A320 Why managed speed reduces when selecting V/S?

[IFLY_INDIGO]

I have observed at many occasions that when we were in OPEN DESCENT and we we select V/S, managed speed reduces to 250 kts [we are still well above FL 100]

I know about mode reversion but this is different than that

I couldn't understand why it happens. FCOM has no mention about it


Has anyone also faced the same situation? Do you know why it happens?

[BEL1000]

It happens when reaching very close to FL100 (10,000 feet) to meet/follow the speed restrictions as aircraft are not allowed to exceed 250 KTS under FL100 unless requested to ATC the good thing is that Airbus planes such as A320 does that with the help of the MCDU/FMGC. I am not sure about Boeing..

[Uplinker]

I have never seen this happen, and we are frequently instructed to descend from cruising FL to FL340 before a certain waypoint inbound to the UK, which I usually do at V/S -500'/min.

[Metro man]

V/S has priority over IAS in mode reversion so speed will be whatever is required to achieve the selected V/S. Cost index will vary descent speeds with CI20 giving around 270kts. Below 10 000ft default speed is 250kts.

[SmokeAndNoise]

The fmgc calculates deceleration to the next speed restriction (i.e. 250 kts at fl100) based on the vertical speed you select. If you select a high VS the 'deceleration segment' becomes longer and if you select a low VS it becomes shorter.. Thus if you select a high VS the managed speed target will come back to next restriction long before the actual waypoint or pseudo waypoint because the fmgs calculates that your deceleration will take a long (if not infinite) time/distance at current VS. Hope that makes sense.

[Uplinker]

@Indigo: Did you mean that the managed speed bug changes to 250Kts, or that the speed decays to 250 (if required to achieve the V/S) but the bug stays where it was originally?

[FlightDetent]

I have seen the bug drop to 250 after v/s had been selected. About 6000 ft higher than i would have had expected. Same thing down to 220 at about 7000 AAL.

I do not remember much, fms version and such. Apart from the fact that it was about three times a year and I did not see the reason behind it.

[EcamSurprise]

I have seen the bug drop to 250 after v/s had been selected. About 6000 ft higher than i would have had expected. Same thing down to 220 at about 7000 AAL.

Yup, same thing.

Descending in VS and the managed speed target changes to 250 around 15,000ft or so.

[Romasik]

The same thing on 330. Occasionally and with no apparent reason.

[SmokeAndNoise]

Gents, sorry if my explanation was not clear, english is not my first language. I'll try with a more detailed example.

A:

Fl150 descending. You are in open des with 270 kts. Vs will be approximately 2000 fpm +/-. If you pull vs the speed bug will come back because the fmgc calculates you need a very long time to get the speed back to 250kts with -2000 fpm (actually you would never get to 250 kts at this rate of descent).

B:

Same scenario as above. You turn the vs knob to -500 before you pull. The speed bug will not come back because the fmgc calculates that you do not yet need to begin deceleration to reach 250 kts by fl100. Deceleration would begin at approximately fl110 in this case as you would expect with the other descent modes.

Make sense now?

[EcamSurprise]

It does! Thanks.

[FlightDetent]

Likewise. I did not read your first post properly to begin with.

[Romasik]

Gents, sorry if my explanation was not clear, english is not my first language. I'll try with a more detailed example.

A:

Fl150 descending. You are in open des with 270 kts. Vs will be approximately 2000 fpm +/-. If you pull vs the speed bug will come back because the fmgc calculates you need a very long time to get the speed back to 250kts with -2000 fpm (actually you would never get to 250 kts at this rate of descent).

B:

Same scenario as above. You turn the vs knob to -500 before you pull. The speed bug will not come back because the fmgc calculates that you do not yet need to begin deceleration to reach 250 kts by fl100. Deceleration would begin at approximately fl110 in this case as you would expect with the other descent modes.

Make sense now?
What's the big deal if it takes longer time?
I think it may have something to do with thrust. OPEN DESCEND - idle thrust. If we just pull the VS slector and target speed remains the same, the thrust will be fluctuating around keeping the current speed. To make sure the thrust remains idle, FMS sets the lower target - the next targed of 250. That's what I normally do when I select VS and target speed stays where it is and does not drop to 250 - I select 250.

[lurkio]

Smoke and Noise has it spot on in my experience.

[vilas]

IFLY INDIGO
It is given in FCOM DSC-22_30-75 P 1/10




INTERACTION BETWEEN LATERAL MODES,


VERTICAL MODES, AND MANAGED SPEED PROFILE






When NAV mode is engaged:


The FMGS guides the aircraft along the flight plan and considers the constraints attached to the


F-PLN waypoints. As a result:


‐ Managed CLB and DES modes are available


‐ The managed speed profile includes: V2 - SPD CSTR (if applicable) - SPD LIM - ECON CLB


SPD/MACH - ECON CRZ MACH - ECON DES (MANAGED SPD) - SPD/MACH - SPD CSTR


- SPD LIM - HOLD SPD - VAPP/GS MIN.


It is valid for all vertical modes, except EXPEDITE  .

[IFLY_INDIGO]

Gents, sorry if my explanation was not clear, english is not my first language. I'll try with a more detailed example.

A:

Fl150 descending. You are in open des with 270 kts. Vs will be approximately 2000 fpm +/-. If you pull vs the speed bug will come back because the fmgc calculates you need a very long time to get the speed back to 250kts with -2000 fpm (actually you would never get to 250 kts at this rate of descent).

B:

Same scenario as above. You turn the vs knob to -500 before you pull. The speed bug will not come back because the fmgc calculates that you do not yet need to begin deceleration to reach 250 kts by fl100. Deceleration would begin at approximately fl110 in this case as you would expect with the other descent modes.

Make sense now?
It is a satisfactory explanation I was looking for!

It happens only below FL150 or so, when the time would not be enough for the older DECEL point to work with the current V/S. Therefore, FMGC calculates new DECEL point and airplane is found to be already beyond that new DECEL point.