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VNAV ALT -> VNAV PTH

Started by paojack, Mon, 6 Nov 2017 08:17

paojack

Hi Everyone,

I have a question about VNAV, here's the scenario:
A/C cruising at FL310, within 50nm from T/D, pilot initiate an early descend to FL200, at FL200 FMA change to VNAV ALT, and the aircraft is well below the path (with VTK well above), if the pilot set the MCP altitude window to a lower altitude (without pushing the knob), will the aircraft descend automatically when intercepting the path? (VNAV ALT change to VNAV PTH automatically?)

I tested it in the big sims, one of them worked but the other don't, In real aircraft, it worked but I only tried it once, so I'm not sure and couldn't find answer in the FCOM.

P.S: I found an article from Atlas website about "on-approach", it says "* "On Approach logic" allows us to do three things: (1) Auto transfers from [VNAV ALT / SPD] to [VNAV PTH] (2) Allows us to descend in [VNAV PTH] with the MCP speed window open. (3) Allows us to rotate the altitude selector to a higher altitude than the aircrafts present altitude, such as our MAP altitude without leveling the aircraft off. 
"
Obviously the scenario above is not definitely NOT "on-approach", but I'm wondering if the said "on-approach" feather is same as in aerowinx?


Thanks,
P.J

emerydc8

I don't think it will switch to VNAV PTH and descend further unless you roll in a lower altitude and altitude intervene (push in on the altitude knob) as you capture the path from below. It is not in "on-approach" logic.

When you tried this in the real plane, was it a 747-8 or did it have the NG software?

Britjet

It depends on whether you have the speed window open at the time.
(of course you have to be approaching the descent path from below as well).
If the window is CLOSED the aircraft will transition to VNAV PTH without you taking any action.
If the window is OPEN it will remain in VNAV ALT and would require a press of the ALTSEL to change to PTH.
This was comprehensively tested by a friend of mine in a real BA 747 a few years ago.
Peter.

Hardy Heinlin

... and after that comprehensive test this has been implemented in PSX as well.


|-|ardy

emerydc8

QuoteIf the window is CLOSED the aircraft will transition to VNAV PTH without you taking any action.

Thanks, Peter. I wish I could test it tonight, but there is no VNAV ALT on the 767.