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pcubine

Tutorial 1.5 Page 52

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From the Topic Title document

 

"Leave the autopilot out of VNAV until crossing TUDEP, otherwise, as soon as VNAV is selected, the FMC will re-command the ECON speed. "

 

I thought the idea on crossing the North Atlantic Tracks was to maintain a constant speed and altitude from entry to exit.

Shouldn't  the word track replace TUDEP. If I return to VNAV after crossing TUDEP, the speed changes from .83M  to .833M.  Is this possibly a misprint?

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I thought the idea on crossing the North Atlantic Tracks was to maintain a constant speed and altitude from entry to exit.

Shouldn't  the word track replace TUDEP. If I return to VNAV after crossing TUDEP, the speed changes from .83M  to .833M.  Is this possibly a misprint?

 

You might want to go back and re-read it, as I think you missed the earlier entries where you force the speed at TUDEP at the bottom of page 49.

 

The whole point of that line is to ensure you're at M0.83 prior to TUDEP, so you enter SPD INT and command M0.83. After passing TUDEP, VNAV recognizes there's a speed restriction from TUDEP onward, which is why you remain in SPD INT until passing TUDEP.

 

In other words:

Speed prior to TUDEP: ECON (VNAV will re-capture ECON if you leave SPD INT)

Speed at and after TUDEP: M0.83 (VNAV will remain at M0.83 if you leave SPD INT)

 

Therefore, in order to ensure you're at M0.83 prior to TUDEP, we enter SPD INT using the speed of the restriction to adjust speed prior to that fix. At and after the fix, VNAV recognizes the speed restriction, which is why it says "[l]eave the autopilot out of VNAV until crossing TUDEP [...]."

 

The practice I showed is way more nit-picky than it needs to be (going from ECON to M0.83 right at TUDEP would be operationally fine), but I wanted to push people off of LNAV/VNAV a few times to get people thinking outside the box.

 

No idea what replacing 'TUDEP' with 'track' would get anyone. TUDEP is on the track, it isn't the track itself.

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