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Author Topic: Question VNAV DES page  (Read 276 times)

double-alpha

  • Join date: Mar 2015
  • Location: NTAA
  • Posts: 262
Question VNAV DES page
« on: Fri, 15 Jan 2021 01:31 »
Hi everybody,

I noticed something strange (for me) during a descent with the basic 017 - 10 nm to TOD . situ, maybe I don't understand the VNAV DES page.

Imagine ATC tells you to start your descent at your actual Mach number, and to keep 260 kt at the crossover altitude.
I enter in VNAV DES page 2L, e.g. :   .844/260 and EXEC.

The magenta Mach Indication (e.g. : .844) on the PFD changes to Airspeed (e.g. : 276).
At the TOD, the magenta Airspeed indication changes from 276 to 260.

I thought it would be first a descent at M.844 (indicated in magenta on the PFD), then at the crossover altitude a descent at 260.
Am I wrong? and why

Hardy Heinlin

  • Moderator
  • Join date: May 2009
  • Posts: 12233
    • Aerowinx
Re: Question VNAV DES page
« Reply #1 on: Fri, 15 Jan 2021 04:10 »
Hi, on that STAR into EHAM there's a 280 kt constraint at FL240. I don't know exactly when the real 744 would change to 280, but PSX changes to 280 at the T/D already as the deceleration before FL240 will start very soon anyway. This makes the path prediction more robust. If the SEL SPD is set to 260 already under such conditions, PSX will set 260 instead of 280.

If you delete that 280 constraint before the DES starts, the FMC will switch to IAS when M.844 rises to 260 KIAS. However, this is already the case at CRZ ALT. When you descend with a fixed Mach, the IAS will rise. If you set a higher IAS, say, .844/290, the FMC will keep .844 until the IAS rises to 290 kt. In this scenario, the IAS at CRZ ALT is below 290 kt. So there's some headroom for the rising IAS during the descent.

The crossover is smooth. There's no speed bug jump. This is because the FMC sets the crossover altitude at the .844/290 crossover, not at the fixed crossover altitude which the MCP SPD uses. The FMC will not rise the IAS and then suddenly jump back to 260.


|-|ardy
« Last edit: Fri, 15 Jan 2021 04:37 by Hardy Heinlin »

double-alpha

  • Join date: Mar 2015
  • Location: NTAA
  • Posts: 262
Re: Question VNAV DES page
« Reply #2 on: Fri, 15 Jan 2021 04:33 »
Hi Hardy,

Understood , that makes sense.

Thank you for your quick reply !

double-alpha

  • Join date: Mar 2015
  • Location: NTAA
  • Posts: 262
Re: Question VNAV DES page
« Reply #3 on: Fri, 15 Jan 2021 04:48 »
I re-checked, everything looks good....

I am a bit tired : I didn't see the 280 kt constraint at DENUT

In addition, I meant a M.83/290 kt descent speed instead of the M.844/260 I wrote above...

I should go to bed