VNAV Challenges...informational post

[Guest D17S]

Interesting thread. Vnav is one of my favorite topics. There

[FOD 23]

SamAt last!To date it has appeared that the attitude to VNAV has been "its too difficult, we don't understand it so lets ignore it and use the simpler AP modes". And, as you say, some of the comments written have been incomprehensible.Much money and effort has been put in by engineers etc to develop this mode for long range, 2 man crew operations for some very good reasons. So for me, to get the best out of the beast, I would like to master all systems - including VNAV.As to PSS - and I am sure all the other big developers - they also have put alot of effort into getting the FMC right so, again, my aim to to use it in the way it was designed.Indeed, the only problem I have had with VNAV is the flap glitch in CLB mode discussed in another thread. I am sure that this is a minor bug which slipped through the testing net and can be easilty rectified in their next release.John R

[baw716 0]

John,Well, I am a man of my word and when I say I'm wrong I admit it.John, I apologize. I got this wrong. I just heard from one of my pilots who also is a simmer and he told me some very interesting details regarding the FMC and VNAV mode. When I went on to fly the aircraft this am, I got the same thing...no return to VNAV mode; just advanced straight to descent mode and that's it. This is NOT how this works in the real world. We can therefore assume based on his expertise (not mine) that there is indeed several problems still with the FMS, not the least of which is the issue of VNAV.That said, the short term workaround is the procedure I discussed; use LNAV or HDG SEL plus SPD/FLCH above 400ft in the climb and retract the flaps. Once the flaps are retracted, then you can safely go into VNAV and it should behave more or less correctly. Given VNAV is unreliable with regard to flap extension (or possible retraction), I would suggest the same procedures as I have before; VNAV down to the point where you need to start extending flaps, then switch to FLCH, then if you are doing a CDAP approach, use V/S over the FAF (about -800fpm should do it at 140kts) and stay on the a/p to about 100 feet. A/T should come out after the a/p, so perhaps a/p out at 200 feet, a/t out at 100 feet, then retard on landing.The other point: Do not use full thrust on takeoff unless you are at or near MTOW. The power is too much and you'll blow past 200kts before you can hit the FLCH button. I've been using TO2 for flights less than five hours; probably TO1 in the 50-80% MTOW range and TO above that.I will discuss with PSS today about this and see what their programmers have to say about it.Again, I was wrong about this aspect of the FMC and I apologize. I still believe, however, that even with this glitch, the aircraft is flyable. The only major problem is the progress page which still does not work and that is a killer unless you are very familiar with plotting your position and doing the estimates by hand (not a bad practice, but a lot of work during the cruise phase of the flight).I'll be back on after discussing with PSS.Kind regards,Dave Lamb

[FOD 23]

Hey Dave,No apologising - I flew for 35 years and I know I was wrong a lot more times than I was right. That's what flying is all about!But I never flew the new big Boeings and, as I have flown PSS since the original Airbus series, I was particularly interested in the new 777. My aim is to:1. Learn all I can about the beast and how modern pilots OPERATE the aircraft.2. At the same time - if possible - help PSS to improve for next time - that helps us all.As to the FMC vertical modes, to me FLCH and V/S have been with us for a long time and I know exactly what they offer and will use them when necessary. The big difference now is the VNAV mode - it is designed to give the most effective vertical profile and, therefore, to reduce the pilots workload. And - other than this flap glitch - it does seem to work very effectively for some of us. I have just done a full VNAV departure, 14 hr cruise and landing via the published STAR with no problems. I stayed in VNAV the whole time. IMHO PSS have done a great job with this 777 release - lets forget all about the November release - and it can only get better as we learn more about what is actually programmed into the system.Back to this thread - your original summary was a great idea and will help a lot of people. In my mind that is what this forum is all about.Stay in touch and best wishesJohnPS As far as I could see, the ETA on the progress page issue had been resolved. I will have a look again tomorrow.

[Guest D17S]

The crews I work with agree that nothing should be engaged or even armed for TO except the autothrottles. Generally, they will hand fly speed on pitch at V2+ (~) 10 until acceleration altitude then engage the AP in Vnav. This is generally 1000 feet AGL. At acceleration altitude, Vnav will target airspeed away from V2 (+ whatever) to the first speed limit while still in THR REF/TO (i.e., still maintaining a speed on pitch climb). This is usually the existing flap setting

[Guest D17S]

FLCH's rate of descent/climb is based on the amount of descent/climb requested. At < ~ 2000 foot change request, ROC/D will be ~ 1200 fpm. This ROC/D will increase as the MCP is dialed toward > ~ 8000 foot change request. FLCH uses its thrust partner "THR" to adjust power to target a greater of lesser ROC/D. Select FLCH and dial up and down the MCP altitude window. Watch thrust ramp up and down . . . . and the corresponding and resulting ROC/D increase/decrease.Note: FLCH SPD's real deal is that it's a timing issue. FLCH wants to "get there" in 125 seconds. For less than a 2000 foot request, it will maintain a kinder, gentler 1200 fpm climb. It will increase rate by adjusting thrust as the target altitude gets further away. It's thinking "Woah, I better hurry up!" But if it's too far away, it will just max out thrust for a climb -- or pull it all the way back to idle for a descent -- and do the best it can.Give it a try. V/S can still be a better choice. These are all just tools in your box.

[baw716 0]

Sam,Excellent post. You've hit a lot of points right on about the 777s automation. If you operate this 777 correctly, even with the glitch in the VNAV system, you can operate the aircraft in VNAV from T/O to touchdown and it will do it correctly every time. PSS did a great job with the patch.A couple of points just to keep this discussion interesting:While VNAV does wonderful things, it can do one thing that is potentially dangerous: Make pilots lazy. I agree that the use of VNAV on the climb out will give the proper power and speed bug up settings for flap retraction, which cuts down the workload considerably, especially for we virtual pilots, who often do the work of two real pilots.With that said, each airline has written their own set of procedures and they run the gambit from LNAV/VNAV arm on the ground to not using any automation prior to takeoff. Of the carriers operating on this side of the continuum, United is one of the carriers that require pilots to work the takeoff absent VNAV until after flap retraction and in fact, I believe I read somewhere that they do not use VNAV below 10000ft on the climb (different on the descent due to different STAR altitude restrictions). With this VNAV, if you inadvertently extend the flaps (easy to do if you do not have a programmed joystick axis), then there are some real problems that occur; the principal one being that the VNAV mode moves to the DES mode, which makes VNAV worthless for 2/3 of the flight. This can have serious implications on fuel burn and if a pilot is inexperienced and does not know what to do if VNAV fails, things can go bad quickly.This is why I have advocated the use of FLCH to manage speed up to flap retraction. Once the flaps are retracted, then VNAV can be engaged and can be in control (as long as the MCP altitudes are correctly set) for the remainder of the flight. I also do not advocate VNAV for approaches under 5000ft, since again, pilots need to be able to manage this aircraft from the MCP. FLCH is best for making small adjustments in altitude. If in V/S, there is no speed protection so airspeed must be watched like a hawk. That said, the airlines that require the use of CDAP approaches (like United) require the use of the V/S on the glideslope within a certain range (-800fpm in round numbers) on the autopilot. For me, using CDAP approaches has made my handflying better, since I use this approach consistently, but kick out the autopilot much earlier than the 100ft advocated in the procedure. I kick it out at 1000ft, then handfly the aircraft to the runway, kicking out the a/t at 100ft so everything is in my hands as I touch down.Of course, I stress this is one approach, one procedure. If you use CO as the guide, they are totally at the opposite end of the spectrum, requiring their pilots to arm LNAV/VNAV on the ground prior to taking the runway. This way, they have autothrottle control in the climb and the ability to use the FD to handfly to a safe altitude (but even then, CO states that by 1000ft they must have one autopilot running).Thankfully, there are workarounds for the various quirks of the PSS FMC. Our continuing discussions with them serve to help them know what we are experiencing and how to tweak (or repair) in some cases, the FMC to get it to do everything expected of it. While they aren't quite there, at least we have something that we can fly.I, for one, am very happy about that.Best,Dave LambCat IV Senior CaptainUnited Airlines Virtual

[Guest D17S]

All excellent points about becoming too reliant on automation. The pilots I work with actually got a "letter" stating "Fly the airplane more!" And it's not just the AFS. The MD11s have such an automated fuel panel that if something goes wrong, they have problems even finding which boost pump to use. This systems automation theme has its place, but I really like Boeing's philosophy of forcing the crew to press a button every once in a while. Someone familiar with an old, manual DC-8 or 747-1/200 would have an MD11's fuel system figured out by the second glance. . at least I did anyway.All the other Boeing FMC's allow a return to cruise mode by selecting the FMC's cruise page and entering a new cruise altitude. The T7 doesn't? That's odd.There are CDA pamphlets (for this airport and that) scattered all over the crew lounge these days. I haven't looked yet, but it will be interesting to see how they integrate VNAV PATH descents into the mix. The VNAV PATH tool would be just the thing to get to the FAF at idle. Then, I would assume the CDA would end at GS capture. They could use a closer-in approach fix, but I think that GS capture would have to come into the picture somewhere. Gotta look. That'll be my project for next week.Good stuff.

[FOD 23]

Dave"With this VNAV, if you inadvertently extend the flaps (easy to do if you do not have a programmed joystick axis), then there are some real problems that occur; the principal one being that the VNAV mode moves to the DES mode, which makes VNAV worthless for 2/3 of the flight"Yes but only in the PSS 777 and I am pretty sure it is a bug and never intended.Other than that, I haven't found anything else a problem with the FMC system? The latest release seems to work as advertised - my last two flights included descents from FL410 via a STAR into ILS and I stayed in VNAV from TOD to glidepath lock.John R

[clayton4115 56]

John you state there is no problem with FMC?try thisprogram a route into the FMC then you fly this route, then at cruise go an cancel a waypoint which is AFTER your T/D and press the execute button then tell me what happens with your T/D.I bet you you won't have one, you will only have a E/D and your left to your own devices at FL400 to work out your T/D!bug major BUG!!!

[Guest NormanB]

>I bet you you won't have one, And I bet I will ;)The issue you are seeing is one already identified where the waypoints after TOD ONLY include those from a STAR. PSS are working toward a fix for that.

[FOD 23]

'Fraid Clayton lost his bet here as well!As per Norman - deleting post-TD nav waypoints whilst in the cruise causes no problems. I have done it on most long flights whilst sorting out the approach path and tried it again just now.As to when the waypoints post-TD only include those from STAR, I Have now done 4 flights into Hong Kong using the DUMO1A from FL 390 to runway 25L. In each case my last nav waypoint was well before DUMOL. I tried deleting STAR waypoints before the last nav wp and between that and DUMOL.In the first case, I thought I had lost the TD, I reset the runway and STAR and the Td was back; tried again and could not reproduce it - everytime I deleted a wp, the TD was still there.On the three other fights, whatever I did, the Td remained - and I tried deleting at different times and different wp. I have not been able to reproduce the loss of the Td and now I even suspect that the first case was a false alarm - I was expecting it to be deleted and, when it moved out of the range set on the ND, I assumed that it had disappeared.Any comments on just how to reproduce this?John R

[clayton4115 56]

as long as this is rectified in the upcoming patch, good!

[Guest divettri]

I have experimented with VNAV but although aircraft climbs & accellerates & cruises as programmed, when it comes to descent profile, the aircraft fails to respond and hence continues at cruise altitude & speed. Why?Also, what does green "T/D" indicate on EFIS at FMC programmed decent point?

[clayton4115 56]

T/D means Top of Descent the point where the FMC computer calculates the time to commence the plane for descent to arrive at your airport in a smooth descending manner.