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VNAV path mode ...

Started by Egyptair687, Tue, 8 Jun 2021 17:39

Egyptair687

Hello guys,

I just wanted to ask : Is vnav path mode always aiming idle thrust during descent ?
That means, is the T/D set that way (including all information like wind, weight and so on) that vnav path tries to remain in idle thrust all the way down to the FAF ?
How flexible is vnav path ? Does it take into account, that there may be constraints and from then, it tries to idle thrust the descent ?

Thank you

Ahmed

Jeroen Hoppenbrouwers

Yes, it tries to descend at idle thrust, or slightly above idle so it has a bit more room downward in case it meets an unexpected tailwind or less headwind than forecasted.

The descent algorithm attempts to meet all altitude and speed constraints and will warn if it cannot do so without assistance. Extending speed brakes is not automatically done. Adding thrust is automatically done.

An idle descent is the most fuel-economical path so this is why. Most of the FMC is about fuel savings.


Hoppie

Hardy Heinlin

Quote from: Egyptair687 on Tue,  8 Jun 2021 17:39
Is vnav path mode always aiming idle thrust during descent ?
That means, is the T/D set that way (including all information like wind, weight and so on) that vnav path tries to remain in idle thrust all the way down to the FAF ?

Down to the first hard altitude constraint. Thereafter it's usually a shallower path which requires more than idle power, thus the A/T mode will change from HOLD to SPD.

You can see the calculated, predicted idle path down to the first hard altitude constraint on Instructor > Analysis > Navigation. Gross weight, wind data, OAT data, and planned anti-ice activation altitude will influence the path (see the settings on the FMC DES FORECAST page).

The idle path system is a feature of the legacy FMC. The NG FMC always plans a shallower descent with off-idle thrust, i.e. the NG FMC sets the T/D a bit earlier.


Regards,

|-|ardy

Egyptair687

Thank you Jeroen & Hardy

Ahmed