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Flaps30

Avoiding a VNAV Disconnect ?

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I love the tutorial by Tim M. and like using it to practice certain phases of a flight. One thing I can't seem to nail down is an annoying VNAV disconnect between LBV and RSW in Flight 3. I enter the constraints and descend toward LBV at 315 knots, and it barely makes LBV at 12000. The plane then speeds up and levels off, then starts to descend toward RSW, then I get the disconnect bell before RSW and the "unable" message. I even changed the speed between LBV and RSW to 250 instead of 240 per a suggestion by a forum post to steepen the descent, but the vertical descent gauge drops to the bottom and then it disconnects with an "unable..." message. There is no way it wants to hit the 7000 mark at RSW. Any ideas? I think I am doing everything right. Could someone walk me through this? Thanks. regards, Tom

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Not familiar with the tutorial offhand, but the best thing to do IMO is disregard the VNAV for descent and go manual (MCP) by the STAR, IAP, ATC, and/or mathematical calculation so you hit those constraints.


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Hello sir,Preceeding VNAV disconnect, do you get FMC messages "drag required" ?Tips to avoid VNAV disconnect:- don't use it for descent :)- start descent well in advance.- if you have tailwind: tough! start descent sooner then planned- increase drag: use speedbrakes, increase speed, ... (flaps, although not good for wear and tear)- if your'heavy, it will be more difficult to descent on the descent path and stay at the desired speed.I heard from a real life pilot: VNAV is not often used during descent.Best Regards,Bert.

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Hello Bert,Yes, as a fact in real life it's pretty much ATC who handles descents so I suppose LVL CHG or VS is prefered.Now, I always use VNAV down to around 5000 feet and it works perfectly once you understand it's limitations. It's very simple. 2 ways to do it, you can either stick to the altitudes on the charts(STARS) if you use them or go with the 3.3 rule. In case of a tail wind you can increase it up to around 4 since below 10000 feet it's hard to descend and slow down at the same time.Of course, all this works when FS real weather doesn't go mad with sudden wind direction changes, in that case hit the VS button.Happy landings

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I'm not sure how the NG handles VNAV but I find it a much more powerful tool for Descent then FLCH or V/S In most cases. If you just let the aircraft do it's own thing in VNAV 99% of the time ATC will require you to do something else to what you preplanned. However if you keep updating it with your crossing restrictions eg expect Level at SCUPP 11000 etc the FMC will reduce your workload. This is Realworld or online, FS ATC is too rigid.Rob


Rob Prest

 

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This is an example of why VNAV is not always the best method.LBV to RSW is 27.3 nm; the change in altitude is 5000 ft, which requires a descent rate of 183 ft/nm. At 240kts (4nm/min) that would be 732 ft/min descent - but, the ground speed is actually higher because of the altitude so say ground speed is 300kts (5nm/min) still only requires a 915 ft/min descent. These are relatively shallow descent rates.The problem with VNAV is it doesn't descent through 12000 at LBV but instead levels off and then starts to descend to the next fix after crossing LBV. This causes two things, first a not-very-real-world flight experience and secondly it decreases the distance available to descend to the next constraint.Assuming the transition from level at 12000 to establishing a steady rate of descent to RSW takes 30 sec., now you're only 22 nm to the fix and you need 1,000 fpm. Okay, that is going to work for you except you have all this extra airspeed that you need to bleed off that the 737 can do at 1,000 fpm but not a clean Queen.The concept of going faster to descend quicker seems like a good idea until you consider your goal is not to get a higher fpm descent rate but to get on the descent profile of 183 ft/nm. Now the less ground speed you have the easier it is to stay on profile with a moderate descent rate.My preference is to use the MCP SPD and V/S to control my descents with the VNAV indicator on the ND providing a reference (but I don't chase it).The 183 ft/nm descent profile is actually relatively shallow and should not be a problem if you got the right technique.


Dan Downs KCRP

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Yes Dan,I agree, that VNAV indicator does help as a sort of a reference. With some practice and experience I think VNAV on the 737 works fine. Yesterday with real weather at Stockholm I had to land on RWY 08 with no visibility, for those of you who don't know this a LLZ only runway, I entered the altitude constraints in the legs page according to the DME's, it took me all the way down safely to the threshold. Just perfect.Happy landings

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I've found that the PMDG 737NG VNAV works fine and I use it all the time. It works best if you input the forecast winds on the DES (descent) page, prior to reaching TOD (top of descent).There is a small bug if your descent has multiple altitude constraints. With no intervention VNAV will level off at each constraint and will not continuu descending after passing the waypoint. The simple work around is to simply deactivate and reactivate VNAV immediately after passing the next constraint waypoint. Then descent will continue just fine.


Dave Paige

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I've flown the real -700 for about 7 years. I don't participate in the forum too much but I like to chime in about Vnav subjects. I can tell you that a Vnav Path descent is the preferred method of descent down to about 5000 to 10,000 feet or until ATC starts messing with you too much. Vnav Path descent mode during an approach is now the prefered method, It's safer and more accurated than the old VS dive and drive in. It works great, less work to a point for the pilot and much more accurate, but you must learn to master it. I can also tell you that the PMDG NG's Vnav Path mode is very accurate and works just like the real thing, espealy the first flight to a desdination. It's fine to switch to a VS or LVG mode if you need to , but don't come to the conclusion that Vnav Path descents are faulty is the simulation. I don't see much descussion of forecast winds. Headwinds or tailwaind during a descent will play a key role in FMC Vnav Path calculations. You must go to the Descent Forecast page in the FMC and insert known winds so the FMC can take this information into account when calculating a Path descent. Vnav Path descents take time to master, and even the masters can experience problems and have to revert to a simpler descent mode such as VS or LVG.You've hit the nail on the head you ever you are above. You can actually see when that nasty Vnav Path bug is in control of Vnav. The VSI will show a magenta target symbol. In Vnav Path mode you should never never never see that. Only using VS mode should you see a magenta target symbol on the VSI.Like the guy above said, to get rid of the bug and get your NG back on Vnav Path mode, just cycle the Vnav switch.I'm really disappointed that PMDG has not come out with a fix for this bug yet or a fix for some of the bugs in the FMC that we've been dealing with for many years. If PMDG would fix this bug, there probably would be much less confusion about Vnav Path descents.I look forward to the new NGX. I'm sure it will be another great product!John Floyd


John Floyd

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>>if you have tailwind: tough!Per my observations in my brief PMDG experience of only a couple of months (747-400X), this is the biggest VNAV killer ! Interestingly enough and what a coincidence, the last time the tail wind did it to VNAV, I too was in the LBV/RSW area (FORTL.FORTL4).Zach


Zachary Kapsiotis

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>>if you input the forecast winds on the DES (descent) page, prior >>to reaching TOD (top of descent).Would love to do this. What provides this information ? If it's ATC, does an addon like RadarContact do it ?Thanks,Zach


Zachary Kapsiotis

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'dive and drive...' ROFL, I've never heard it called that before.:-hah I like it!I agree with everything you've said...it's a little bit of an art form in FS sometimes, but for the most part it does work pretty good.Like I've said before, I've even used it to cheat-simulate my way down an RNP/GPS APPR at some airports w/o an ILS...hehe.FYI, I think I heard the crew say they are working on enhancing the VNav for the new NG. Don't quote me on that though.

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I use the forecast winds from AS6.5 or FSFlightkeeper ACARS.Rob


Rob Prest

 

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I use only MSFS generated winds. I have a rough Idea of the winds from surface to 50,000 feet and use that. Understand, it doesn't have to be exact. A rough estimate is better than no estimate. When flying the real NG, I use the flight plan provided by dispatch and take winds off waypoints with altitude restraints. For example, flying into KLAS on the SUNST TWO RNAV star, at FUZZY, it says to cross at 16,000' at 250 KT. It may be important for the FMC to know winds at FUZZY, especially in a fast tailwind to make this restriction. I take the winds at FUZZY from the flight plan, say they are 120/65, and insert this in the descent forecast page at 16,000'. It works every time. The airplane will cross FUZZY at 16,000' at 250 KT. If you don't insert this information in the descent forecast page, then the FMC will calculate a descent path that starts too late. Because it starts a descent too late the AFDS pitches down more to stay on the Vnav Path, and speed will build up possibly to VMO/MMo at which point you would have a Vnav disconnect. In a Vnav Path descent the AFDS stays on the path and the pilot controls speed with thrust or speed brakes. Target speed is just that, a target. The FMC has to some given data to calculate a path. Once the Vnav Path is started, the AFDS makes no attempt to remain at target speed. But, if you give the FMC a good estimated wind to work with, target speed will remain usually within 5 KT of target speed.John Floyd


John Floyd

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Thanks for all the feedback, guys. In response to the FMC message, once or twice I get a "drag required" message in VNAV but not in the example here. I forget the actual wording, but the message begins with "unable to". I didn't enter any wind forecasts because weather and winds are disabled in the tutorial. One question though; if I want to avoid a situation like this will increasing the speed on the legs page between LBV and RSW do the trick? I like the point of the bug in causing the level off before the descent resumes. I will retry this time shutting VNAV off then on again to see if it helps. Thanks again. Tom

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