Jump to content

Sign in to follow this  
Guest flightgamer

NG VNAV, is excellent for descents, if used correctly...

Recommended Posts

Guest flightgamer

Well, i fly an average of 15-25 hours a week for SWvA... i found, given the time period, PMDG did a remarkable job on the NG's (700/600) VNAV Logic. i have seen many skeptics who did not enjoy, or did not get good results, flying the descent using VNAV. i heard many say entering winds accurately, or just plain using LVL CHG to do they're descents, which i find very labor intensive. anyways, what i do, is i just go into my STAR's chart and i look at the intersections/VOR with "expect ATC to clear you to *here*" or whatever, and i enter those into my FMC as seen... below is a picture of my edit to the MADN4 STAR FMG Trans...1st Page of the FMChttp://www.uploadhouse.com/fileuploads/517...c26d6088aa4.jpg2nd Page of the FMChttp://www.uploadhouse.com/fileuploads/517...efa6314d1f8.jpgThe MADN4 (Madwin 4)http://www.uploadhouse.com/fileuploads/517...71f874859de.jpgAs seen below, this was my descent AOA the whole way down to 3000 for ILS 29 in Oakland.... the ride was beautiful, no VNAV disconnects, no "UNABLE NEXT ALT" messages, no spoilers needed no Early flap/gear deployment needed.... it was great, i do this every time i fly... And it takes but a few moments to open the chart and punch in "14000" or whatever and hit the corresponding key..... very easy and safe...I held a respectable VS of 2600, i held my Airspeed, i was holding a neutral throttle level of 50% (not bad) which is great... http://www.uploadhouse.com/fileuploads/517...3e7c782ad23.jpgSo as you can see, a simple, 2 minutes before your flight, can make a much more enjoyable experience for your and your passengers....Go have fun and Fly!!!

Share this post


Link to post

Very nice job explaining the topic.The disadvantages to VNAV don't crop up in the basic STAR you used, you'll have problems with CIVIT at KLAX due to the numerous bracketed contraints, or GLASR at KSEA (and several whose name I don't remember at KATL) where you enter downwind at 12000 and the FMC just can't make a smooth descent and deacceleration. Biggest problem is the FMC treats all constraints as though the aircraft should level off at the indicated altitude, which it tries to do, and then continue the descent after the fix. This is very unrealistic, when the constraint is "cross at" it's pretty easy to maintain a smooth descent and cross the fix at or near that altitude while descending using MCP control. Another problem is the deacceleration to 240 at 10,000 that I don't like especially in situations where the current constraint is 250, which is the KIAS speed limit, this is minor but in some cases you end up loosing the distance need to maintain a reasonable descent angle because of it.There's a few more but I think you get a sense of the arguments against VNAV in the terminal area (i.e, below 18,000), but enroute descents are fine.Once again, nice job with the pics.


Dan Downs KCRP

Share this post


Link to post

Kylan:I too use VNAV for inital descents, however, after FL180 I usually rely on LNAV and VS- with sequential changes to the MCP ALT and SPD INTERVENE.I'd venture to say very few people know as much about SIDs and STARs as Dan Downs and Steve Bell, and both (and most real world a/c captains) use MCP control for final descents. If you use that, along with a /XX command on the FIX page to draw a circle around various FIXES, you can execute ultra precise vertical navigation profiles with a bit of practice. Then when ATC mixes it up a bit, keeping you high and fast- its just a matter of knob twisting- no frantic typing- heads down in a crowded sky.VNAV works great for secondary climb (usually switch from MPC control to VNAV passing FL100, and sometimes even as late as FL180 at the US transition) and initial descent (until FL180)- I stick to MCP SPD/VS for the rest. Nice job on the posting, keep in mind there is not just one right way; and there are various ways to skin a cat!Best-Carl F. Avari-Cooper BAW0225http://online.vatsimindicators.net/980091/523.png


Best-

Carl Avari-Cooper

Share this post


Link to post
Guest flightgamer

Hi Guys,Thanks, i did my best explaining how you can exclusively use VNAV for descents, i know that you can use/rely on many other ways to descend, i had just seen in another post someone was saying descending via VNAV was hard and they couldn't seem to hold a nice pitch for descent so, i thought i woudld post ;-) now, i want to thank you guys for explaining that to me, because i am a VATSIM'er of 1 1/2 years, so i take as much criticism, or in this case, explanation as i can.... anyways i want to thank you guys for explaining that because i never knew real pilots used MCP for Descent around/below 18000. is it because of ATC altitude assignments, its just easier and less of a hassle to spin the wheel than punch the buttons? thats just my guess. but in either case, i appreciate you guys explaining that to me and everyone else.... i have a few questions tho:Do all real pilots usually flip off VNAV by about 18000 and just fly via ALT/MCP?Do some fly via VNAV?and just explain some things you think we could all benefit from....Thanks again for taking time out to write :)

Share this post


Link to post

Kylan,I agree with both Dan & Carl...I invariably use a mode than VNAV during the decent...especially when flying online & in crowded terminal areas.A wonderful book which I would recommend you purchase is the Big Boeing FMC Guide buy Bill Bulfer. The intricacies of VNAV are fully explored & explained in plain english.As Bill explains...VNAV & the FMC Could be regarded as a 'strategic' tool, it tells you when you will arrive at a certain point in time & space, it gives accurate fuel estimates & many other pieces of info such as predicted T/D etc.The MCP could be regarded as a 'tactical' tool - you have absolute & positive control over what you aircraft is doing...essential in a busy sky when ATC could give you very precise speed & alt instructions...this is the time when you should be 'eyes out' & watching your PFD & NDs closely for conflicting traffic etc.e.g. 'Speedbird 100 cross BNN VOR at & maintain flight level 70, speed 220 kts...'IMHO it is much easier to select V/S or FLCH modes on the MCP & adjust your rate of decent by using the green arc on your ND than it would be to go head down to your FMC & start typing!!!I'm not a pilot BTW!!Kind Regards


Steve Bell

 

"Wise men talk because they have something to say.  Fools talk because they have to say something." - Plato (latterly attributed to Saul Bellow)

 

The most useful tool on the AVSIM Fora ... 'Mark forum as read'

Share this post


Link to post
Guest flightgamer

Cool man, ill look into it, THANKS :)

Share this post


Link to post

I think it's important to keep in mind that the real world FMCs are all that good at VNAV descent either, and therefore require much assistance from the pilot to get it right. This is probably one more reason why most will simply use the MCP to descend once vectored off the plate. It's just too time-consuming to bother with the FMC when the MCP is so much faster and easier. The real plane is also a pain to slow down too, so the PMDG is accurate in this regard in terms of spoiler deployment.I see a lot of complaints on various forums about FMC descents, when in reality, the user is simply asking too much of the FMC, and trying to defy its capabilities.


- Chris Jefferies

 

Asus Maximus VII Hero motherboard | Intel i7 4790k CPU | MSI GTX 970 4 GB video card | Corsair DDR3 2133 32GB SDRAM | Corsair H50 water cooler | Samsung 850 EVO 250GB SSD (2) | EVGA 1000 watt PSU

Share this post


Link to post

I agree VNAV descents seem far too slippery in the PMDG 737 800/900 and using an earleier than predicted MCP/VS method seems a much better approach than the full VNAV descent from t/d. The PMDG 744 handles this much better automatically however. I reset MCP Altitude to 500ft before T/D and leave it in VNAV all the way down - it is nearly always spot on the numbers at 1500 where I disconnect the AT and the 500ft mark where i Disconnect the AP. What is the difference? Is the 744 one simply modelled better? I ask because soon I will have a chance to fly a full motion 737 simulator and I don't want to stuff up the approach particularly as it is being recorded on DVD for the rest of the family to see.regardsGerry

Share this post


Link to post

>I ask because soon I will have a>chance to fly a full motion 737 simulator and I don't want to>stuff up the approach particularly as it is being recorded on>DVD for the rest of the family to see.Gerry - as someone who has had the chance to fly a full motion simulator myself can I suggest that instead of using the FMC you take the opportunity and actually FLY the plane yourself? It is THE most incredible experience the first time you get a chance like this and to just sit back and watch the computer fly the simulator is really a wasted opportunity. Fly it! It's FAR more satisfying to find that you can actually handle "the real thing" yourself. It's much nicer being able to say "_I_ landed it on one engine in a crosswind" than "it landed itself".Just my opinion obviously!


Bill Casey

wpigeon.jpg

Share this post


Link to post

Dan,The trick to the CIVET5 is waiting until ATC gives you your initial crossing fixes (usually 2), if memory serves me right, then just replace those above and below with the ones ATC wants you to fly. Rarely do you get to fly as published on any arrival into KLAX until after GRAMM.Just take out the 2 (and others for non ATC flying) in the FMC and replace them with the ATC restrictions (i.e. 170 for GRAMM), and VNav works just fine.Also; for others, make sure you're not using 'all runways' arrivals waypoints. Sometimes you have too many fixes which causes confusion on some STAR's thus your aircraft is going to be trying to loose to much altitude in too little time.The hardest part about the CIVET5 is actually the top of the approaches where you have 2 miles laterally with 2K feet to drop, so you might want to be close to VRef if ATC isn't screaming at you to keep up your speed.It only took me literally about 100 times to get it down...LOL:D On paper the CIVET5 looks just like any other arrival until you start flying it!In any case the Vnav should work just fine in the decents as the OP states. I think most people are just flying the STAR's to fast.Edit for all: I would say that if they improved the FMC logic for the next 37 series to include better Vnav of multiple ALT constraints, that would be a serious enhancement and a bonus feature, not a bug fix.

Share this post


Link to post
Guest tomahawk_pa38

Slippery it certainly is ! I've used VNAV a lot and sometimes no problems at all and other times - problems. Even though I use real world weather and input forecasts - sometimes it misbehaves. For example recently EGKK - LEIB CORDA2N STAR for landing runway 24. This calls to cross ADX (Andraitx on Majorca) at or above FL120. No problems with winds (000/00) aircraft descending nicely from FL350 passess ADX at FL120 on target. After that a slow descent - so slow in fact that the next thing I notice is that I'm about 10 miles from landing and I'm still at 8000ft with no error messages or any warning like "Steep Descent after XXX etc". Anybody any idea why it wouldn't go down ?Second question, LEIB has an ATIS frequency of 117.8 - how do you pick this up please and is there a way of knowing what ATIS frequencies are stored in FS9 as sometimes the ones on the published charts don't seem to work.

Share this post


Link to post

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  
  • Tom Allensworth,
    Founder of AVSIM Online


  • Flight Simulation's Premier Resource!

    AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!

    Click here for more information and to see all donations year to date.
  • Donation Goals

    AVSIM's 2020 Fundraising Goal

    Donate to our annual general fundraising goal. This donation keeps our doors open and providing you service 24 x 7 x 365. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. We reset this goal every new year for the following year's goal.


    2%
    $705.00 of $25,000.00 Donate Now
×
×
  • Create New...