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CRJ AP Altitude Hold at Low Speed/Power Settings


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I've been experimenting with the CRJ 900 at slow speeds recently, and it doesn't seem like the autopilot has enough pitch authority to maintain altitude. In the real aircraft, the autopilot will continue increasing pitch in order to hold altitude until the stick shaker activates. Once the shaker activates, the autopilot will disconnect automatically and the cavalry charge aural will sound until one of the pilots selects AP/SP DISC on the control wheel. In the MSFS CRJ, the aircraft begins a slow descent almost immediately upon power reduction. Speed continues to decrease until reaching ~150 KIAS and then stabilizes with the aircraft descending somewhere near 3,000 fpm, but never reaching the low speed cue (red/black checkerboard) and stick shaker.

 

Video of the behavior beginning from M0.77 and 37,000 ft around 75,000 lbs:

 

 

 

An oddity that suggests this might also be related to thrust occurs at 1:10. The thrust levers are increased and the aircraft stops its descent and begins a slow climb at 200 KIAS.

 

Steps to Reproduce:

1. Engage ALT or ALTS on FMA and allow VS and airspeed to stabilize

2. Reduce thrust to idle

3. Observe decreasing altitude and descent rate while airspeed stabilizes around 140-160 KIAS

4. Observe aircraft does not follow flight director above ~7.5 degrees pitch nose up

 

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  • Aerosoft

This kind of behavior is known to us but it's highly unusual to fly an aircraft at such slow speeds in normal operations without extending some flaps. We will see if we can improve the altitude hold further at some point but right now it isn't the highest issue on our priority list.

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  • 2 weeks later...
On 5/3/2022 at 11:48 AM, Secondator said:

This kind of behavior is known to us but it's highly unusual to fly an aircraft at such slow speeds in normal operations without extending some flaps. We will see if we can improve the altitude hold further at some point but right now it isn't the highest issue on our priority list.

(Read: this will never be addressed)

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10 hours ago, Richard Dastardly said:

How's the real jet at slow speed on AP, if anyone's actually got it in that state?

 

As long as the AP is activated, the real aircraft would never start a descent or climb due to airspeed changes. It would increase/decrease pitch to maintain altitude. At low airspeeds this would go as far as stick shaker onset. The same is true for climb with V/S or PITCH mode.

 

When the aircraft approaches a high angle of attack, the stall protection computer will:

  • warn the crew of an impeding stall through the stick shaker,
  • activate the engine auto-ignition system and
  • disengage the AP

 

On 5/3/2022 at 3:48 PM, Secondator said:

it's highly unusual to fly an aircraft at such slow speeds in normal operations

 

Agreed. But overall, the AP/FD is relatively sluggish and inaccurate both in lateral and vertical modes.

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Quote

As long as the AP is activated, the real aircraft would never start a descent or climb due to airspeed changes. It would increase/decrease pitch to maintain altitude. At low airspeeds this would go as far as stick shaker onset. The same is true for climb with V/S or PITCH mode.

 

When the aircraft approaches a high angle of attack, the stall protection computer will:

  • warn the crew of an impeding stall through the stick shaker,
  • activate the engine auto-ignition system and
  • disengage the AP

This is accurate (and verbatim from the FCOM no less).

 

 

On 5/17/2022 at 6:03 PM, Richard Dastardly said:

That sort of behaviour turns up in a lot of aircraft - whether it's an asobo AP problem or just a lot of people not tuning the AP right I don't know.

 

How's the real jet at slow speed on AP, if anyone's actually got it in that state?

We train to this specific configuration and scenario during our Extended Envelope Training in the CRJ.

 

 

On 5/16/2022 at 11:26 AM, jstnj said:

(Read: this will never be addressed)

 

I believe Aerosoft will address this eventually. They have higher priorities with the aircraft at the moment and that's ok. The majority of end users aren't testing the flight model and AP at the edges of the aircraft's flight envelope. I primarily brought this forward because I felt it might hold some additional information regarding other autopilot issues users have been experiencing lately.

 

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Assuming you mean the real aircraft (I haven't tested this with turns in the sim), yes it's much the same. The autopilot does a surprisingly nice job of holding altitude at high AOA.

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  • 1 month later...

As of update the A/C will do it's best to hold attitude down to stall speed, but then it starts sinking instead of disconnecting the AP. Add power & it'll overshoot the selected altitude before settling back down again - the trim runs awfully slowly. It is a little improved, i think - certainly couldn't get down to 120-ish kts on the AP ( in a -900 ) before it started dropping, previously.

 

Edit: I take that back, it's sinking at 140 kts again.

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