NGX VNAV Implementation and Winds Aloft

[masneoquil 66]

I remember a retired 757 captain telling me that he almost never used VNAV in descent, due to the inaccuracy. Always in the climb, but almost never in descent.It would be interesting to know if, as simmers, we are obsessed with using the feature despite it's use being used less often in the real world.Martin Wilby

That doesn't means that none of the real world captains don't use it. Maybe the 757 VNAV was inaccurate.

[Joel 10]

That doesn't means that none of the real world captains don't use it. Maybe the 757 VNAV was inaccurate.

Also, during descent, ATC can begin vectoring you to your destination instead of having the VNAV do it. The VNAV doesn't know the traffic.

[masneoquil 66]

Also, during descent, ATC can begin vectoring you to your destination instead of having the VNAV do it. The VNAV doesn't know the traffic.

That's different. ATC is vectoring you.

[Joel 10]

That's different. ATC is vectoring you.

What I meant was that the Vnav finds the best descent path to your destination but that may change due to other restrictions especially with traffic.

[michal 44]

What I meant was that the Vnav finds the best descent path to your destination but that may change due to other restrictions especially with traffic.

I would say traffic is secondary. The most important factor is the design of the airspace and need for so called step-downs. This is something that can't really be programmed into FMS. So there is no way, specially around busy airports to leave your cruise altitude on VNAV and get all the way to approach phase. You will transition a few sectors and they have LOAs (letter-of-agreements) with adjacent sectors to deliver you at certain altitudes. So typically you could be be flying portions of your descent on VNAV but you will have hold your altitude on certain segments.

[masneoquil 66]

I would say traffic is secondary. The most important factor is the design of the airspace and need for so called step-downs. This is something that can't really be programmed into FMS. So there is no way, specially around busy airports to leave your cruise altitude on VNAV and get all the way to approach phase. You will transition a few sectors and they have LOAs (letter-of-agreements) with adjacent sectors to deliver you at certain altitudes. So typically you could be be flying portions of your descent on VNAV but you will have hold your altitude on certain segments.

That's something i alwais wonder. If ATC doesn't give you clearance to descent below an altitude and in the approach you have ALT restrictions, then you have to catch up with the descent path otherwise you would miss the restriction. Is that very common?

[michal 44]

In the US it is very common. This is how traffic is fed into bigger airports - if you saw a typical vertical profile you would see number of -steps-. Not sure about your 'restriction' question though.

[Darwinian 0]

I remember a retired 757 captain telling me that he almost never used VNAV in descent, due to the inaccuracy. Always in the climb, but almost never in descent.It would be interesting to know if, as simmers, we are obsessed with using the feature despite it's use being used less often in the real world.Martin Wilby

I dont know whether im wrong or right but if im been stepped down to the approach via ATC, i usually use the Level Change function. I found that when flying into busy airports VNAV is inaccurate and i end up too high of missing crossing restrictionsMike

[Cras 6]

Yes, either the Level Change, which is probably best when closer to the Earth and speed is lower, or the vertical speed mode. On many youtube videos of approaches, I tend to see most use vertical speed to control the descent. Level Change, or FLCH (fletch), is safest and most practical when passed to tracon. Trying to use it coming out of cruise may result in too steep a climb, since the engines are idle and speed is maintained only by the descent.

[Darwinian 0]

Yes, either the Level Change, which is probably best when closer to the Earth and speed is lower, or the vertical speed mode. On many youtube videos of approaches, I tend to see most use vertical speed to control the descent. Level Change, or FLCH (fletch), is safest and most practical when passed to tracon. Trying to use it coming out of cruise may result in too steep a climb, since the engines are idle and speed is maintained only by the descent.

I have never seen that particular problem myself but ill keep it in mind and use a vs descent on my next flight. Thanks for the advice

[Tabs 2,790]

I dont know whether im wrong or right but if im been stepped down to the approach via ATC, i usually use the Level Change function. I found that when flying into busy airports VNAV is inaccurate and i end up too high of missing crossing restrictions

VNAV can only do what it's actually capable of doing. If you're way off path because of a late descent by ATC then it's going to miss restrictions (the NGX FMC is going to tell you with error messages in the scratchpad like "DES PATH UNACHEIVABLE") There's no inherent limitation where VNAV fails at "busy airports" though - if ATC allows you to stay on the calculated VNAV path, it'll fly it fine.

Yes, either the Level Change, which is probably best when closer to the Earth and speed is lower, or the vertical speed mode. On many youtube videos of approaches, I tend to see most use vertical speed to control the descent. Level Change, or FLCH (fletch), is safest and most practical when passed to tracon. Trying to use it coming out of cruise may result in too steep a climb, since the engines are idle and speed is maintained only by the descent.

The only actual restriction here is not using V/S in climbs. It doesn't protect against stalls and will happily keep climbing while airspeed decays away. There's no problem with using V/S in descent - it will respect and capture the MCP altitude. Using V/S is very common on non-precision approaches too.There's no such thing as a descent that's "too steep" out of cruise - people feel acceleration, not speed. You want to ease into it (many pilots start down with DES NOW or V/S a few miles prior to T/D to avoid a sudden acceleration), but once the descent is at a constant vertical speed, it doesn't matter if it's 1 FPM or 3000 FPM - they feel the same. LVL CHG and VNAV PTH are both going to give you an idle path angle on that first leg of the descent, the resulting V/S is what it is and it will always be fairly "high" when descending from high altitude.