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E/D definition (VNAV)

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Moderator
Registered: May 2009
Posts: 3430
Good evening,

the VNAV mystery is never-ending ...

Does anybody know what the term "descent" means in VNAV language?

(A) - Descent is the path between T/D and the first waypoint altitude constraint, i.e. the path that is planned to be flown at idle thrust. E/D is the first waypoint that has an altitude constraint.

(B) - Descent is the path between T/D and the lowest waypoint altitude constraint; this can be the runway's altitude constraint if no other constraint exists. E/D is the waypoint that has the lowest altitude constraint.

(C) - Descent is the path described in (A), and when this is passed, it's (B) and the E/D will jump forward accordingly.

(D) ... ?


Cheers,

|-|ardy
Moderator
Registered: May 2009
Posts: 2178
Location: KTMB
I vaguely remember that the FMC calculates the longest path it can fly at idle without busting an altitude constraint.

If there is a waypoint "at or below FL240" followed by a waypoint "at 10,000", and the second waypoint causes a glide through the first waypoint at FL200, the E/D will be the second waypoint.

Not that this helps you much...
Moderator
Registered: May 2009
Posts: 3430
This sort of "tunnel finder" is also incorporated in PSX (tunnel through window constraints and A/B "hurdles"), but, as far as I understand it, it should be applied only to the non-idle part, i.e. to the part after the first constraint (approach part, usually).

E.g. if the first constraint is a B type, the idle path won't be computed at all (just dashes in the LEGS, no predictions). Therefore, the idle path is terminated at the first A-constraint or "at"-constraint after T/D.

Recently, in an old Kai Tak video, I saw the E/D on the OM. The OM wasn't the last waypoint. The route included the MM and the runway. I assume the OM was the first constraint after T/D, i.e. the end of the idle descent.

The video didn't reveal what happened after the OM overflight. I assume the E/D then jumped to the runway waypoint. I.e. in the approach part, the E/D is no longer at the next constraint, but at the lowest constraint, which is typically the runway.


|-|
Member
Registered: May 2011
Posts: 101
What is shown in the FMC "END AT" (descent page) is the lowest altitude constraint (B). Does the 747 have an offpath des page? The DTG here is also the distance to the E/D point which is the lowest altitude constraint.

As for calculations: E/D is 50ft above runway threshold for all approaches except for VOR approaches. For VOR approaches it is the missed approach point. During cruise, an E/D is also created when al altitude constraint is entered on the LEGS page on a downstream waypoint (which is described above, it's the lowest)

(so even visual approaches have an E/D 50ft above runway threshold)
Moderator
Registered: May 2009
Posts: 3430
Yes, the 747 has an offpath des page.

But why did I see an E/D on the OM in that Kai Tak video (IGS Rwy 13)? Perhaps the runway had no constraint, but that's unlikely.

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