the commanded and the flown rate of altitude change of more than 50 fpm. But if you climb in FLC, encounter the discussed issue with the botched altitude hold, and then try to bring the plane back down to the desired altitude in VS mode, you’ll suddenly find a deviation of up to 1000 fpm. So commanding a descent at 500 fpm will make the plane climb at ~400 fpm instead, commanding -1200 fpm will see you descend at only ~400 fpm and so on.

What’s also odd is that when you climb in FLC into your desired altitude at a relatively low rate, the moment mode changes from FLC to ALTS the autopilot commands a quite agressiv pitch up, increasing the rate of climb to up to 1200 fpm just 150 ft before level off, only to then pitch down again just as agressively at the desired altitude, usually overshooting it a bit by up to ~60 ft.

So whether directly connected to this bug or not, the G1000 autopilot definitely has problems of its own that at the very least contribute to some degree. (It’s the same in lateral naviagtion, where the autopilot will turn extremely agressively into GPS or LOC paths without any necessity, somtimes at almost perpendicular angles.)

2 Likes
9

This is an even better description of what I’m seeing. In watching it closely, it also see the pitch up after crossing the set altitude, and the VS being much different than what is set when you try to descend. I also see the VS issue when descending to meet a VPTH

2 Likes
10

I get the problem in Career as well. One issue that is consistently in Free Flight is that the 408 won’t respond to the barometer set keybind, except to switch between STD and BARO. But I’ll need to research that one a little further.

1 Like
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After a number of tests and flights, the problem seems to be related to trim and elevator logic with the AP. As the airplane approaches the set altitude, it starts to slow the climb by changing the trim. But it does not react fast enough. So as the plane hits, and then exceeds the altitude, tghe plane has started to level out, increasing airspeed as it is not climbing as steep. One would expect, at a minimum, a trim adjustment related to the airspeed and rate of climb. But the trim responds slowly or not at all, so as the aircraft increases speed, it also begins climbing again.

It then continues to climb and eventually, when airspeed starts to even out, starts lowering the trim, but very slowly. In most cases, it can be 5-15 secs (sometimes more) for just 1% of trim change. by the time you reach 0% trim, you;ve overshot the altitude by anywhere from 2k-5k ft.

It then continues to trim down at the same slow rate, eventually getting to about -25% trim and a usable rate of descent back to the desired altitude.

I have found this both in FLC and VS modes. In VS modes, you can set a descent speed, but it may take minutes for it to heit that descent speed because the trim responds so slowly. And the reason I mention elevators is that it seems it is climbing or descending primarily due to trim and not elevators, as if the AP is not changing elevators at all and only adjusting trim. So it appears there is a lack of connection between the AP and the elevators. If this were not the cas3e, you would start descending much more quickly than waiting for the trim to make the change.

Hope this helps to further identify the cause.

1 Like
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Another data point. I am currently flying the 408 straight and level. The below is where the trim is in level flight:

image
image691×442 19.1 KB

This does not seem remotely correct.

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At least in SU1 Beta, I can tell you that VS is much better and I use it regularly to climb and descend. I’ve had problems in the past with FLC just like you, but I haven’t tried it lately with the SU1 Beta. I’ll have to try FLC to see whether that is working better in SU1 Beta.

2 Likes
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I’m in the beta, and have been exclusively flying the 408. I get this issue every single time I fly it. I’ve gotten used to it, but my altitude graph always has a huge peak at the beginning where it overshoots. And for me it happens with either VS or FLC. Interestingly, my last flight I watched the trim while it descended on VNAV, and the trim responded much more quickly. So there’s a disconnect somewhere, I just don’t know where yet.

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As far as others have documented, there have been no changes made to the 408 in the beta at all so far.
(Which actually has a good side, in that the SkyCourier doesn’t suffer from other problems introduced with the beta.)

Personally, I have made a habit of climbing in FLC at 150 kt unless obstacles require a quicker ascent.
At that speed the level off always works perfectly fine, climb rate is still decent enough, and if you look at flight data of real 408s it actually seems to be a quite common speed for the climb to cruise (which in turn is usually only in the range of 8000 to 10000 feet due to the unpressurized cabin).

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I also find C208 and C408 wobbling and overshooting. Maybe the developers do not use the PID-mathematics correctly on course and altitude changes. A good understanding of what is going on can be read here:

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Are you setting your trim before takeoff because it does not start out in the takeoff range. Try adjusting your trim to between 0 and -0.4.