Sign in to follow this  
sotiris

Flight level change and VNAV mode.

Recommended Posts

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.

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

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 by Ephedrin

Share this post


Link to post
Share on other sites

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 by sotiris

Share this post


Link to post
Share on other sites

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 by Ephedrin

Share this post


Link to post
Share on other sites
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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

Andrew,

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

Anything about Master FD light ?

Share this post


Link to post
Share on other sites

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,

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

  • Like 2

Share this post


Link to post
Share on other sites
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".

Share this post


Link to post
Share on other sites
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.

Share this post


Link to post
Share on other sites

On the PERF INIT page??

format is FL350 or 35000. this should always work regardless of the flight mode. 

Share this post


Link to post
Share on other sites
43 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".

Using an imported route file?

32 minutes ago, Ephedrin said:

On the PERF INIT page??

Correct. PERF INIT is for pre flight, not during flight.

Share this post


Link to post
Share on other sites
2 minutes ago, scandinavian13 said:

Using an imported route file?

No, I create it myself with Jepp charts.

Share this post


Link to post
Share on other sites
1 minute ago, scandinavian13 said:

Correct. PERF INIT is for pre flight, not during flight.

Sure.. it‘s been several months since I‘ve flown the 737 the last time but as far as I remember I‘ve always done it there as the CRZ page is deleted as soon as you started a decent.. of course it‘s possible that I mix it up with something else... I’ve flown the Jumbo and A2A almost exclusively for 2 or 3 months... 

Share this post


Link to post
Share on other sites
6 minutes ago, Ephedrin said:

Sure.. it‘s been several months since I‘ve flown the 737 the last time but as far as I remember I‘ve always done it there as the CRZ page is deleted as soon as you started a decent.. of course it‘s possible that I mix it up with something else... I’ve flown the Jumbo and A2A almost exclusively for 2 or 3 months... 

It does remove the info, but there's a reason the page is still accessible, and has the open boxes for altitude entry.

10 minutes ago, sotiris said:

No, I create it myself with Jepp charts.

In what format are you entering the altitude?

Share this post


Link to post
Share on other sites

I did one more test flight and this time it worked going to five box CRZ page. It accepted the new lower FL as MOD CRZ.

I have no idea what went wrong at that flight.

Thanks for your time.

Share this post


Link to post
Share on other sites
3 minutes ago, sotiris said:

I did one more test flight and this time it worked going to five box CRZ page. It accepted the new lower FL as MOD CRZ.

I have no idea what went wrong at that flight.

Thanks for your time.

Welcome!

If you catch it happening and can find a specific way that it doesn't work, let us know.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this