View Full Version : Descending from VNAV ALT?
turnandburn
29th Jul 2010, 01:44
Does anyone have any manuals/books/articles/info for the parameters and trajectory for when the 747-400 will descend from VNAV ALT.
This is not a question of if, as already know it will but have not found the criteria in which it will.
Or any detailed reference about VNAV.
I have the FCOM so something else.
Apologies FROM vnav alt, long duty last night.
Intruder
29th Jul 2010, 03:04
It won't.
To reach VNAV ALT, the altitude was constrained by the MCP altitude setting, and the FMS put the airplane effectively in "Altitude Hold" on reaching the constraint. You have to enter a new MCP altitude and push the altitude setting button to break out of VNAV ALT.
turnandburn
30th Jul 2010, 07:55
It does but only in very few situations. Watched it do it last night during an approach but could not define why it differed from another approach where it just remained level.
an3_bolt
30th Jul 2010, 08:18
Watched it do it last night during an approach
...possibly you were in Vnav Alt, intercepting the descent path inside the capture region for vnav approach logic, with the MCP set below current vnav alt altitude....:ok:
Wizofoz
30th Jul 2010, 12:58
Nope. Still won't leave Vnav Alt in that situation.
The only thing i can think of is G/S capture. Was this an ILS?
Watched it do it last night during an approach - I'm not sure what you are asking about. When did it 'do it' and what was the 'VNAV ALT' and MCP ALT setting? What exactly is a 'trajectory'?
As far as I know, and I am not a novice on the B744,but stand to be corrected, A/C only will descent out of vnav alt if locked in approach mode, to descent out of vnav alt mcp alt has to be changed and button pushed. Then A/C will start descent only if there's no alt constraint in the FMS limiting the descent. In constraint case mcp pushbutton has to be pushed untill FMS alt is lower then mcp alt.
Automatic descents after alt. constraints will only happen if A/C is in vnav path, as done in non prec. approach.
Greetings.
No references just by trial and error.
If you can reprogramme the FMC to redraw the vertical profile to show you low on profile with a lower alt set in the MCP while in VNAV ALT it will go to VNAV PTH as you join the profile from below.
Eg. Descending with HOLD & VNAV PTH say to reach AAA at FL290 when ideally the crossing alt would be say FL330.
As soon as SPD appears press the MCP alt knob still with 29000 in the window this will delete the 290 resriction in the FMC and will draw a new vertical profile and you'll get VNAV ALT, then reset the MCP window for your lower alt when the new vertical path comes down to meet you the A/C will transition to VNAV PTH and descend.
I've been told pushing the alt knob within 2nm of the waypoint will clear the Alt restriction in the FMC with the same alt set in the window (SPD activates usually within 2nm) I think I tried it at 1.9 still in HOLD and PTH and can't remember if it worked.
It also works by entering a new restriction in the FMC that once again redraws the profile above the A/C. This new alt may have to be in the MCP before entering it into the FMC.
I too would like to know what parameters allow it to go from VNAV ALT to PTH automatically.
Never works in the sim either?? Well not our ones anyway.
turnandburn
30th Jul 2010, 22:22
Was 4500 level.
Mcp set at 4500.
So In VNAV ALT
Yes aware of and watched GS capture while in VNAV ALT.
Did not set lower alt did not hot the hit the button
Have asked a variety of sim ins, trainers and line pilots and opposing answers.
About 50% split. (some of these pilots 15- 25 years on type say it will descend)
i can only surmise that there is coding in FMC that is hidden to me that this will occur.
So no books or ifno on the triggers.
Found some research papers that talk around the six different descent trajectories and the 32 parameters it uses to fly the VNAV descent but have not really found hard rules as to what coding to look at or combinations of modes and speeds that trigger this.
Only been i 744 for 4 years but been boring holes in sky for 26 years now.
turnandburn
30th Jul 2010, 22:27
I have also been looking through the maintenance manuals in ISO view which we have access to in dispatch.
No luck yet.
turnandburn
30th Jul 2010, 22:29
When approaching TONGA at FL290 in taipei knock it out and get new profile. Seen that done that.
an3_bolt
30th Jul 2010, 23:23
Nope. Still won't leave Vnav Alt in that situation.
The only thing i can think of is G/S capture. Was this an ILS?
GS will always do it once you capture regardless of MCP alt selection - I refer to it as a "hard" mode.
However - if you are inside of the capture region of the VNAV approach logic and in VNAV ALT with a lower selection in the MCP - the modes will transition to SPD/lateral mode/VNAV PTH and fly away from the previous VNAV altitude once it captures the descent profile. Things such as VNAV speed intervene whilst remaining in VNAV PTH mode and the ability to set a higher MCP Altitude and remain in VNAV PTH mode are now enabled.
Of course the usual caveats apply such as using an approach from the FMC database that is compatible with the VNAV approach logic.
Works pretty well on GNSS approaches:ok: