VNAV Challenges...informational post

[FOD 23]

The following was provides by Dave Lamb back in Spring 06. Its one I shall refer to again and that I feel we should not lose when the other forums are deleted.""After having read a number of books (flight manuals, FMC Guides and Mike Ray's sim book -- which I think for simmers should be required reading), I think I have compiled a set of procedures for takeoff and approach profiles which should help overcome the VNAV issue with the PSS777.There are many schools of thought about when to arm and what to arm before and when after takeoff. Is it LNAV/VNAV? FLCH, HDG SEL, and what if I mess up retracting the flaps? Will it change modes? Unfortunately for us, my discussions with two pilots on this subject rendered essentially the same answer, which does not square with the flight manuals (CO especially) or the books on the subject of when, where and what to do.The Heart of the VNAV Problem,Quoting Mike Ray: "If you are changing from a NON-VNAV pitch mode such as FLCH or ALT (for example after a go around), or de-selecting speed intervention while in VNAV mode; and simply push the VNAV selector button, the AIRSPEED COMMAND BUG is likely to slew up or down to some airspeed that has no meaning to the human operator. In most cases, the engines either come on with a sudden burst of power, or worse yet, go to idle and the airspeed starts dropping below thet selected flap speeds.Anyway...NOT GOOD and this unexpected event causes pilots all over the world to start pushing buttons, clicking off stuff, twiddling knobs, and generally getting all excited while trying to regain control of the jet. This is usually followed by some comment like: WHAT THE $@#*&$ IS IT DOING NOW???"So, based on the number of issues with VNAV, I am going to present a procedure which is basically what the PILOTS have told me to do; not the books. Since pilots fly the aircraft and not the books, they have the most experience in dealing with situations, especially those as complex as VNAV.Mike Ray writes in his 747-400 Sim Guide, "the VNAV function is far too complex for a mere human airline pilot to understand, so let's accept the fact that we will NEVER fully understand VNAV. We must be constantly aware of what it is doing and confirm that it complies with what we want it to doing; however, MAINTAIN SITUATIONAL AWARENESS."That is difficult to do if you are futzing around with trying to change the speeds in the CLB, CRZ or DES mode. So, what I am going to suggest is the conservative use of VNAV and more use of SPD, FLCH for speed and altitude control.It is important to note that we are talking about VNAV only, and not LNAV; it is VNAV that will cause your aircraft to do things you do not want it to do if it is permitted to do what it is programmed to do.So, from a procedural perspective, here are the steps, from before you take off to 10000, and from 10000 down.I. Take Offa) before you take the runway, set up the heading you want to fly once you have cleared 400 feet. In other words, if you are going to fly runway heading, then put the runway heading in the MCP. Of course, your MCP initial clearance altitude should already be input as should your speed for the initial climb: V2+20kts.:( do not arm up any flight mode before takeoff. Once the flight director has been turned on and the autothrottles engaged, the aircraft will "track" down the runway and through the rotation and into positive climb. If you are using autothrottle for takeoff (and you should, since it will give you maximum engine control), TOGA will set thrust and hold it through the rotation and initial climb. The critical part after rotation and positive climb is the pitch attitude you will maintain to hold V2+20kts. If you have set up the FMC properly, the flight director will help you hold the correct pitch attitude to maintain the speed dialed into the MCP.c) at 400 feet, engage either HDG SEL, or LNAV as appropriate. If you have a lot of turns in the SID, LNAV is the better choice. If you are on "radar vectors" after departure, you must use HDG SEL, since the headings you will be using after takeoff will be dictated by ATC.d) DO NOT ENGAGE VNAV at 400 feet.e) At the acceleration height or 1000 feet, engage FLCH and autopilot. At this point, you will be in HDG SEL or LNAV and SPD/FLCH. As you retract your flaps, you increase speed in the SPD window, e.g. call flaps 1, speed 200.f) Once you are clean (flaps up and 250kts), then you can engage VNAV to manage your remainder of your climb. Remember, the VNAV mode moves forward, not backward, so if you do something dumb like extend the flaps and you are in VNAV, it will advance to the descent phase...and you cannot go back.Once you select VNAV and you are given higher altitudes, change the MCP altitude then push VNAV again. This will reset the FMC to the increased altitude value. VNAV will not override what is in the MCP, so even if you have a 290kt climb, if you do not set the altitude above 10000 feet, it will continue to hold a 250kt speed (because of the speed restriction that is set in the VNAV CLB mode).Another point: if you are not doing step climbs, it is almost better not to use VNAV at all. FLCH is preferred for altitude changes, as it provides better speed protection than VNAV. VNAV is based on a complex formula, FLCH is simple. It will adjust the pitch of the aircraft to hold the speed you set in the MCP up to the altitude you enter. II. On descent:Once again, if you have set crossing altitude values, then VNAV is very helpful in achieving those speeds and altitude values. However, if you have to deviate from that at all, then use FLCH, especially if speed limitations are an issue. If you are in descent mode and you have to go back to VNAV, then simply enter a new MCP altitude and hit the VNAV button. It should recapture your crossing altitudes, adjusted based on your current altitude in the FMC (check the legs page for the impact of doing this before you go back to VNAV from FLCH).On approach and landing: Most pilots do not use VNAV for landings, even if it is recommended by Boeing. Reasoning: Control. If CDAP approaches are required by the airline (United is one of them), then V/S is used for the approach from just outside the FAF to the DA or 50', depending upon the situation. In this case, your SPD is set, flaps are set for landing, and you fly a vertical speed approach not exceeding +/- 300 feet of the required vertical speed to achieve crossing the TDZE. For most 3 degree landings, this is -800fpm at 140kts. You should be on autopilot down to the DA; however, for UVA checkrides, you will be on the FD only...so set up your approach, have it set in your head well before the FAF, because at the FAF, you should be in to your -800fpm profile. Flying on the FD with the needles in the crosshairs is the best approach, with managed speed, all you need to do is stay on the needles and you should be able to fly yourself down to the DA or 50'. Remember to disengage autothrottle before touchdown, so you have reverse thrust control, otherwise the a/t will try to hold the SPD set in the MCP. I usually do this at 500 feet (if on the FD only), since once the a/t is released, the throttles will stay where they are until you retard them.It is my opinion that most pilots, at least the ones I've spoken to dislike VNAV unless flight economies demand step climbs.One of the problems with not using the "arm VNAV on the ground approach" is the fact that if you do not use a derated take off, most of the time, the aircraft will accelerate so fast that before you can think, you are beyond V2+20 and playing catch up with the SPD control. Part of the reason that the flight manuals and some airlines advocate this approach is simple: Using VNAV for take off automatically controls the speed, pitch and altitude through all phases of the flight under which it has control. That said, its almost like you can "set it and forget it", since at 400ft, VNAV will kick in and manage your acceleration and pitch up to the altitude set in the MCP.There is one very big flaw in this thinking: the "set it and forget it" approach is a very BAD HABIT to develop. If you make a mistake, say accidently forget to pull up the flaps or the landing gear, it will do things you may not want it to do and you may find yourself wondering why something is happening when your attention should be concentrated on FLYING THE PLANE!!This is a debate that will rage on for sure, since many airlines and flight manuals advocate this approach, while many line pilots actually don't like this approach at all. So what is best for you?My advice is to listen to the pros. Maintaining situational awareness while flying is critical and to do that, you must have control over the aircraft, even in autoflight mode. While VNAV will disengage if you do something contrary to its logic, it may not show up on the FMA and before you have figured it out, it may be too late to do anything about it.So, in closing, basically, VNAV is for long range cruise when situations dictate step climbs and descents to manage fuel economy. Other than that, using the active modes in the MCP gives you greater control, and when flying by yourself (as we do most of the time), we need all the control we can get.Acknowledgement to Mike Ray's Unofficial Boeing 747-400 Simulator and Checkride Procedure Manual (there is not one for the 777 yet and since the 777 and 747-400 share the same FMC (with some minor differences), the applications are still the same.I am sure we will have lots of discussion on this point, but if you are going to post an alternative procedure, please state the source of your information...if the information is company confidential, please so note on your post. That will help us to understand your company procedures (if you are a pilot or dispatcher), if you are a virtual pilot only...the source would be helpful so we can check out the procedure variance to see for ourselves if your procedure works better (which it very well might).Remember, I am only suggesting procedures based on what I have been told by two real world pilots, plus information gleaned from different FMC Guides, the Sim Procedure Manual and two 777 flight manuals I have in my possession. This does not make what I say "Gospel". There are other procedures that are totally fine. I'm just suggesting the ones that will keep you out of trouble as you fly your climbouts and landings...and is one man's approach.I hope this provides some guidance and insight into the complexities of the VNAV system on these aircraft. At a minimum, it should make things a little easier for the pilots who seem to be having such difficulty with VNAV on the PSS777.Kind regards,Dave Lamb""John R

[Brian Doney 127]

Hi John.One problem I've always had with Mike Ray is his explanations and instructions almost always seem to imply that the pilot/simpilot is just too dumb to understand the actual concepts at hand.For instance:"Quoting Mike Ray: "If you are changing from a NON-VNAV pitch mode such as FLCH or ALT (for example after a go around), or de-selecting speed intervention while in VNAV mode; and simply push the VNAV selector button, the AIRSPEED COMMAND BUG is likely to slew up or down to some airspeed that has no meaning to the human operator. In most cases, the engines either come on with a sudden burst of power, or worse yet, go to idle and the airspeed starts dropping below the selected flap speeds."Complete rubbish, IMHO, that can only be true if the pilot in question has absolutely no concept of how VNAV works, and how to monitor it's current state. I actually have to wonder what aircraft he's actually talking about here, because some of the above CANNOT happen under VNAV control.I've never been able to understand why, rather than explain reasons the speedbug could, depending on the situation, slew away from current speed; rather than explain how to monitor the situation so as to always know what to expect, he instead pushes the disconnect even further by advocating unnecessary workarounds that only add to the workload, and effectively remove one of the most powerful tools on the flightdeck from the pilot.The whole post seems to say, "Oh, don't worry if it doesn't do what you expect, you couldn't understand it anyway...", which I totally disagree with.Look forward to any feedback.

[FOD 23]

Brian,The original post was an honest attempt to help those who were new to the FMC - it was generally very well received at the time and set off a long and seemingly sensible discussion on the subject of VNAV and Mike Ray's approach. Certainly no one suggested that it was complete rubbish - which is why I wish to refer to it in the FAQ that I am drafting.I do not know Mike Ray and I have none of his books. However, I am sure that your post {and anything else that you can contribute to our knowledge of VNav in the real world} can only help anyone reading this up for the first time. RegardsJohnPS Did you receive my reply to your e-mail; I was hoping we might eventually take things forward.

[Jim Harnes 13]

To add some perspective to Dave Lamb's post, the procedure he recommends was vetted by the senior T7 captain from a major US airline. His comments are in the original thread signed as "Leo".Works for me :).Jim Harnes

[Brian Doney 127]

With all due respect, there is a significant number of pilots that do indeed have a very difficult time coming to grips with many of the concepts in the above post.It is ABSOLUTELY understandable why an airline might adopt SOP's that help address this, and even without that, why a pilot would use the tools available to him/her in a way he/she is most comfortable with.I knew Leo A.(UA T7,retired now I believe) well, I say it that way as it has been years since we last chatted. I used to pick his brain when I had the opportunity about some of the very issues raised in this thread.It isn't the idea that the workarounds are or are not effective that I take issue with, more that it's a disservice, to both pilot and aircraft, to just let it rest there. The discussion never seems to continue.There's way more to the story than "VNAV is complex"...which in and of itself is very true. It is NOT an impossible riddle, however; let me tell ya, if I can get it, anyone can. It is entirely possible to have all VNAV functions work for you, in a stable, predictable manner. If one still decides to use other, more comfortable methods, to achieve whatever goal, great; but to just write it all off as too complex is selling yourself short, IMHO.P.S. - John, I have something for you re: the 757 and problems with it's lackluster pitch authority while accelerating in any speed controlled climb. Please, e-mail me at bjdoney(at)comcast.net and I will get the modifications to you ASAP.

[FOD 23]

Looking at these posts in more detail, I think there are good old coms problems here.As I understand it, Mike Ray's guide was an attempt to explain the VNav system to newcomers to FS and the FMC - it is not the kind of manual with the level of knowledge which a rw 777 pilot would be required to study when he was going for his captaincy. Further, I suspect that the quote Brian highlighted was a bit of poetic license on Mike's behalf - to impress the FS newbie with how difficult the subject was. It certainly doesn't help anyone's understanding of VNav. I totally agree with Brian's fourth para - and if I saw the speed bug move to a new setting for no apparent reason I would find out why very quickly.To close this - I would say that Mike Ray's point about situational awareness is the nub of the whole discussion. I suspect that SA is as important in operating a 757/777 cockpit as it is to a fighter pilot in a many v many combat - and you will not be able to maintain SA in a 777 cockpit unless you know how the VNav programme works and what it is trying to achieve.John RooumBrian - thanks; e-mail on its way.

[Guest Mike Phyrio]

I've never had any real trouble with VNAV....other than the old saw about it not making alts when it was supposed to.... Meh, you work around it.I fly exclusively online with VATSIM so oft times, there's controllers to contend with.Lettuce say I'm taking off from KLAX 25R on my way north. I'm gonna get a rwy hdg, init alt (usually 5000) and a squawk. So, I sets me alt for 5k, VSPEED for 2500 or 3k, whatever I happen to feel like, my speed for 250 and take off. I hand fly the thing (in this case, the 757) up to whatever altitude I'm at when I've finally got the gear up and locked and one click off the flaps, then switch on the AP. Depending on the traffic, the controller will clear me for another alt. I set that alt and kick back. Sooner or later, he'll clear me for my final alt. This is where I turn on VNAV and I have yet to be disappointed in it.I don't even bother with FLCH because.....I mean, what for?Coming back down is pretty much the same deal. I use VNAV to get down to the inintal descent alt and then alt and VSPEED the rest of the time down to the ILS. That, of course, is where the fun begins.I still love the 757!!!!!! Yay it!!!!

[Brian Doney 127]

"I don't even bother with FLCH because.....I mean, what for?"Well, selecting FLCH at THR RED altitude will select CLB thrust, and will also give FD pitch commands for current IAS. Seems to me to be a bit less work.But lets not confuse things too much here, as there are indeed some differences between the 777, which this thread primarily addresses, and the 757.

[kapapilot 0]

The Problem I have is the preparation before the flight, I am confused on setting up the cdu for vnav, I get the vspeeds in I just don't understand how to setup the aircraft to use vnav during the flight. If anyone could explain the setup process, that would really help me out.thanksAnthony

[FOD 23]

Anthony,You put this same question a couple of days ago in another thread and I replied to it. See below.Could you please stick to one thread for one topic as it is v difficult to follow your problems as you move from thread to thread.If you cannot follow the tutorial flight, please explain just where the problem lies.John Rooum

[kapapilot 0]

I'm sorry John. I just went to this thread because I am studying for a 777 checkride and a member of that airline directed me to this forum. I appologize. So first I do not understand what vnav really does. Second, I am unsure of which steps in the tutorial are related to vnav.I appreciate your help and I once again appologizethanks JohnAnthony

[Jim Harnes 13]

Anthony,I pointed you to this thread as it gets into the details of operating the PSS T7. Don't want to cut John (FOD) out of the picture, but if you'd like to have a discussion about how VNAV in the PSS T7 operates, I'll be happy to talk with you in the UVA addon aircraft forum. As you know, help from the real world is also available at UVA.See you there :).Jim Harnes

[FOD 23]

Anthony,Don't apologise - just stick on this thread we can try and get you to your check ride.VNAv = Vertical Navigation. = Generically seems often to cover all vertical FMC/MCP height modes {ie VNAv, FLCH etc} = Specifically is the FMC mode which directs the aircraft's height to provide the most efficient operation on a given route. To do this, the FMC takes into account many factors including aircraft weight and payload, aerodynamic and engine performance, route, height / speed restrictions on the route etc etc. Note that these are the items input by the crew; see pages 21 - 33 of the 777 tutorial. But this tutorial is solely designed to help you get the PSS 777 airborne - it is not a full guide to VNAV.VNAV itself is a complex programme and a full understanding will take some research. Try the follwong:1. AVSIM forums - there are many threads on our Boeing forum, on the PMDG forum and on the generic forums whcih discuss VNAV2. Many of the VA have excellent web sites with briefing on subjects such as VNAV. Also if you are contemplating a check ride, you should talk to your Training Captain.3. Do not forget our old friend Google. A quick search found this:http://www.ae.gatech.edu/people/jpclarke/c...ions/Miller.pdfFinally, nothing can replace practice in the air {or simualtor}. Don't rush straight into long haul routes just because its a 777; fly several short routes {1 -2 hrs} looking at how the VNAV is used in the different profiles which ATC and/or aircraft performance restrictions impose.Good LuckJohnEdit:The above crossed with Jim's post.Jim: If you can get to him one to one in a similar time zone I am sure that you could get him up to speed far more quickly than I can at the other side of the pondBe my guestBest WishesJohn

[Jim Harnes 13]

John,Roger that and Happy New Year!Jim

[kapapilot 0]

Thanks for the help JohnAnthony