LVL CHG??

[martinlest]

Not too many visitors here these days, but I'll give it a go :-) ...

Is there a way to use the LVL CHG option in XP11? If I engage it to descend from cruise altitude, the aircraft more or less falls from the sky - it is descending at 6000ft/min (or more: that's as high as the gauge shows) within five seconds. Even if I reduce the SPD value first, the descent rate is basically 'plummet'; and of course, overspeed quickly follows. I always have to resort to engaging and setting V/S to descend. 

Thanks.

 

Edited April 30 by martinlest

[Litjan]

You are doing something wrong.

[Aaron737]

Hi Jan, 

I don't have XP12 since my computer can't handle it but I have always enjoyed flying your -300 in XP11. I recently got employed by a -300 operator so I now have the pleasure of flying the real thing! Beautiful plane to fly, I'm sure you have fond memories. 

To be fair to the original poster, I too had the same issue with LVL CHG in XP11. It would aggressively trim nose down to 6000fpm+ when commanded from cruise altitude -  Exactly how they are describing it. I doubt it is user error since I have the same issue.
I've noticed in the real bird LVL CHG can be a little sluggish at times but nothing to this extent. 

Might be worth looking into? Maybe it's a plugin conflict or something?

Just thought I would give my two cents. 

Cheers

[Cameron]

2 hours ago, Aaron737 said:

Hi Jan, 

I don't have XP12 since my computer can't handle it but I have always enjoyed flying your -300 in XP11. I recently got employed by a -300 operator so I now have the pleasure of flying the real thing! Beautiful plane to fly, I'm sure you have fond memories. 

To be fair to the original poster, I too had the same issue with LVL CHG in XP11. It would aggressively trim nose down to 6000fpm+ when commanded from cruise altitude -  Exactly how they are describing it. I doubt it is user error since I have the same issue.
I've noticed in the real bird LVL CHG can be a little sluggish at times but nothing to this extent. 

Might be worth looking into? Maybe it's a plugin conflict or something?

Just thought I would give my two cents. 

Cheers

Can you provide a video of this since you've decided to join the topic? We haven't seen or had this widely reported before.

[Litjan]

What I believe happens is that people engage the LVL change while in MACH mode. This will make the speed control chase the set Mach number, and since the indicated speed rises rapidly for the same mach number in a descent (as temperature gets warmer and the speed of sound increases as a consequence), the plane will pitch down rapidly to achieve the necessary acceleration. If you disregard the automatic reversion to indicated speed (by setting the MACH mode again after it reverts to SPEED in the descent, for example), you can actually overspeed the aircraft easily, as at some point the set Mach number will exceed the VMO of the aircraft.

Descent speeds in excess of 5000 feet per minute in LVL CHG while in Mach mode is something I have observed regularly in the real aircraft, especially when paired with a decrease in headwind in the descent.

PS: Many pilots did not use FL CHG for the initial part of the descent for this very reason, rather using V/S to initiate a shallow(er) descent until the speed reference could be switched over to IAS and the FL CHG would work more stable.

Edited July 5 by Litjan

[Aaron737]

On 7/4/2024 at 8:37 PM, Cameron said:

Can you provide a video of this since you've decided to join the topic? We haven't seen or had this widely reported before.

I will take a look in the archives. I believe I have one from a while back. 

 

On 7/4/2024 at 10:11 PM, Litjan said:

What I believe happens is that people engage the LVL change while in MACH mode. This will make the speed control chase the set Mach number, and since the indicated speed rises rapidly for the same mach number in a descent (as temperature gets warmer and the speed of sound increases as a consequence), the plane will pitch down rapidly to achieve the necessary acceleration. If you disregard the automatic reversion to indicated speed (by setting the MACH mode again after it reverts to SPEED in the descent, for example), you can actually overspeed the aircraft easily, as at some point the set Mach number will exceed the VMO of the aircraft.

Descent speeds in excess of 5000 feet per minute in LVL CHG while in Mach mode is something I have observed regularly in the real aircraft, especially when paired with a decrease in headwind in the descent.

PS: Many pilots did not use FL CHG for the initial part of the descent for this very reason, rather using V/S to initiate a shallow(er) descent until the speed reference could be switched over to IAS and the FL CHG would work more stable.

I totally agree. We rarely use LVL change from cruise though the few times we have done the aircraft handles it quite well. We actually used it recently from cruise to catch our path from a late descent clearance in which we also reached a 5000-6000 fpm rate albeit the AP did this gradually with no more than -5 degress nose down. What I think the OP is experiencing and the same thing I have seen is an aggressive pitch down to 6000+ fpm in a matter of seconds with the speed blowing past the target speed.
I'll try find my video.

[Litjan]

It could well be that the autopilot of the real 737-300 has some additional gain parameters that limit certain pitch rates in certain speed regimes or something like that.

I just want to be honest with you to let you know that we are probably not going to spend considerable time implementing and testing such further parameters to a custom autopilot that already works much better than the default X-Plane autopilot (in my opinion) - especially given the fairly small discrepancy from real behavior which also can be mitigated with an easy selection of alternate modes or reverting to a continuous speed (vs Mach) descent, if so desired.