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VNAV Challenges...informational post

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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

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hi Davethat was really excellent reading, if permission grants id like to copy and print your message and put it into practice!rgds

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No problem. As I told Clayton, if these procedures help, then, by all means, please use them, tweak them (post if you do, please) and hopefully, they will be as successful for you as they have been for me overcoming some of the quirkiness of the T7.Mike Ray wrote a 777 manual for the Brand X T777 (generic for simmers), but the 747-400 manual is written for pilots. Its pricey, but explains things in plain english rather than the technospeak that is contained in some of the flight manuals. Since the T7 and the 400 share the same FMC, the information applicable to one (and procedures) and applicable to the other.I am posting this on the forum so others will also take advantage of this information (why I wrote it), no need to ask for further permission --this applies to all pilots. I did this for you.All the best,Dave LambCat IV Senior CaptainUnited Virtual Airlines

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Well for me on decent the FLCH doesn't work right. If I'm going say from FL380 to FL260 the FLCH has me at 6000 fpm decent. From say FL260 to FL140 FLCH has me at about 4800 fmp decent. Below that it's around 2500 or lower if I'm slowing down. So at high alts I have to use v/s.

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If your recall my post, if you use VNAV above 10000ft, you probably won't have the sink rates you are experiencing. With this said, you need to set up the FMC VNAV modes correctly before departure, since this will make a significant difference in descent rates.One of the problems with FLCH at high speeds is the tendency of FLCH to want to get you to your altitude based on speed. A speed descent will, by definition be a high V/S descent due to the pitch required to maintain the speed. It is for this reason that VNAV is recommended above 10000ft, then FLCH or V/S underneath that. One of the dangers of using V/S is busting through your altitude if it is not set up properly in the MCP; that and the fact that you have no speed control in V/S.Quoting Mike Ray's book again: "FLCH..this mode provides some (speed and altitude) protection. It will fly toward a selected altitude and it will not climb or descend beyond that limit. It has a shortcoming, however. It relies on the airspeed set in the MCP for control and if you are making a large climb/descent, it does not take into account any factors that adjust for altitude. This could drive the airplane into an overspeed situation".FYI, this happens as well in the 757/767; the 777 is a little more problematic because of the power of the engines and its sheer mass. It will drop like a rock if the speed is not well maintained.His recommendation, as is mine, is to use FLCH for climb/descents of a few thousand feet or less. If you are descending from altitude to 10000 feet or stepping down, CRZ, FL240, FL100, then use VNAV and set up your descent profile before you start your descent (in the FMC under VNAV). This should prevent the large V/S excursions you are seeing in your descents.Let me know how it goes.ThanksDave LambCat IV Senior Captain (B767-300)United Virtual Airlines

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That was very informative, Dave. Thanks for the time and effort put into that post. 'Much appreciated indeed. Take it easy, and have a good weekend.-Ely

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Just a reminder...if ATC wants you to start down before your TOD profile, just reset your MCP altitude, then hit VNAV. Here, you want to use VNAV, since it will give you managed descent through the crossing altitudes you have set in the FMC. All you have to do is tell it what altitude to continue to descend to. Below 10000, then flip over to FLCH, since the descent rate will be much less dramatic and speed control will be easier to maintain. Just make sure that your crossing altitudes are set correctly on the legs page and that the 250/10000 restriction (if in the USA) is set in the VNAV DES page. I'm sure there will be more discussion forthcoming, and I'll try to stay on top of the subject.Best to all,Dave

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Hi Dave"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."Assume from this quote that you managed to confirm what Clayton and I were seeing with flap lowered in a VNACV CLB mode? Of course, it does not advance if you do this in other VNAV modes.RegardsJohn

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hi Davejust doing a flight from Gatwick to JFK and i used the FLCH for departure and it worked really well, ha, just as well i used FLCH as i again pressed the wrong F key for FLAP up, but bc i didnt have VNAV engaged all went smoothly and after she was clean at approx 7000 i engaged VNAV and things went greatso from future will be using your method rather than LNAV and VNAV immediately.

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Hi Dave,Nice reading and very insightful. Thanks for the effort to put this together for all here.Its very encouraging to see the spirit of this community where everyone engages to help each other out. I've also observed how engaging real world pilots are in discussing an enthusiasts' opinion and queries. Thanks & best,Jay

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John,That would be correct. This happens on the real 777 FMC, which is why I made the post the way I did. The problem is, we are talking in theory, since the pilots I have spoken to have said emphatically that flap extension in the climb is something that does not happenin real flight operations due to the notch detents on the flap handles. Therefore, the conclusions regarding advancing VNAV to the descent mode is based on logic and some educated guessing. We know VNAV goes in one direction only, CLB, CRZ, DES. If this holds correct, then a flap extension might create advancement to descent mode.The way to correct this is to operate the balance of the climb in FLCH, CRZ will not be affected and once you are in the descent at the appropriate speed, you should be able to engage VNAV after resetting the MCP altitude (for lower).One big suggestion I would make is (if you can afford the spend) is to get the CH Throttle Quadrant. You can then program the flaps into an axis, put enough null zone in it to prevent you from accidently hitting the handle and extending the flaps. This is the setup I have and it worls pretty well. Of course, like everyone else, I sometimes bump it too much when throttling back and inadvertently extend the flaps, but I've learned to be very careful and the instance of accidental flap extension has lessoned considerably.Using the keyboard with buttons, if you are not careful and you hit the wrong button (at night this is a real bear), it is possible to do all kinds of dumb things). This is the reason that I have mapped as many functions to the buttons on the CH Yoke and Throttles. Any thing else I do with the mouse. It cuts down on "hitting the wrong key".I'm still working on getting reverse thrust to work with a joystick axis; I'm almost there...Programming one of the axis on the Yoke (usually prop or mix) in reverse and tying it across with FSUIPC is the best approach. Of course, I'm going WAY off topic here, but some of the things that I am mentioning cut down on the amount of accidental keystrokes that are fatal if you get it wrong. One big one with the 777 is NOT to use the keyboard Z for autopilot engagement/disengagement. Even if I map it to the joystick, it still acts off the FS setup and it just creates havoc with the autopilot.Using the mouse to connect and disconnect is far better, although a tad more cumbersome.If anyone has good suggestions about how to map joystick functions through FSUIPC, I could use them. A lot of what I'm doing is trial and error (mostly error).As I said in one of my earlier posts; VNAV is very mysterious in the way it works. We can only draw conclusions from what we see and from what our pilot friends tell us (which I accept as fact). This is the area in which flying technique is everything, and will likely avoid some of the issues you have seen.Best,Dave Lamb

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Clayton,I'm very happy to hear its working for you. To all other pilots who are experiencing this problem, I think it worthwhile to consider using this procedure vs. what is in the PSS handbook (and the CO flight manual if you are using that for procedures). Arming LNAV/VNAV on the ground is just a bad idea. An additional thought, if you write down the sequence of actions from TOGA through to after takeoff checklist, you may avoid doing something in the wrong order, and increase the chances of vastly improving your climb out performance (and improve your peace of mind). Takeoffs are the most stressful period of the flight for we simmers, since we are doing the work of two people (unless you are using something to command a psuedo F/O).Best to all,Dave Lamb

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Dave, Very many thanks for staying with this - particularly as I have never flown a modern FMC, I find your posts on use of the FMC and the options used by different airlines very interesting. However, as to the problem over flap selection, I think we have our wires crossed. My interest is:1. Is the PSS 777 FMC working as PSS intended?2. If so, is this a reasonable simulation of the real aircraft?In my view, in the case of the particular scenario of selecting flap in a VNAV CLB mode, I believe the answer to both is NO. So I will have a word with the PSS experts for their view. Turning to your post, we still do not know how the real 777 FMC reacts in this situation - if it is not in your book, I guess the only way to find out would be in a 777 simulator or in the air. C'est la vie! Also, appreciate that the 777 has notch detents but that certainly does not stop flaps being lowered in the regimes we are discussing - that's why there is an airspeed limiter which stops the flaps travelling even when selected above a safe speed. Below this speed, there is no reason why the pilot cannot select them if he wishes. Agree that with the detents, they are unlikely to be selected by mistake but, again, if it can happen - it will - which is why we continue to have accidents!As to joysticks, I use X52 - my fighter pilot background - and have no problems with accidental flap extension. Also, all my buttons/axes are mapped using FSUIPC including reverse thrust. I found this easy using the FSUIPC "buttons" tab - let me know if I can help if you are having problems.Spent today with Rick Piper's latest - absolutely superb! Back to the 777 tomorrow.Very Best WishesJohn

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