VNAV SPEED Control off or on?

[bravolima]

Hi all,

in the MCP.INI you find the section [VNAV] with one line

DescentSpeedControl=

The description read: when set to off, pilots controls speed with the throttle

I´m fiddling around with pm´s capabilities of VNAV conducted descents; for the VNAVPATH descent I found a more or less working work around. The first and the last waypoint of my descent are hard altitudes (bolded). Unfortunately we cannot fill in weather data in the forecast page, probably depending on the weather the result is good or bad. SPD INTV and ALT INTV helps sometimes. But sometimes in transition from descent to arrival, the speed bug runs up to 280 oder 320 ktn... that´s not good, if you are distracted by other things, it spoils you up your arrival.
What is the reason behind this configuration "DescentSpeedControl"? Does it refer to special 737 type variants for instance without VNAV SPEED mode or something other?

regards

Bernhard (LSZH)

[Thomas Richter]

Hi all,

in the MCP.INI you find the section [VNAV] with one line

DescentSpeedControl=

The description read: when set to off, pilots controls speed with the throttle

I´m fiddling around with pm´s capabilities of VNAV conducted descents; for the VNAVPATH descent I found a more or less working work around. The first and the last waypoint of my descent are hard altitudes (bolded). Unfortunately we cannot fill in weather data in the forecast page, probably depending on the weather the result is good or bad. SPD INTV and ALT INTV helps sometimes. But sometimes in transition from descent to arrival, the speed bug runs up to 280 oder 320 ktn... that´s not good, if you are distracted by other things, it spoils you up your arrival.
What is the reason behind this configuration "DescentSpeedControl"? Does it refer to special 737 type variants for instance without VNAV SPEED mode or something other?

regards

Bernhard (LSZH)

Hi Bernhard

"Descent Speed Control" means that the PM AT controls thrust in descent (VNAV) or not (=No).
The normal way is =Off because the Pilot has control, the way it works in real live is (VNAV)
- AP comands descent
- AT shows RETARD (green)
-->Throttle levermove to iDLE
--> AT shows ARMED (white)
- now PF has control of Throttle
Now in this situation the AP is in N1 mode and tries to keep the AC speed with pitch.

Reality:
There is a big problem between Controller and Pilots!
Controller like to lower your speed at the same time you descent or in your flight plan you setted this up, lower ALT together with lower SPEED.
But there is a very big BUT because if you lower the speed when you descent the AP doesn't command may be any negative VS because we are in N1 mode and that means the AP tries to keep the speed with pitch.
So if your speed is e.g. 300kts in FL320 and next way point is FL100 with 250kts the AC doesn't descent at all because it needs to get first the speed before it can descent. It also can happen that it pitches up to lower the speed.
To get it right you would need to intervent speed to descent.

Back to PM:
But this is not a problem in PM or of PM because it does this exactly.
The problem for those situations are the controler you have to deal with and tell them that doesn't work for you and you need adifferent APP or a high speed descent.

[bravolima]

Thanks Thomas for your quick answer.

The background of my question: I´ve learned in a CBT-program from a big airline, there are two ways to conduct a VNAV descent. Secondly descent planing with or without VNAV capabilities of the 737 is a very hot topic among real 737 pilots.
Beside real life ATC issues, it is exciting for me to see how it goes and how my pm-system deals with it.

First VNAV-PATH (path is given, PF controls speed) and VNAV-SPEED (descent speed is given, PF controls path). Controlling Speed goes with speed brakes, FMC descent page, leg page.
VNAV PATH goes with altitudes restrictions and V/S in the descent page i think (when VNAV-SPPED is selected). Quickly and temporarily you can modify the descent with SPD INTV or ALT INTV ( in the VNAV SPEED Mode). For this you need for instance the CPflight Pro MCP.

As far as I see now, controlling speed in the descent modes goes always via FMC (first descent page, probably leg page (when and why?, perhaps in the preparation during cruise) never directly with the throttle. I think this is why the option
--- DescentSpeedControl= --- is set to "no". For a moment I was not quite sure about it.

Another question came up to day, when changing altitudes in the descent they always work as restrictions and they are bold. Can I "debold" them somehow? VNAV PATH descent works best at my setting, without any bold or hard altitudes.

comment on that?

regards

Bernhard (LSZH)

[bravolima]

I could give the answer by myself: to "debold" a constraint in the Leg-page, press "delete" and then to the right LSK-line. Very easy....


cheers

Bernhard (LSZH)

[saabpilot]

CP Flight have Speed and Altitude Intervention in all models of the MCP (even the OLD RS232 unit)

- Push VNAV button for more than 1/2 second to engage SPEED Intervention
- Push ALT HLD button for more than 1/2 second to engage ALT Intervention

You only need to d/l and upgrade your MCP firmware from them.

Bjorn