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Climb performance over FL300


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This happened to me today in the CRJ700. Never had this problem in the CRJ550. I'm curious which one you were flying when you ran into the problem? Too bad I didn't turn on dev mode, and checked the value for visual icing when this happened. I'll follow the discussion here, and see if I can contribute later.

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7 minutes ago, Breezewind said:

This happened to me today in the CRJ700. Never had this problem in the CRJ550. I'm curious which one you were flying when you ran into the problem? Too bad I didn't turn on dev mode, and checked the value for visual icing when this happened. I'll follow the discussion here, and see if I can contribute later.

I'm pretty sure it was the 700 when I had the problem but probably will occur with both versions.

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I had this occur to me in the CRJ700 the other day.

First flight I had done in the CRJ above FL300. 

Got to about FL350 towards FL380 and then as described in this thread occurred. I had to disengage AP, nose down, re-trim and select a decrease in altitude to FL280 which is when the aircraft became stable again.

I'm pretty sure I had anti-icing on but not wing anti-ice.

 

Hopefully switching icing to 'visual only' will help. 

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I encountered this problem a couple of times already when climbing at a 250/290/0.74 profile, and best I can tell, it's ultimately an autopilot issue.

 

Details:

When selected mach is 0.74 it seems the autopilot does not follow this properly, and oftentimes continues pitching up even if mach speed is much lower than 0.74 already, all the way until stall if no manual intervention is done. 

 

I expect in speed mode that the plane should pitch for speed, right? So if I have mach 0.74 set, I expect the plane to pitch down as necessary to maintain mach 0.74. Instead, it keeps its pitch up, climbing at 2000+ft/min even if that means continuously going below the selected speed.

 

The workaround I use is to instead climb at a 250/320/0.77 profile, because at least even if AP can't follow selected speed to the letter, the speed it lands at is still high enough to continue the climb, and it wouldn't eventually lead to a stall. 

 

 

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My way for that bug for now: Turning on ''Live weather'' , take off, climbing to let's say FL250, switching weather to ''Clear sky'' and continue flight. When desending, the same procedure. For now, three flights without any issue. Now I'm going to check if using ''Unreal weather" engine solving that problem or not.

 

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10 hours ago, theclumsygeek said:

Details:

When selected mach is 0.74 it seems the autopilot does not follow this properly, and oftentimes continues pitching up even if mach speed is much lower than 0.74 already, all the way until stall if no manual intervention is done. 

I have never seen this happen in testing, and I sometimes fly the aircraft 10 hours per day in all kinds of weather and weight scenarios. Once switching from IAS to Mach mode around FL280-290 it occasionally drops a couple of knots below the speed bug, which is an area that needs some refinement, but I have never seen it spontaneously pitch way up while still in speed mode and start losing airspeed.

 

The only thing I can think of that might cause this is if some external controller is changing your pitch trim. Do you have a rotary trim wheel or any kind of axis assigned to the trim function by chance? Or an axis assigned to spoilers?

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Thanks for looking into it. I don't have any axis assigned to trim, but I do have an axis assigned to spoilers. It's fully retracted, though. Do you think that could be a factor? I also have a quite noisy yoke (I see it vibrating in the virtual cockpit the entire flight). It doesn't break autopilot for the most part, but maybe it's enough to cause those weird AP behaviors?

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Please delete the spoiler axis as this may interfere with the spoiler simulation. We are working on a solution to elominite the issue.

As for noise on the yoke or rudder axis. Again that may cause an issue in the control cycles. It is easy to widen and maybe also shift the center point so that the neutral position is not any longer giving input. Up to 5% do not disturbe the feeling when handflying the aircraft. 

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I think that did the trick!

 

I set a 5% deadzone on my pitch and roll axis, removing the shake on the yoke.

Did a flight with a 250/290/0.74 climb profile on close to max GW. Plane climbed beautifully. At selected mach 0.74 it was climbing at around mach 0.733, which was close enough.

 

I guess the micro movements from the yoke were disturbing the AP and causing those occasional pitch up movements eventually leading to too low speeds and therefore mediocre climb performance / stall.

 

I'll continue monitoring in my future flights and report if the issue comes back.

 

Thanks for the tip!

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On 4/3/2021 at 10:22 PM, Adam Kawecki said:

everything is ok until turning AP back, then immediately trim goes up again

Yes, a known issue that is well documented. There is a workaround so AP works again.

 

Personally, I can only trigger a stall by climbing higher than the current weight based on CG, and conditions allows. The highest I can get with any safety at max-weight in the 700 is FL370. With some care I can get to FL390. After that it gets easy to stall and certainly is not safe.

 

Have a look starting from:

 

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Update: Issue is unfortunately still not fixed. I still think it's related to the AP issues mentioned above.

 

Details:

Did another flight, and this time even with the yokes not shaking (5% deadzone applied), the AP does not follow the selected mach properly.

It's just a very minor deviation, like 1 degree higher than what the FD needs, but it is enough to go below the selected mach of 0.74, and left alone the gap increases as the plane loses climb performance on slower speeds.

 

Here's a screenshot showing selected mach of 0.74, but the AP happily keeping mach speed at 0.717 (and getting lower bit by bit). I had to increase selected mach to around 0.76 to force the AP to pitch down a bit.

243826580_MicrosoftFlightSimulator04_06_20211_28_08PM.thumb.png.fc89d216d8afa5f3c18eb72d6d6a95a0.png

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I assume you are pretty heavy and you just get borderline to the aircraft capacity. There the Mach SPD control is not working well anymore. If the aircraft cannot maintain 500 fpm at nach 0.74, you are at the limit of its climb capacity. The proper reaction is to change to VS and stabilize for a cruise. 

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TOW was 68940 (out of MAX TOW of 75000?).

What's weird was it was continuing to climb at 1700 fpm as in the screenshot, even if it was slowly going way below the selected mach. It could have pitched down a little and slowed down climb rate to catch up in mach speed.

 

Ok thanks for the tip. I will do that workaround in the meantime, until default autopilot improves and becomes more accurate.

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