If, say, it is required to descend 1000ft lower than the declared CRZ FL to FMC, then the page CRZ is not available any more, VNAV mode light goes out and MACH/SPEED mode has to be used to continue cruise on the lower FL. During this event, I noticed that Vertical deviation symbol shows on ND, which means the aircraft understands the start of descent. This level change is required in some airspaces like Spain. So, in that case, the availability of CRZ page is lost in the real thing too, or this is a glitch ?

Thanks for your time,

Sotiris Stavrakis.

That‘s exactly what ALT INTV is for.

dial in your new or temporary altitude on the MCP and press the button next to it. The airplane will remain in cruise mode.

Edited December 5, 20187 yr by Ephedrin

For how long can this action remain ?

As I recall, the "intervention" did not hold for long. (I used speed intervene once in the past and after a while the speed reverted to the FMC speed).

Edited December 5, 20187 yr by sotiris

Sorry Sotiris, I was wrong.

I've never actually used altitude intervention so I was misinterpreting it.

I've looked into the FCOM and this is what stands there:

Push – (during VNAV cruise)•if MCP altitude is set above current FMC cruise altitude, FMC resets cruise altitude to MCP altitude and initiates a cruise climb•if MCP altitude is set below current FMC cruise altitude, an early descent is initiated. Lower FMC cruise altitude cannot be entered using ALT INTV switch.

 

look here: FCOM, 4.10.10

 

however, when the airplane has entered decent mode you should be able to return to cruise mode by recycling the flight directors.

Edited December 5, 20187 yr by Ephedrin

OK thanks Marc. I ' give it a try.

7 hours ago, sotiris said:

For how long can this action remain ?

As I recall, the "intervention" did not hold for long. (I used speed intervene once in the past and after a while the speed reverted to the FMC speed).

LVL CHG is exactly what you should be using in this case. Start on FCOM 4.10.8:  Select new altitude (descent), push LVL CHG, set desired speed (speed intervention is a different subject); then autothrottle will retard to idle and pitch will hold speed.  Note this does not delete FMC altitude constraints but ALT INTV does.

Thank you Dan.

Marc, recycling the FD did not have any effect on FMC CRZ page restoration.

Another thing I noticed while FD off and on action, is that captain's FD MA light remained illuminated even if I switched captain's and first officer's FD switch off-on.

Alt Intv is fine if you're relatively close to TOD; it'll enter an early descent but will capture the MCP altitude.  

If this is well before TOD though, the answer is to simply enter the lower cruise altitude on the FMC cruise page.  Just because that value disappears doesn't mean you can't put it back ;-).  If you came out of vnav for the descent, just enter your new cruise altitude and reselect vnav.

Andrew,

The case is that the CRZ page in FMC goes out and doesn't accept any FL.

Anything about Master FD light ?

Dan or anyone else in the knowing,

Can you please tell me why it is necessary that the FD needs to be recycled when, f.i., shooting a RNAV approach?

Thanks a lot,

RNAV approaches are non precision approaches. When you‘re on an ILS you arm and activate tr approach mode which will guide you down to the runway via the FD bars. However there are RNAV approaches that require to go visual at a certain altitude or waypoint and then you don‘t fly after the FD bars anymore. And on the PFD you only want to see what you need. And to get out of the APP mode you need to recycle the FD, you can‘t deactivate it otherwise, except for the TOGA buttons. But I don‘t think that there are any technical requirements to do that.

OK, understood. Thanks Marc

 

For what it's worth, in order to get back into CRZ, just click the CRZ page button on the FMC and re-enter a cruise altitude - in this case, the altitude that you just descended to. Done.

Don't let the computer outsmart you. A lot of people assume an FMC is some complex thing. It's not. It's old technology with (occasionally) new software on it, but it's still, at its core, a dumb box that runs calculations and not much more. When it comes to interacting with it, imagine the simplest way of doing something, and that's probably the answer.

1 hour ago, scandinavian13 said:

For what it's worth, in order to get back into CRZ, just click the CRZ page button on the FMC and re-enter a cruise altitude - in this case, the altitude that you just descended to

If I click on CRZ page, I 'll see five "unticked" boxes. Trying to fill them out with the new FL it returns a message "INVALID ENTRY or VALUE".

5 minutes ago, sotiris said:

If I click on CRZ page, I 'll see five "unticked" boxes. Trying to fill them out with the new FL it returns a message "INVALID ENTRY or VALUE".

Go to the PERF page and enter your flight level there.