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Jambo Buana
15th Nov 2006, 15:30
Could someone with access to "my boeing fleet" try and see if there is any progress or info on what VNAV is doing these days? I know that these constant descent gradient features are in, but heard lots of airlines are not liking it. Are there any updates to make it better or is the plane still slowing to flaps up speed after passing first at or below wpt (sometimes)?
Thanks:ugh:

Rainboe
15th Nov 2006, 16:04
This has been mystifying me- you have to watch the darn thing in the descent because the command speed bug suddenly winds back on you. So it's an 'at or below' waypoint glitch in the software is it? I have seen on the descent page it can suddenly think the flaps are extended, and could that be why it does a panic slow up?

MorningGlory
15th Nov 2006, 16:10
Yep still doing it..

Rainboe - No, on a vnav 'path' descent if you put in a restricting altitude due to terrain/airspace etc (ie therefore making you high on profile) the fmc commands a slower speed to cover less distance while you are high, but will not command less than clean speed, and then cocks up your profile..

Seems to be taking ages to fix it.. In the meantime good old fashioned dme, vs and level change will have to do.......

stator vane
15th Nov 2006, 18:07
it is good to program and use for an aid, but for all the years i have flown the 737--10,200+ hrs total and 8200+, the vnav descent has always been quite useless. i use v/s or level change to match the projected descent rate. the winds and temps change too much during the descent perhaps.
we should make the aircraft do what we want it to do when we want it to do it and how we want it to do it. how ever you wish to do that is up to you.

BOAC
15th Nov 2006, 18:23
Seconded - 737 VNAV is the classic 'what is it doing now' function and is not reliable. I see F/Os programme all the winds/QNH/etc into the box and it STILL screws it up. I prefer to screw it up myself:) . I can reach the 'barber's pole' just as well as it can.

Rainboe
15th Nov 2006, 20:48
The 747-400 VNav, so similar it's almost identical, works pretty well. I hardly ever found it gave inaccurate information, yet the 737NG VNav can actually start giving some very erroneous information. It always miscalculated flying to Leeds from the SSW, saying things like 8,000' high when 80+ miles away and descended early to 20,000'. I could never establish WHY- even putting in direct CF with no intermediate constraints, it would still miscalculate, then later in the descent recompute it and come up with the right answer. Never saw this on the 747.

FlyTester
15th Nov 2006, 21:39
Could someone with access to "my boeing fleet" try and see if there is any progress or info on what VNAV is doing these days? I know that these constant descent gradient features are in, but heard lots of airlines are not liking it. Are there any updates to make it better or is the plane still slowing to flaps up speed after passing first at or below wpt (sometimes)?
Thanks:ugh:

It's called U10.7 and it's coming soon to an FMS near you.

:oh: That's all I'm at liberty to say. :oh:

CaptainSandL
15th Nov 2006, 22:05
Here are the changes to VNAV in U10.6:

1. Improvements to Basic VNAV- U10.6 software incorporates several improvements to address in-service VNAV operational issues.

The “VNAV DISCONNECT” message will not be displayed when VNAV transitions to a missed approach. The message will continue to be displayed for other VNAV disconnect situations.

All automatic switching to speed descent mode have been removed. Speed descent mode can only result from crew selection of SPEED prompt on the DES page. Note that when the optional Common VNAV feature is enabled, the SPEED prompt is not displayed on the DES page.

Path Descents will always be available as long as there is at least one waypoint in descent. Prior to U10.6, Path Descents would be disabled once the END of DESCENT point was passed even though additional descent waypoints may be in the flight plan.

During a point to point Geometric Leg in descent, VNAV will always track path, even if SPEED in descent is active when the leg becomes active. In this case, the descent mode will automatically switch to PATH.

2. Common VNAV Option – U10.6 FMC software adds an available option that can be enabled by FMC OPC to set the FMC to use VNAV SPD as the speed on elevator mode for over-speed reversions instead of disengaging VNAV. This option removes the selectable SPEED descent mode and the PATH or SPEED designation from the FMC CDU descent page titles (for example ACT ECON DES instead of ACT ECON PATH DES).

The benefit of this option is that it uses VNAV SPD as the speed on elevator reversion mode for path descents with un-forecasted tailwinds to prevent over-speed conditions and provide commonality with other Boeing model FMCs. Previous 737 FMC versions disengaged VNAV for these cases.

3. Geometric Path Descent Option Changed to Basic – U10.6 software changes the basic descent mode to Geometric Path Descent. Geometric Path Descent was an optional feature in prior FMC software, activated by FMC OPC. U10.6 now supports Geometric point to point path descents as basic in lieu of “stair step” descent paths. This change provides commonality with other Boeing Airplane models and allows VNAV to be used for approach operations at non-standard temperatures.

This is the constant descent gradient feature that Jambo mentioned but it is only applicable on approach, not for the whole descent.

Notice that the Common VNAV Option (speed reversion) is only an option, so your airline may not have this feature activated.

For me the main problem seems to be the transition from the NG’s higher cruise Mach number to the descent profile. It does not seem to anticipate the ToD or the programmed descent Mach/IAS very well. The Classics certainly seemed to cope much better.

Personally, I hope that U10.7 comes soon and contains some big improvements to VNAV.

S&L

Jambo Buana
15th Nov 2006, 22:55
Thanks SL. Having real trouble trying to teach this stuff as it seems to keep changing without the airline giving any info or FCOM2 either. Boeing flight training dept need a kick up their backsides for not publishing better info on this. As we can see in the rest of this thread most pilots have given up on the VNAV which is simply because its sooo badly taught, FCOM thin and poor in general!