Avoiding a VNAV Disconnect ?

[rocketfs 2]

Question for John Floyd:Is the PMDG 373 actually capable to fly VNAV approaches?Another real life pilot tried it and he said that the thing went crazy...Thanks for the explanation and tips by the way!Best Regards,Bert.

[downscc 1,327]

VNAV is one of the hot-button subjects in this forum. We even have line pilots on both sides of the issue."Increase speed?" Between LBV and RSW you're decending through the 10,000 transition, it's best to descend at or below 240 kts otherwise the FMC will level off at 10,000 for deacceleration. Otherwise, as I pointed out above, you have plenty of distance for a smooth descent of 900fpm (or greater with tailwinds).Absolutely must get rid of the magenta bug on the VSI if it appears in VNAV mode. I've noticed this bug appears whenever passing the constraint slightly above, for example, 12,020 crossing instead of 12,000 seems to lock it up every time but a 12,000 crossing works as intended. The 737NGX FMC is going to be totally new, looking forward to it with much anticipation.

[zubart 5]

It really does sound like the Vnav bug may be causing the problem for Flaps30. Can anybody tell me where I can get a copy of Tim's tutorial?John Floyd

[dave_paige 11]

>It really does sound like the Vnav bug may be causing the>problem for Flaps30. Can anybody tell me where I can get a>copy of Tim's tutorial?>PM me.

[Flaps30 3]

Dan, I'm not familiar with the "magenta bug". Does it show up when a Constraint is entered? How do you get rid of it? I am familiar with the green arc that predicts where you will be at a certain desecent speed. Thanks. Tom

[Flaps30 3]

John, I just did a search for the tutorial for you but could only find the french version in the AVSIM files. I could not find it at the PMDG site either, only a tutorial for the 747. Try a post in this forum. I'm sure you could find it somewhere. It sure helped me get familiar with the 737, and is well written. Regards, Tom

[downscc 1,327]

The bug appears on the vertical speed tape. It's normal when using V/S mode, if you set 1000fpm the bug will be at that value. It's abnormal when it appears during VNAV mode, usually after crossing a fix with an altitude constraint. When it appears, the aircraft will no longer follow the planned vertical path. To get rid of it, press VNAV button once or twice (twice I think).

[Flaps30 3]

Thanks Dan. I'll remember that. Sounds like disengaging and reingaging solves most problems.:-) Regards, Tom

[engwerda 0]

I don't have this problem with the Level D 767 or the PMDG 744 for fs9 but I do with the PMDG 738. Now sorry if this has been mentioned already but the way I still use VNAV for descents is to go to the the DES page in the FMC and set tgt spd at /280 (enter it like that and ignore the mach number that comes before it). This way it will slow to 280 in the descent to 10,000 before you reach the TOD and hangs on to it down to FL100. As someone else said here putting a wind figure in the Forecast part of the DES page also helps. I usually put in the runway heading and 10 or 15 knots. It seems to help but it does not help below 10,000. It slows to 240 knots OK but by the time I reach 8000ft the speed is back up near 260 knots even with the speed brakes out. I have tried disengaging the AT but to no avail. I also put the flaps out early and then finally the gear but who wants a dirty configuration and a 15 mile final?Any ideas for slowing the beast down below 10,000? I am using FS9 and resetting the speed in the MCP with AT engaged is no help whatsoever.I have not set any crossing restraints in the FMC.Gerry

[Mats_J 2]

Gerry,What do you put in the FORECAST page and what winds do you have facing you? Hope it helps,

[engwerda 0]

Mats,I put in the runway heading and 10 knots. That is not an actual forecast as I set winds to zero in the weather set up - cheating a bit I know but I don't have any weather add ons.I tried a flight from KUL to PEN (nice and short as I have just flown in an A320 with Air Asia on that same route).In the cruise at FL210 the winds were 271/25 HDG 336.In the descent below 10,000 the prevailing winds were swinging around towards head on at about 18 knots. On short final it was on the nose at about 7 knots before it disappeared.One interesting thing. VNAV did a great job down to 10,000 as expected and then it accelerated back to 250 kts at which point VNAV disconnected and LVL CHG engaged. From that point I could leave the AT connected and set a slower speed in the MCP and hey presto the nose came up and the speed reduced nicely with a normal approach and landing made. So, for me from now with the 738 it will 280 in the descent,set a mild headwind in the DES FORECAST page, down to 10,000 via VNAV and then LVL CHG from there on in.I hope that someone finds this useful.gerry

[Mats_J 2]

Gerry,Depending on your route towards the runway and the present winds it might work and it might not work with your technique. If you do not have the winds info *or entered the wrong winds info* you might end up with strange results as you've previously encountered.Another thing you need to know is that you have to manage the speed as the PIC when in VNAV PATH. Speed is not managed by the AFDS. Use speed brakes and throttle (successive presses of F4 has been reported to work to ump the throttle to gain speed, or override the throttle in the PMDG menu).I usually use VNAV down to IAF without any disconnects even when winds are not accurately predicted so it is possible. Hope it helps,

[zubart 5]

I PMed you.If you have sent me a PM back, I can't get on my user profile because something is wrong with it. I've contacted Avsim but nobody has responded yet.Thanks,John Floyd

[Flaps30 3]

Gerry, Just curious, but do you experience the leveling off/delayed resuming of the descent with multiple constraints as mentioned above, and if so, have you tried to disengage-reengage VNAV? It has helped me. Tom

[Flaps30 3]

Gerry,When you enter the /280 constraint on the Descent Page, are you in an Economy Path or a Speed Path descent. I'm under the impression that speed constraints aren't emphasized in Path mode, just the descent angle. My tutorial says to have the plane adhere to speed constraints, you use Economy Speed. Is this how it slows to 280? Thanks, Tom