Who uses VNAV to descend .... and who doesn't?

[Guest meyomyx]

Following a recent post and reply from Brad, I'm curious. I find VNAV too unreliable to perform a descent/letdown. Let me make it clear that I'm sure that it is working as it ought and, according to them that know, it is a remarkably faithful representation of the FMC.Now, I may be wrong, but I understand Tim M advocates (and I for one prefer) resorting to LVL CHG/VSPEED and let down as per the chart, letting LNAV do its bit.Brad says VNAV is fine (and he should know).I find that, not on every occasion; but on many descents, you end up in a mess with the thing disconnected - set what descent forecasts/ wind velocities you like. Often you're not entirely sure why. Is it an incorrectly writted STAR? A gust/wind pattern? It would seem to be more of a problem, the more restrictive the descent is.Try the 3 times dist to go plus 10 descent strategy. VNAV won't even have thought about starting your descent yet.Nope, for me, VNAV descents take your mind off following a letdown as published .... and on many occasions will leave you at the wrong altitude or speed.Discuss :)

[crvm]

Don't use the default VNAV PATH with restrictions, insted go VNAV SPEED if you desire to stay in VNAV. Also, there is a BUG (yes I said it) that happens on descents in PATH with certain wpt restrictions causing a 500 FPM descent that ends up kicking out of VNAV. Try a descent without restrictions above 10000' and see how it goes at least to 10000'. Best Wishes,[h4]Randy J. Smith[/h4]http://www.rawbw.com/~bdoolin/shinault/Animation1.gifCaution! Not a real pilot, but do play one on TV ;-)AMD 64 3200+ | NEC LCD 1980SXi 19" | ASUS KV8 DELUXE | GFORCE 5700 ULTRA @535/1000 | Maxtor 6Y080M0 SATA 80 GIG | 512 DDR 400 | Windows Xp Pro |

[Guest meyomyx]

Funnily enough - thats what I find seems to work OK and it ensures I don't forget to slow to 250kts!

[crvm]

And in case you are not aware, if your speed becomes greater than 15 knots below 10000' VNAV will disconnect. Must stay on top of it below...Best Wishes,[h4]Randy J. Smith[/h4]http://www.rawbw.com/~bdoolin/shinault/Animation1.gifCaution! Not a real pilot, but do play one on TV ;-)AMD 64 3200+ | NEC LCD 1980SXi 19" | ASUS KV8 DELUXE | GFORCE 5700 ULTRA @535/1000 | Maxtor 6Y080M0 SATA 80 GIG | 512 DDR 400 | Windows Xp Pro |

[Guest tmetzinger]

>Now, I may be wrong, but I understand Tim M advocates (and I>for one prefer) resorting to LVL CHG/VSPEED and let down as>per the chart, letting LNAV do its bit.I advocate deciding which part of the pitch equation (forward speed or rate of descent) is more important and then selecting the appropriate mode.As Randy's said, there is still a BUG in VNAV PATH that you may encounter rarely when you have multiple altitude constraints above 10000'. Some STARS can cause this.I generally find that if I'm flying without VATSIM, that VNAV PATH or SPD descent works just fine (I only get disconnects in the bug identified above or if I don't respond when the FMC tells me DRAG REQUIRED). If I'm flying with ATC, though, they usually give me a descent that's different than the VNAV-calculated one, and then it's off to other pitch modes.The key thing is to understand what mode the autoflight system is in, and what values it's using to fly the airplane (particularly in pitch). When you understand that you can switch modes with confidence and cope with anything that comes up. For instance I've gone from VNAV to LVL CHG as part of an arrival, but then when cleared for a non-precision approach I went back to VNAV for the vertical profile, and as long as I got the drag (gear and flaps) out in time if flew it flawlessly.

[Lairyliam]

i have found that i reduce my speed to 275kts in the fmc and i can go all the way down to 5-7000ft with out much worrymight not be best practice but works for meLiam

[Guest Paragon]

I generally fly VATSIM so I get almost all the time a descent before the FMC does. I have a question though...usually I'm told to descent to a certain flight level at a certain point, and I punch it into the FMC and hit the "DES NOW" button, which works fine. If I want to use the LVL CHG and hit a certain level at a certain point, is my only reference the green arc? The FMC does a good job but I would like to be able to use the LVL CHG button more as I feel more in control.

[Guest lamont]

I fly the majority of my flights online with VATSIM and usually let VNAV handle the initial descent to the first or second published altitude restriction.I do pay particular attention to the speed and if "drag required" pops up I apply it. Once I get near to the final waypoint in the star and expecting vectors I use either LVL/CHG or VS for the remainder of the approach. Never had a problem using this procedure.

[Guest zip]

I let VNAV do the climb, cruise and initial descend for fuel efficiency, but I usually end up disconnecting on approach below FL180 because I invariably end up going too fast and too steep.Like Tim, I very seldom end up following an FMC loaded STAR on approach due to instructions by the APP controller and non VNAV command modes seem more appropriate.

[Orlaam]

I still have VNAV issues sometimes as well. The other day I programmed a flight into KABQ from KPHX and didn't modify any of the altitude or speed constraints. However, when the plane hit TOD, it got to about -5000 and got going damn fast. I let it go until it disconnected because I wanted to see how far it would take it and if it would correct itself. I basically just hit VNAV again and it started working right. No STARs either. Just a straight approach.

[crvm]

I let it go until it disconnected because I wanted to see how far it would take it and if it would correct itself. You must keep in mind that in the default PATH DESCENT vnav does not "correct" anything other than the path profile, speed will only come into play in vnav path if one becomes too slow. In a sense vnav path does "correct" itself when going to fast by reversion to LVL CH or CWS. If you expect a tail wind, make sure to be "liberal" in the forecasts page...Best Wishes,[h4]Randy J. Smith[/h4]http://www.rawbw.com/~bdoolin/shinault/Animation1.gifCaution! Not a real pilot, but do play one on TV ;-)AMD 64 3200+ | NEC LCD 1980SXi 19" | ASUS KV8 DELUXE | GFORCE 5700 ULTRA @535/1000 | Maxtor 6Y080M0 SATA 80 GIG | 512 DDR 400 | Windows Xp Pro |

[Guest pamber]

A simple question -how do you change frOM VNAV PATH to VNAV SPEED Bernie Lonnen

[crvm]

Before OR during descent push the SPD prompt > on the DEScent page...Randy J. Smith

[Guest Kosta]

I'm using VNAV PTH and never had necessary to change to VNAV SPD. I noticed though only one problem, which is actually accurate, and is my error, if you put descent point too close, it will say DES PATH UNACHIVABLE. OK, that one I get (I don't think that's the bug guys here are talking about). The rest appeared to be working OK! VNAV PTH all down to 5000ft usually...

[Wolfko]

I use VNAV to decend till that point, when the default ATC vectores me away from the FMC-programmed flightplan. Then, off course, one must disconnect VNAV, because all the data VNAV uses to calculate the decend are not valid any more.On the 2 or 3 flights so far, on which I did not use the default ATC, I had no problems to use VNAV all the way down until intercepting the final ILS course.Wolfgang