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JSACKS

VNAV climb assistance

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I'm still having some trouble managing the climb out phase of the NG in VNAV with total success. I often can engage VNAV fine, at say 2000 AGL, but I sometimes get kicked out of it by the first or second waypoint, at 3000 or 4000' and into CWS during a turn on LNAV. When I program a route, I observe that the FMC usually assigns or suggests speeds of 250 starting lower than I might have imagined and I often reduce those initial waypoint speeds to 210 or 230 as a result. The AP will hold those lower speeds successfully but then suddenly I find I'm off VNAV after having engaged it successfully. I switch to LVLCH then and usually am able to re-engage VNAV OK when I am going somewhat faster.I've noted that in many cases, it is suggested that one fly the route to at least 3000 AGL and flaps up and be accelerating to IAS 250 prior to VNAV engagement. (I've wondered what happens to waypoints one passes on the way up to such a profile. Do they then become inactive or what?) So I still engage VNAV at something below 3000 AGL.Or are my results to be expected in the circumstances and should I not even try to engage VNAV until I am clean and accelerating to 250?Appreciate guidance, thanks!Jonathan


Jonathan Sacks

Dell XPS Gen 4, Pentium IV Northwood extreme 3.8Ghz, 3Ghz RAM, eVGA 7900 GTO,

12 GoFlight modules plus MCP-PRO AP and EFIS, GF pedestal, CH rudder pedals,

CH throttle quadrant, 42" LG LED, 24" DELL LCD, Windows XP, FS2004, FSUIPC 3.96

FS Autostart 1.1 (Build 11), FS Navigator 4.6, UT, FE, GE, REX, PMDG, Level-D, PSS, etc.

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Jonathan,As I stated in another thread (sorry for repeating myself) it helps watch some DVD video shot from a real cockpit and see how these guys manage their climbs and descents. Real life is full of surprises and pilots must use whatever works for them best at the moment. So for example this Virgin Atlantic (747-400) crew flying out of Heathrow engaged LNAV at 50 ft, then VNAV SPD at 400 but very soon they were climbing out in the V/S mode, then VNAV SPD, then VNAV ALT, then .. I hope you get the point. ;-)Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2http://www.precisionmanuals.com/images/forum/747400.jpg


Michael J.

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Jonathan,If you get kicked out to CWS it is probably due to spikes from your controller. I don't think the A/P should revert to CWS unless yoke is moved. Someone please correct me if I'm wrong! ;-)Hope it helps,


Mats Johansson
PMDG Flight Test Dept
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Mats:You might be right. I have learned not to touch the yoke after connecting VNAV (although actually on previous flights, I have done so, gently, with no ill effects and with VNAV still connected). But here's what I think I have discovered in this particular procedure:If I program a SPD/ALT restriction that is apparently unattainable for the NG shortly after liftoff, when I connect VNAV, it often fails to connect at such a time and the FMC scratchpad often says immediately UNABLE NEXT ALT and kicks me into CWS. But if I then suddenly lower the altitude and/or speed restriction in the FMC and execute the command in response to such an annunciation and hit VNAV a second time, VNAVC connects and stays connected OK. I also find that if I simply accept the "predicted" spd/alt data in the FMC (i.e., the small fonts in the FMC on the right side that are generated after completing the PERF INIT and TAKEOFF pages), VNAV also connects normally and stays connected. The reason for this, it appears, is because the predicted constraints are dynamic and because of this, everything is flexible and more attainable. (This is often the case but not absolutely always so.) My reason for posting was because I was experimenting with doing a 180 deg shortly after departing EGLL 27R (or L), trying to simulate a very challenging departure. And initially, every time I connected VNAV, it would connect normally half of the time at 1000-2000 AGL but as I approached the first waypoint, it would say UNABLE NEXT ALT, kick me into CWS and put me into a shallow dive (which I assume was from a joystick signal because the joystick must have been pointed slightly downward--the MS Force Feedback 2 stick does NOT have self-centering).So the results of all this experimentation seem to have shown that if I either leave predicted spd/alt constraints in as generated by the FMC, and if I program attainable altitudes and speeds in the FMC for the NG, VNAV can be connected fairly early on and will "hold" fine. If not, then it's CWS time.What I have settled on doing is programming slightly lower speeds and altitudes for the initial waypoints during climbout and the 180 turn. I will assign for e.g., 210/2700 to a waypoint that has a predicted 250/3100 data in it because that is in fact not necessarily attainable (I guess a lot of this depends on your power settings too and I prefer R-TO with CLB 1 or CLB 2 because things happen a little less dizzyingly). And so on. It works perfectly for me so far that way.Jonathan


Jonathan Sacks

Dell XPS Gen 4, Pentium IV Northwood extreme 3.8Ghz, 3Ghz RAM, eVGA 7900 GTO,

12 GoFlight modules plus MCP-PRO AP and EFIS, GF pedestal, CH rudder pedals,

CH throttle quadrant, 42" LG LED, 24" DELL LCD, Windows XP, FS2004, FSUIPC 3.96

FS Autostart 1.1 (Build 11), FS Navigator 4.6, UT, FE, GE, REX, PMDG, Level-D, PSS, etc.

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