use of VNAV.......

[captainflyguy]

according to a KLM captain, he tells me that he disconnects VNAV below 10,000 ft is this the norm? I know some captains that disable it altogether. Can someone give some more insight on this please?

[DE727UPS]

VNAV does some weird stuff, sometimes. The best way to describe it is to say sometimes it just doesn't do what I would do, or what I think it should do. It's really great for meeting a crossing restriction on an arrival. You gotta watch it, cause if there is a tailwind, it might be a bit behind, but for the most part, I'm amazed on how efficiently it meets a restriction. Much better than I could do hand flying a 727.

It's pretty standard to NOT use VNAV below 10K. It was even mentioned to me in F/O IOE. In an active ATC vectoring environment, you can't really use it cause ATC doesn't want you "on path" like VNAV wants. I think that's the main reason guys don't mess with it. It does work pretty well for non-precision approaches, though. Makes it a lot like an ILS, just a lot more setup and button pushing.

[Doug Taylor]

Exactly. VNAV is good up in the flight levels or in a cruise descent, but went you're getting vectored around and need to descend on command, it's flight level change all the way.

[captainflyguy]

O:K..... thats kind of what I was thinking; just wanted to make sure that I had a clarified view of its proper use.

[seagull]

However, disconnecting it also means you lose many of the protections that it can add. If the routing is pretty close to what the actual will probably be, the FMS on the later aircraft (4th generation onward, for sure) can do a pretty decent job of keeping you from descending too early, or too late.

The two reasons not to use it would be:

1. You have no idea what the actual "miles to run" are, and/or, ATC needs to you at an altitude immediately;

2. More commonly, the crew does not understand the FMS well enough to program it such that it will provide useful information, possibly procedures that lead to not having a useful routing in the system before arriving in the critical phase of flight (I am NOT advocating mucking with it below 10k!), or, too often, not understanding what the system is doing well enough to know that it was going to do a fine job if you just left it alone! All are really training issues.

[Velocipede]

If I'm getting vectors to an ILS, then the VNAV is off below 10K. If I'm doing RNAV RNP or any non-precision approach (VOR, NDB) then LNAV/VNAV path works pretty slick.

[Doug Taylor]

RNP *is* slick!

[Cptnchia]

Originally Posted by Doug Taylor

RNP *is* slick!

So is doing the Kermit One CDA approach into ATL. Although, only those lucky enough to fly redeyes domestically get to do it. It will take you from MEM all the way to the outer marker at close to idle thrust in VNAV/LNAV. All you have to do is configure at the speed changes and depending on the wind, maybe crack the boards a little bit to help out. It's funny watching the captains who don't trust the system having to refrain themselves from switching over to V/S or FLCH.

[BobDDuck]

Flight idle from over MEM? I dunno about bigger stuff but we need at least 1500fpm at idle to keep 250. Even if you are in the high FL3XX at idle it would have you down in 20 minutes. I'm thinking it takes a lot longer to get from MEM to ATL then 20 minutes.

[seagull]

Originally Posted by Velocipede

If I'm getting vectors to an ILS, then the VNAV is off below 10K. If I'm doing RNAV RNP or any non-precision approach (VOR, NDB) then LNAV/VNAV path works pretty slick.

It can work fine with vectors to an ILS, really just depends on what you have in the box and how you edit. I would say it's about 50/50, as ATC sometimes does things that you really can't predict.

[Doug Taylor]

as ATC sometimes does things that you really can't predict.

You mean the 'expedite down to 12,000 and slow (from 330) to 180 knots' request? NOO!

[DE727UPS]

I can't wait to do one of the new west coast NDA CDA arrivals into SDF. It's VNAV to GS intercept. All pre-programmed into the box. Don't mess with it. ATC is playing along. We're gonna save tons of gas.

I'm cool with it. Just hope it works as advertised...

[Doug Taylor]

We did the RNP into Quito in the sim. Once during nighttime, another time during day VMC.

Holy cow, check that actual nav performance and check it twice!

[DE727UPS]

This is more of an arrival thing to start your descent at the last minute, save gas, and spool them at 1000 agl. They got it all figured out. And it might even work as long as ATC plays.

The RNP approaches we don't do.

[Cptnchia]

Originally Posted by BobDDuck

Flight idle from over MEM? I dunno about bigger stuff but we need at least 1500fpm at idle to keep 250. Even if you are in the high FL3XX at idle it would have you down in 20 minutes. I'm thinking it takes a lot longer to get from MEM to ATL then 20 minutes.

Sorry, I guess I should clarify. The CDA starts at MEM, not the descent portion. But as was posted by someone else, once you start down, the power is way back.

[BobDDuck]

Makes sense. I know Boeing's are gliders, but I was thinking that was pushing it a bit.

[Mr.Maximus]

Personally, I love VNAV - most of the time. I'll use it alot on arrivals and then on most non-precisions including visuals. Pre-programming crossing restrictions / speeds for arrivals and sit back and enjoy. Once in the terminal environment though, I a strictly VS guy until on an approach. Not real big on FLC though, although some guys swear by it. Gotta love the snowflake.


Max

[Doug Taylor]

Vertical speed, ORLY?

What type of aircraft?

[my_controls]

Don't know, I think with VNAV you get what you put in. If you program it well and give it the proper data with temps winds and so on it's pretty slick. I've found myself going "what's it doing?" myself but mostly it's my lack of understanding the system (737-800).

Once programmed it can take you nicely from cruise all the way down to being established on an ILS for example. (Assuming of course you set everything else up). I'm doing my sim training for VNAV non-precision approaches soon and it looks pretty cool compared to the way we do it now.

However for the other types of approaches it's still company policy to use something other than vnav below fl100. It's also not suitable of course for radar control and vectoring because you don't know what you're going to get next. The VNAV mode in my mind was meant to be used with LNAV as a pre-programmed route ending in an approach. Left alone with the correct parameters it will do that nicely.