VNAV speed management/speed constraints

[ahinterl 1]

Sorry if this was answered before already, my searches didn't yield the desired results.Something I came across those long years of simming is speed constraints during VNAV descents on add-on planes like LDS 767, PMDG 737NGX, iFly 73NG, Wilco E-Jets v2 and probbly many others s well.Because the PMDG MD-11 is still my favorite plane that I now fly for a long time already, I'm accustomed to the advanced automatism that this aircraft provides. This includes the capability to meed altitude and/or speed constraints along the lateral flight path almost 100% each time, as well as the ability to nicely manage deceleration during descent.When I recently "reverted" back to the PMDG 737NGX (sorry, but regardless of all the eye candy and the level of realism it doesn't beat my MD-11), I wondered why this plane seems to disregard speed constraints during descent (I have the assumption that the target speed becomes the constraint speed after passing the constraint) completely. My LDS 767 and the iFly plane do the same, as does the Wilco E-Jets v2, so I suppose there's a "logic" behind this.I must admit that I never made a descent fully on VNAV only in the past on my planes. I use this feature since I have the MD-11. It's so intuitive and understandable that at first, I was kind of shocked when I saw my 737NGS cross a 200/4000 constraint with 240kts.I guess in real life it wouldn't make a difference whether a plane could do VNAV speed management like the MD-11 or not since it would be under ATC control most of the time where the VNAV path would be violated anyway. But, I'd like to know whether the optional entries for speed constraints e.g. on the LEGS pages of the Boeings are "just a reminder" but have no real effect on the calculation of the descent path.

[Guest bstolle]

Haven't installed any of these planes right now, so I don't know if they really disregard these constraints but the real ones certainly don't.1. This includes the capability to meed altitude and/or speed constraints along the lateral flight path almost 100% each time, as well as the ability to nicely manage deceleration during descent.yes2. I wondered why this plane seems to disregard speed constraints during descentThey shouldn't3. I have the assumption that the target speed becomes the constraint speed after passing the constraintThe speed is met at the constraint IRL4. I'd like to know whether the optional entries for speed constraints e.g. on the LEGS pages of the Boeings are "just a reminder" but have no real effect on the calculation of the descent pathNo. they are no reminders. At the constraint the plane will be at that speed and altitude. During the descent e.g. the 767 deviates + 15kts from it's target speed to maintain the VNAV path at idle if needed.If the 15kts speed increase aren't enough, the FMC message 'drag required' appears.PS: Da ich ja auch in Wien wohne, können wir uns ja mal treffen wenn Du willst..LGBernt

[idahosurge 2]

Sounds like a question you should ask in the PMGD NGX support forum and since they made both the MD11 and the 737NGX they should be able to give a good answer.

[ubersu 78]

Sorry if this was answered before already, my searches didn't yield the desired results.Something I came across those long years of simming is speed constraints during VNAV descents on add-on planes like LDS 767, PMDG 737NGX, iFly 73NG, Wilco E-Jets v2 and probbly many others s well.Because the PMDG MD-11 is still my favorite plane that I now fly for a long time already, I'm accustomed to the advanced automatism that this aircraft provides. This includes the capability to meed altitude and/or speed constraints along the lateral flight path almost 100% each time, as well as the ability to nicely manage deceleration during descent.When I recently "reverted" back to the PMDG 737NGX (sorry, but regardless of all the eye candy and the level of realism it doesn't beat my MD-11), I wondered why this plane seems to disregard speed constraints during descent (I have the assumption that the target speed becomes the constraint speed after passing the constraint) completely. My LDS 767 and the iFly plane do the same, as does the Wilco E-Jets v2, so I suppose there's a "logic" behind this.I must admit that I never made a descent fully on VNAV only in the past on my planes. I use this feature since I have the MD-11. It's so intuitive and understandable that at first, I was kind of shocked when I saw my 737NGS cross a 200/4000 constraint with 240kts.I guess in real life it wouldn't make a difference whether a plane could do VNAV speed management like the MD-11 or not since it would be under ATC control most of the time where the VNAV path would be violated anyway. But, I'd like to know whether the optional entries for speed constraints e.g. on the LEGS pages of the Boeings are "just a reminder" but have no real effect on the calculation of the descent path.

It depends - constraints in small type are computed and may or may not be honored. Constraints in large or colored type should be honored; you will generally see a DECEL mark in the flightpath preceding the waypoint that marks the point where the aircraft will begin to slow to meet the constraint.DJ