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Speed intervention and VNAV DES page

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Author Post
Member
Registered: May 2009
Posts: 324
Hardy,

I was doing a LNAV/VNAV approach last night.

While on speed intervention (with "142" dialed in), the VNAV DES page (titled "ACT MCP SPD DES") was showing "240" in the "SEL SPD" line.

Shouldn't it show the dialed MCP value like it does in the "CLB" and "CRZ" pages?

Thank you again for giving us PSX!

Mariano
Moderator
Registered: May 2009
Posts: 5140
I think this is correct.

Your FMC speed mode can be LRC, ECON, E/O, RTA, or SEL. SEL is not MCP. The FMC speed values are displayed accordingly under LRC, ECON, SEL etc. on the CDU.

SEL SPD is not the same as MCP SPD.

The selected MCP SPD value is shown on the PFD. The selected MCP SPD value is not copied to the speed value on the CDU displays.

Or am I misunderstanding your question? You are wondering about the "SEL SPD" line only, not about the page title?


Cheers,

|-|ardy
Member
Registered: May 2009
Posts: 324
Hardy,

What I was trying to ask was that shouldn't VNAV DES page line 2L didplay the speed value dialed on the MCP during speed intervention?

I am just asking; I always assumed it did, but I was probably wrong all along.

And I was wrong about the CLB and CRZ pages; I was thinking about manually-entered speeds.

Thanks

Mariano
Moderator
Registered: May 2009
Posts: 5140
MCP SPD is not the same as SEL SPD. Just like MCP ALT not being the same as an FMC altitude.

SEL SPD is an FMC speed, selected in the FMC.

MCP SPD is an MCP speed, selected on the MCP.

The line title at 2L on the VNAV page can be "SEL SPD", but never "MCP SPD".

"MCP SPD" can be the line title only at 5L on the PROGRESS 1/3 page.


Cheers,

|-|ardy


If the line title at 2L on the VNAV page would read "MCP SPD" and would display the MCP speed value, you wouldn't be able to program the FMC with SEL, ECON, E/O, LRC speeds whenever the MCP speed window is open; the MCP value would always overwrite the values displayed on the CDU. That wouldn't make sense. You need to see the data programmed in the FMC -- and the data set on the MCP. And with the white round MCP SPD knob you switch between MCP data and FMC data. It would be paradox if the MCP data was part of the FMC data. MCP intervention wouldn't be possible anymore. You couldn't even restore the previous SEL SPD value because it would've been replaced by the MCP SPD value.

"MCP SPD" can be the line title only at 5L on the PROGRESS 1/3 page because that line doesn't allow manual entries. It's just a display. It displays the active mode like the VNAV page titles do.
« Last edit by Hardy Heinlin on Sat, 08 Aug 2015 00:49:55 +0000. »
Member
Registered: May 2009
Posts: 324
Hardy,

It makes sense now.

Thank you for the clarification.

Best regards,

Mariano

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