mode “VNAV”

[wsixpo]

Hello,

We discover the every day the operation of the FMS and today we have a problem that we are not explained.

I describe the situation :

1) We are in flight after the top off descent at 250 in mode “VNAV” and “LNAV”.
2) After the passage under the 10000 feet, the FMS applies the speed of restriction programmed on 190.

>In order to understand operation and to test the unit we engage the speed by the MCP while pressing on the button SPEED of the MCP.

The speed indicated to the MCP corresponds at the speed of the plane (190) (good). This speed can be adjusted by the coder of the MCP (good).

Our problem if one passes by again in mode VNAV the FMS programs the speed of 250 and not the speed of 190. It seems that the FMS does not take again any more the speed of restriction, but which he takes again the speed of the last point (I think...).

This operation is a problem (bug) or is it normal?
Do you have the same reaction with your system?

Thank you for your help.

[Sean Nixon]

I don't have PM, and I'm not that familiar with the operation of the FMC, but it sounds normal. If you want to return to VNAV, but with a new speed restriction, you need to input that speed restriction into the FMC, otherwise stay in MCP speed mode.

[wsixpo]

Thank you for your reply .

Yes, We thought that this new speed it's the new speed limit for a FMS.
But, the speed restriction does not change on the FMS. In our case, it is always 190 .

We have tried to enter again, but he retains that speed 250.

In my opinion, it should not be at a speed exceeding the limit indicated.

[Sean Nixon]

Remember the MCP and FMC are two different types of autopilot. Changes in the MCP do not happen to the FMC. To get the FMC to fly at 190kts in VNAV, then you'll need to change 250kts to 190kts in the FMC itself on whatever page the restriction is showing. Pilots mainly use the MCP to override the FMC to comply with ATC instructions as it's much quicker to use, but that's not to say the FMC can't be reprogrammed instead, but you wouldn't do that for a temporary change in flight attitude. As you are slowing down for landing, there's little point reprogramming the FMC for a few minutes flight.

[xtremeIT]

Hi !!

Don't forget the Econ speeds on the Climb, Cruise and descent pages.
You can put in your own speeds here.
These 3 pages are only for Vnav.

TC