Switching from VNAV to ALT HOLD

[IndAir967]

Hello Folks,
I am relatively new to TurboFan Flying..
Let s say I have set 36,000 on the Altitude Window..
And FL360 is set as Cruise Alt on my FMC..
Now I have been climbing using Vnav and on reaching FL360 what i ve
observed is that Vnav continues to stay on ..

At this point would you switch to ALT HOLD on the MCP disengaging Vnav and hence speed window becoming active
OR
Let the Vnav on but expect to see ALT HOLD on the PFD without manually engaging ALT HOLD button on the MCP
OR
Just let the vnav be on with out bothering much..

Appreciate your clarification in this regard !
:ok:

[Flying Farmer]

In our ops we leave it in VNAV in the cruise. You will be in VNAV PATH and PATH will be announciated on the PFD. Leave it in VNAV no reason not too.

Hope theres no questions on VNAV descents and why the FMS does what it does :confused:

[IndAir967]

oh ok.. but is it purely ur company's sop to leave it on ? does it wary from each operator to operator.. ? Are there any operators that switch to Alt hold ?
and no i am good as of now with the descent part of it..
thanks..

[Cough]

Couple of points to bear in mind:

When you engage Alt Hold you are not holding the altitude that has been selected in the MCP window. Not ideal for RVSM.

When you use the button, you will loose the flexible speed schedule provided by the FMC - ie. slowing down for a tailwind and speeding up in a headwind because you will then control the speed.

Only ever used Alt Hold once or twice, a year!

[arba]

AirInd967
if the VNAV is doing your intended profile (the speed and/or constraint) leave it in VNAV SPD.

[FCS Explorer]

WHY NOT leave it at VNAV, anyway???? don't see the point of the whole question.

[wiggy]

The only reasons I can think of to use ALT HOLD is if for some reason you suddenly have a requirement to hold a passing altitiude in the climb/descent, and on some aircraft it may be a recommended pitch mode in severe turbulence.

Depending on your aircraft type the mode can have several traps - As others have said if you do insist on using the mode you've lost the FMC speed scheduling and you are not latched to the altitude/FL in the MCP window, you are holding the altitude at which you pressed the button.....

This of course is all subject to the advice your own Company's FCOM - what does that say?

[Kirks gusset]

Without sounding harsh 967 , you need to discuss this with your LTCs as the basic principles of operating in VNAV should have been covered during your line training. Deviation from SOPs and the way the a/c was designed to be flown are the single biggest cause of jet upset, nip this in the bud, ALT hold is not the normal operating mode for many reasons.. most of which are outlined in the FMS/ FMGS sections for Boeing/Airbus. VNAV is part of an intergrated system providing much more than altitude capture.