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

[Guest captbulldog 2]

VNAV is rarely the problem for me. I'm often impressed by it and I usually take it out of VNAV under 10k.However, the guys on the ground doesn't seem to want me to and assigns step des.. annoying!

[Reset XPDR]

VNAV all the way down to the ILS approach for me, and all the way to the dirt without ILS. Yeah, its a bit temperamental with the regular "DRAG REQUIRED" messages, but I find that by the time I get the dress and makeup out to put on, the message has usually gone away ;)Gary

[Guest meyomyx]

Bing bong!Good one :)

[guy]

Initially I had also problems with Vnav. But now I use it all the time for my off-line flying.The problem i had is that with Vnav I arrived always a bit high on finals (1000-2000 feet or so) and vnav disconnected. The Pmdg Vnav calculates a descent path ,but if you have a sharp turn on finals, it does not overfly the waypoints (this is ok), so the real flown distance is shorter. I think the problem comes from there.It should be able to fly the path it has itself computed.What I do know is that I diminish a bit the height of the final approach point (by 1000-1500 feet) and with that I can use Vnav/Lnav until the interception of the ILS. Some times I use also Vnav/Lnav until touch down (I think they call that an Rnav approach).If you go to the PPrune forums you can see that real pilots have also much discussions about Vnav. It is very instructive.Guy

[mauriceb]

>If you go to the PPrune forums you can see that real pilots>have also much discussions about Vnav. It is very>instructive.I visited the PPrune forums and what I found was almost a bit scary. These are real airline pilots kind of arguing about how things work. On the one hand, it made me feel good about my own ignorance about various procedures, on the other hand, I'm now wondering how many other things are the real pilots not really sure about :-)I'm enclosing a copy of a few of the replies in answer to the question below. Remember, these are real airline pilots talking: (If you want to see the whole discussion, you can find it at:http://www.pprune.org/forums/showthread.ph...&highlight=vnavThe question was: "Just wondering if the 737NG goes into Thrust Hold mode if the N1 button is used for takeoff rather than the TOGA switch/s.Can't seem to find an answer in the Boeing Maintenance Manual D&O."Here are some of the various answers:"Here is how I understand it:Try to push N1 before you push TO/GA and nothing happens

[crvm]

its a bit temperamental with the regular "DRAG REQUIRED" messages, Well of course that's relative to your speed. It does not just display DRAG REQ because it's "temperamental" but because your speed is at a point that triggers the message..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 |

[guy]

I Used Vnav/Lnav now to fly the famous approach to the 05 runway at Funchal/Madeira (LPMA). It has a 180

[Qavion]

"If everyone knew this already, my apologies for chiming in. I just saw that NSEU wrote using the N1 button INSTEAD OF TOGA. There is no substitute for the TOGA switches for initiating a takeoff."The scary part is the poorly written (ambiguous) real world manuals...In defence of NSEU, the Boeing Manual says..."N1 Mode The N1 mode is used during these flight phases: Takeoff Climb Maximum thrust go-around. In N1 mode, the A/T controls thrust to the EEC TRA target, calculated by the EEC for the equivalent FMC N1 target. The N1 mode can be selected in these four ways: Pilot manually selects N1 mode from the MCP DFCS requests N1 mode when DFCS is engaged TO/GA switch is pushed for takeoff (on ground) TO/GA switch is pushed a second time during reduced thrust go-around (in the air). When DFCS is engaged in VNAV climb or LVL CHG climb, the FCCs command the A/T to N1 mode."If the N1 mode can be used on the ground, and N1 mode can be selected manually from the MCP, then wouldn't you assume that N1 could be used for T/O?One of the reasons I got involved in flightsimming forums was the interaction between and sharing of information from ALL areas of aviation. Forums like PPRuNe and AVSIM bring together these kinds of people.Not everyone knows everything about Boeing aircraft (not even Boeing).Rgds.Q>

[Guest slats]

We need to realize that both methods are used in the real world. Where VNAV is especially useful, is when one is cleared to decend via a published instrument arrival. In that case, VNAV does an excellent job. case in pint is the KORRY3 arrival into New York LGA.Burley Stinnettslats

[cowpatz]

VNAV is the primary descent method. To not utilise VNAV is like kick starting a motorbike when it is equipped with an electric start or manually starting the aircraft engines instead of using autostart.The main reason for VNAV problems stems fom a lack of understanding of how it works. Believe me when I say this happens in the real world too.There are a few ways of making sure VNAV functions well:1. Use a reasonable cost index (100 to 150) for the descent or program in a middle of the road descent speed say .84 to .86 transitioning to 280 to 320 knots. If the cost index or speed is too high then there is very little speed margin left for the aircraft to increase speed to regain the path. It needs to be able to increase speed above the programmed speed to increase drag to return to the computed path. Likewise if the speed is too low then the aircraft will not be able to reduce speed to reduce drag to reagain the path because it is most likely already at the min drag speed. The only option left is for it to increase thrust or depart the path. Think of the VNAV path as exactly that.....a computed vertical pathway. The only options available to the autoflight systems to maintain the path are to increase or decrease speed or use thrust. As long as the PFD annunciates VNAV PTH then you are on the path. If it reverts to VNAV SPD it means that the autoflight system could not adhere to the path within the programmed speed excursion values. It means YOU need to do SOMETHING.....increase the speed via the MCP speed window, increase drag with speed brake or increase thrust etc depending on your position relative to the path. You do not necessarily have to change modes, although at lower levels it might be easier to select another descent mode.2. Make sure the the programmed altitude restrictions on the descent basically conform to a 3 ml per 1000ft profile. Do not expect VNAV to handle something like a 14000A restriction at a waypoint with only 20 track miles to touchdown.3. Likewise make sure the speed restrictions you place against waypoints is reasonable. To expect it to lose 50 kts in 5 miles is not reasonable and will require pilot intervention.4. Enter the forecast descent winds. For FS9 users without Active Sky knowing what these winds are can be an issue. Just try entering something in there that is reasonable. If anticipating icing conditions then enter this on the Descent page. Even though it requires a pilot entry of the altitude altitude at which anti ice will be used (higher thrust setting will alter the path angle) it is sometimes preferable to enter this as an altitude band value rather than an absolute altitude value. For example you might expect to enter cloud and at an ice forming temperature at say 26000 ft. If you entered 26000 as the icing altitude in the FMC then it will assume anti ice will used for the remainder of the flight from 26000ft and build a profile accordingly. However the cloud might stop at 20000ft in which case the profile should only be vary between 26000ft and 20000ft and not all the way from 26000ft. The band here is 6000ft whereby anti ice is forecast to be used. Therefore enter 6000ft as the altitude. The profile will then be built with only 6000ft worth of anti ice and not 26000 ft's worth. Not really a biggy but might just assist with staying on the path without too much pilot intervention. After all those last crossword, Suduko entries need to be filled in right!!!Just because VATSIM ATC clears you to descend to XXXXft does not necessarily mean that you have to descend there now. Approaching TOD set the MCP to the cleared level. If you are unsure if he means now or at pilots discretion, then ask. If he wants you down NOW then either select DES NOW (if before TOD) or VS. If you use DES NOW be prepared to add a little thrust as this simulation tends to lose it a bit here and go spearing off down for terra firma. You should descend "within the normal descent characteristics of the aircraft" but not less than 500 fpm. Watch the descent path indicator or look at the vert path value on PROG pg2 and see what the closing trend is. When back on the path reselect VNAV if you used another mode.As an example of where VNAV is useful take a look at the CIVET arrival into LAX. This is a STAR that I fly regularly in the RW. VNAV works well here as the procedure has many descent restrictions and can be a challenge to meet on occasion. The secret to flying this well is to make sure you stay on the path. If you do then you can be assured of meeting all the descent requirements. When on the STAR and on the path the MCP altitude window can be set to the commencement altitude of the ILS approach (7000ft) as long as you stay on the path. Of course ATC will speed you up and slow you down for sequencing. The secret here is to keep updating the descent speed in the VNAV descent page. If you had 280 kts programmed in accordance with the STAR requirement and ATC say 320 kts then enter it as the new descent speed. Whilst the FMC is recalculating the new path you can help out. If you are given a slower speed than you originally programmed then you know that you will need drag (sped brake) in order to A slow up and B get down onto the recalculated path which will be shallower. Conversly if asked to speed up then put power on because you wil be below the newly computed path. Once the path reappears then fine tune your actions.Generally when under a final approach controller I will use FLCH or VERT SPD because I can control my descent and speed more positively and I dont need to have my head down reprogramming the FMC. I can refer to the VNAV path indicator as a reference as well as looking at the distance to go to touch down on PROG page 2. I will previously have programmed the FMC with my anticpated speeds and track etc beforehand and updated any ATC track changes with use of the DIRECT or INTERCEPT CRSE to functions via the LEGS page. This will also update VNAV. As long as I am close to the LNAV track then the vertical track guidance will be close enough. If I am conducting an approach requiring VNAV then I can reselect it when approaching the required commencement point.Give these a try and see if they improve the VNAV experience for you.

[Guest Erups]

Select the new FL, switch to LVL CHG and fiddle with the speed.In LVL CHG speed is the "handle" you move to obtain what you need.Then the green arc will move showing the point at which you will reach the selected FL.You can fix the throttles, use speedbrakes, drop some flaps...I actually don't remember if switching to LVL CHG disconnects the SPD mode engaged, as it would be pretty useless to have the A/T command an increase in throttle, if you need to retard to have the AFDS descend...Anyway just experiment a bit.The key is: by deciding the SPEED the plane has to maintain, and basically not allowing it to maintain that speed by engine means, you force it to eighter climb (if speed is lower then actual) or dive (if it is higher the actual).The plane will eventually set on a steady path in which the speed is constant at the defined value.Pretty smart way of flying in my opinion.You just have to get used to it.Edit:didn't realize this was so old....until i actually reread the asking post :(well, at least this info is now public :D

[Krister]

>As an example of where VNAV is useful take a look at the CIVET>arrival into LAX.NOOOO, not the Civet arrival to KLAX!!! I did that on Vatsim last weekend with lots and lots of ATC and traffic and I totally messed up!!! I actually couldn't sleep for an hour since I was going through in my head what I should have done!!I had anticipated due to lack of better understanding an 24R ILS approach and had everything set up. Then ATC gave me the 25L instead so I activated that approach from the STAR page in the FMC. But the approach points for the 24R ILS was still there, couldn't get rid of it easily so I had lots of magenta lines all over the ND. I frantically consulted the charts to get the heights right and ended up handflying the whole vertical and lateral approach from CIVET.I sure wish I had VNAV properly set up then! :) Anyway it was a great flight from KDEN with some 11-12 controllers! And this is of course totally off topic. Seing the name CIVET just triggered my emotions again! lol