Hello!

During descent, the 757-200 blew through an FMC speed restriction at 10,000 feet. Decent up to this point was normal -- VNAV engaged, 260 knots from 31,000 down to 10,000 feet as planned. Passing through 10,000 feet, VNAV pushes the airplane to 426 knots despite having the SPD Trans at 240/10000.

Here's a video of it. I started recording as soon as I heard the engines spool up. I intervened at the 0:14 mark.
https://youtu.be/dG4zh57nlkk

Video of the same flight, but at the top of descent: https://youtu.be/9csYRFQyGl8

Details:
KTPA to KATL
P3D 4.4
757-200 PW2037
version 1.303

Is it any wonder, when you have selected a V/S speed of initially -800 fpm descent then all of a sudden you increase it to -2600fpm descent? Naturally the IAS will shoot up because the aircraft is pithing down at ridiculous  ::)

Colvale wrote on Mar 1^st, 2019 at 8:57am:

Is it any wonder, when you have selected a V/S speed of initially -800 fpm descent then all of a sudden you increase it to -2600fpm descent? Naturally the IAS will shoot up because the aircraft is pithing down at ridiculous  ::)

I think you missed what I said...I intervened at the 0:14 mark. I turned VNAV off, which switches V/S mode on automatically. Whatever the current decent rate is at that moment is selected as the V/S fpm.

At 0:48, I attempt to re-engage VNAV but it again blows through the speed restrictions.

At 0:57, I intervene again because it's pushing the throttles all the way forward. Turning off VNAV switches V/S on again.

On my next flight, I'll get a video of it's attempts to push through the sound barrier with VNAV on (without my intervention), if that's what you'd like.

Mission.Control wrote on Mar 1^st, 2019 at 3:10pm:

Colvale wrote on Mar 1st, 2019 at 8:57am: Is it any wonder, when you have selected a V/S speed of initially -800 fpm descent then all of a sudden you increase it to -2600fpm descent? Naturally the IAS will shoot up because the aircraft is pithing down at ridiculous  ::) I think you missed what I said...I intervened at the 0:14 mark. I turned VNAV off, which switches V/S mode on automatically. Whatever the current decent rate is at that moment is selected as the V/S fpm. At 0:48, I attempt to re-engage VNAV but it again blows through the speed restrictions. At 0:57, I intervene again because it's pushing the throttles all the way forward. Turning off VNAV switches V/S on again. On my next flight, I'll get a video of it's attempts to push through the sound barrier with VNAV on (without my intervention), if that's what you'd like.

No need for a video.

Let me try and get my head around this. Why did you intervene and switch off VNAV in the first place? Did you not trust it to bring your aircraft to the FL you wanted? Yes, I am aware that it switches to V/S, but even so, that rate can be set by the pilot.

I have never had issues selecting the SPD I want or letting VNAV descend to what I set as my ALT.

Kindly give me your routing and I will try this scenario out to see if it manifests itself.

Colvale wrote on Mar 1^st, 2019 at 3:55pm:

Mission.Control wrote on Mar 1st, 2019 at 3:10pm: Colvale wrote on Mar 1st, 2019 at 8:57am: Is it any wonder, when you have selected a V/S speed of initially -800 fpm descent then all of a sudden you increase it to -2600fpm descent? Naturally the IAS will shoot up because the aircraft is pithing down at ridiculous  ::) I think you missed what I said...I intervened at the 0:14 mark. I turned VNAV off, which switches V/S mode on automatically. Whatever the current decent rate is at that moment is selected as the V/S fpm. At 0:48, I attempt to re-engage VNAV but it again blows through the speed restrictions. At 0:57, I intervene again because it's pushing the throttles all the way forward. Turning off VNAV switches V/S on again. On my next flight, I'll get a video of it's attempts to push through the sound barrier with VNAV on (without my intervention), if that's what you'd like. No need for a video. Let me try and get my head around this. Why did you intervene and switch off VNAV in the first place? Did you not trust it to bring your aircraft to the FL you wanted? Yes, I am aware that it switches to V/S, but even so, that rate can be set by the pilot. I have never had issues selecting the SPD I want or letting VNAV descend to what I set as my ALT. Kindly give me your routing and I will try this scenario out to see if it manifests itself.

Again....I intervened because it's pushing way past the speed restrictions. You can very clearly see the throttles pushing themselves all the way forward and the speed increasing past 300 knots when under 10000 feet with VNAV engaged.


Quote:

Did you not trust it to bring your aircraft to the FL you wanted?

Nope, not after that. Another user on a different forum suggested that this was a bug that CS is aware of so I'll leave it to them to solve.

Fair dos.

I'm just on a long flight now, to try and see if I can replicate anything that you seem to have encountered.

Completed a flight in the 757-300 pax, and not a single issue dialling-in the new ALT on descent into the MCP; dialling-in the SPD intervention ,and using VNAV to descend

I can replicate this issue when I change the target speed during the VNAV descend in the FMC.

If ATC demands a different speed and I enter it in the VNAV DES page as the new target speed then this issue happens shortly before passing FL100.

This is a 100% reliable way for me to replicate this issue.

Regardless of whether it is a mistake or not, specifications from ATC are usually regulated by the MCP, ie a speed specification. A change in the entries on the DES page are rare there. For proper descent planning, the inputs are important there to create the descent profile and determine TOD, also subject to restrictions. In the later course of the descent then defaults are set at the MCP, these will predominantly occur only in the last part of the descent for better staggering. VNAV uses fixed restrictions, which i.a. can also be seen on the LEGS page. When connecting VNAV I see an immensely high target speed in magenta on top of the speed tape. What does your LEGS page on this part of the route indicate? You also stay in VNAV PTH mode every time. Speed ​​may be exceeded here, but path is the determining factor.