[SOLVED] VNAV on Descent not following speed restrictions in FMC

[I9311I]

VNAV works great except on descent. It always defaults to 250IAS or less. I find myself manually setting the speed on descent for crossing restrictions frequently in the 280 range, then sometimes forgetting about it. Is that a bug or is it how it works in the real aircraft? 

Edited January 19, 2022 by I9311I
Solved

[leha74ru]

if there are no constrictions on the LEGS page in terms of speed, then it will decrease according to the default profile 0.8/250. You can add these speed/height limits on the LEGS page

[I9311I]

3 minutes ago, leha74ru said:

if there are no constrictions on the LEGS page in terms of speed, then it will decrease according to the default profile 0.8/250. You can add these speed/height limits on the LEGS page

That’s what I’m saying. The speed restrictions, higher than 250, will be loaded and showing correctly in the LEGS page, yet VNAV descent will still automatically go to 250IAS at TOD. 

[leha74ru]

up to 10000 the speed is higher than 250, it is generally in mach  

 

this is logical, you have two restrictions, one of the default profile is 250 and can withstand it because it is smaller

Edited January 18, 2022 by leha74ru

[Tom Stian]

53 minutes ago, I9311I said:

That’s what I’m saying. The speed restrictions, higher than 250, will be loaded and showing correctly in the LEGS page, yet VNAV descent will still automatically go to 250IAS at TOD. 

What is your VNAV descent page says?

 

 

[jetpipeoverheat]

To add to Tom's post, you need to go to VNAV Setup and change your descent speed to be at or above the STAR speed restrictions. VNAV will not speed up to meet a restriction, but it will slow down. Planning a descent at 250 KIAS is painfully slow, both for your passengers and ATC. Generally .78/290 works well.

[sunake]

To add to what everyone else is saying. Speed restrictions on charts are usually at or less. So if the descent page speed is lower than the restriction on the chart, it wont speed up.

[I9311I]

12 hours ago, Tom Stian said:

What is your VNAV descent page says?

 

 

Not sure, actually, Should it be automatic, or should I change the values in here?

Edited January 18, 2022 by I9311I

[I9311I]

9 hours ago, jetpipeoverheat said:

To add to Tom's post, you need to go to VNAV Setup and change your descent speed to be at or above the STAR speed restrictions. VNAV will not speed up to meet a restriction, but it will slow down. Planning a descent at 250 KIAS is painfully slow, both for your passengers and ATC. Generally .78/290 works well.

Ah, I get this now. Will it be persistent or will I have to change it every flight?

[I9311I]

6 hours ago, sunake said:

To add to what everyone else is saying. Speed restrictions on charts are usually at or less. So if the descent page speed is lower than the restriction on the chart, it wont speed up.

Not an accurate statement. Speed is usually a defined value. Altitude is usually a block.

[Tom Stian]

9 minutes ago, I9311I said:

Not sure, actually, Should it be automatic, or should I change the values in here?

I dont remember what the defaults are.

You can change it in the FMS under IDX->Defaults.

Im using .78/290 at the moment for the descend phase.

[I9311I]

58 minutes ago, Tom Stian said:

I dont remember what the defaults are.

You can change it in the FMS under IDX->Defaults.

Im using .78/290 at the moment for the descend phase.

Ok, I get it now. Makes sense, and thanks for the default screen. All set now, thanks.

[sunake]

22 hours ago, I9311I said:

Not an accurate statement. Speed is usually a defined value. Altitude is usually a block.

Ah thanks for the correction. I was thinking about the behavior of another aircraft type and forgot that not all SIDs/STARs with speed restrictions do not include MAX for the speed restrictions.