MCP Problem

[Norbert Woeller]

Good evening,

I try to explain my problem.
When I'm engage VNAV after Takoff (no matter at what altitute), the aircraft autopilot executes a "descent" before climbing again.
I have tried many things. But no luck. Maybe someone can help?

I'm using the PM MCP and the CP MCP Hardware

Thank you
Best regards
Norbert

[Sean Nixon]

I have experienced this too, although it seems random and I haven't yet figured out under what circumstances it occurs. Is it happening all the time for you?

[Norbert Woeller]

Is it happening all the time for you?

Yes, all the time.

Thank you for your answer Sean!

[Peter Nielsen]

Good evening,

I try to explain my problem.
When I'm engage VNAV after Takoff (no matter at what altitute), the aircraft autopilot executes a "descent" before climbing again.
I have tried many things. But no luck. Maybe someone can help?

I'm using the PM MCP and the CP MCP Hardware

Thank you
Best regards
Norbert


Have you checked if your first waypoint altitude restriction is not lower than the altitude you are at when you enhage the VNAV?

Peter

[Norbert Woeller]

Yes, Peter. All checked. Thank you for your answer.

Best regards
Norbert

[flightdeck]

Hi Norbert,
what about the trim?
What is the trim reading, when you engage the VNAV?
Are the trim wheels spinning during the initial dive?
Cheers
René

[Norbert Woeller]

Hi Norbert,
what about the trim?
What is the trim reading, when you engage the VNAV?
Are the trim wheels spinning during the initial dive?
Cheers
René

Hello René,
hm, never have thought about that. Will test it at the weekend. I'm away from home till suterday.

Danke und Grüße nach Wien
Norbert

[bravolima]

Have you checked if your first waypoint altitude restriction is not lower than the altitude you are at when you enhage the VNAV?

Peter

Hello Norbert,

this is the important answer! Have a look at the first waypoints on your FMC, perhaps

XXX 150/2000
XXY 175/3500
XYY 230/5000
YYY 250/10000

if you are between the second and the third waypoint already up to let me say 7000ft and you engage VNAV then AP will force ou down to 5000ft, thats why you dive.
Two things to do:

modify your waypoints before take off[/B when you know you will engage very early VNAV

]XXX 150/2000A
XXY 175/3500A
XYY 230/5000A
YYY 250/10000A

behind the altitudes put an "A", stands for at or above; this means the given altitudes figure like minimums. By the way you can do this as well in the descent. Then you have to put a "B" behind the altitudes. Means at or below.

second way: apply a desired Altitude for instance 15000 activate VNAV [B]and ALT INTV (if you have cp flight MCP pro); this is a second way to modify your climb.

If been told ind daily practice the VNAV is rarely used, usually only in the last tthird or quarter of the climb. Up to there they conduct the flight in MCP mode altitude with SPEED, ALT and V/S.

hope this helps

Bernhard (LSZH)

[bagarie]

Hi Bernhart,

your explications in my opinion is not Procedurally correct. I'll explain why.
If you have in your climb profile an HARD restriction to a WP, like many SID had, you cant'use the A or B (soft Restrictions) because you must stay at a specified Altitude and/or Speed. The same things about ALT INTV, because you cancel the Restriction on that WP.
Finally according to many SOP by different Airline, the Climb phase is flown at VNAV or LVL CHG: That's because the SPD/ALT/V/S don't protect you by the Stall. So normally if you don't use VNAV you must use N1 - LVL CHG.

Normally the VNAV is engaged after cleaned the airplane. You can see that on the Normal Procedure Manual. Or you may use the VNAV at the acceleration Altitute. Ex: on the VNAV CLB Page, LSK 3 set 210/3000 (3000 feet AGL) and on LSK2 250/ as target speed. when you reach Fl100 you resert Trget Spees to ECON (the Speed calculated by the FMC)..

I hope I explained that clear.

Regards
Arnaldo Vacca

[Norbert Woeller]

this is the important answer! Have a look at the first waypoints on your FMC, perhaps

XXX 150/2000
XXY 175/3500
XYY 230/5000
YYY 250/10000

if you are between the second and the third waypoint already up to let me say 7000ft and you engage VNAV then AP will force ou down to 5000ft, thats why you dive.
Two things to do:

modify your waypoints [B]before take off[/B when you know you will engage very early VNAV

]XXX 150/2000A
XXY 175/3500A
XYY 230/5000A
YYY 250/10000A

behind the altitudes put an "A", stands for at or above; this means the given altitudes figure like minimums. By the way you can do this as well in the descent. Then you have to put a "B" behind the altitudes. Means at or below.

Hello Bernhard

thank you very much for your very kind answer. But it's a pitty, I have checked it with a "A" behind the altitutes. So this can't be the reason for the automatic descent.

Best regards
Norbert

[bravolima]

Hello Arnaldo,

what you explain is absolutely right. Given the situation you are bound on a SID, then you should check the SID in your FMS during cockpit preparation and stay with in the limits after take off. Easy said, I find myself often 500 or 1000ft higher than the FMC values in my simflights, but below the initial altitute given by ATC. Needless to say, it is not a good idea to activate VNAV at this early stage of departure.
By the way the departure and the climb-pathes on bigger airports are often variant and not always strictly according to the issued SID, then it is good to know what tools you have to modify your departure climb. Often you can hear on ATC " go dircet to XXXXX and climb FLXXX"....

Bernhard (LSZH)

[eudoniga]

.... ?

Given ICAO noise abatement procedures in force, are you allowed to speed up to 210 KIAS, instead of maintaning V2 + 20, up to 3000 ft AFE ?
I was taught to do the "bug up" and clean the aircraft only after that limit (3000 ft AFE) ...