DES NOW in CLB Mode (747-400)

[cf6-80c2b5f]

Has anyone tried the DES NOW feature while still in CLB mode on the B744? The Bulfer book says it will delete all climb and cruise constraints, but it does not state whether it will force it out of CLB and into DES. Others have claimed it will not work at all while in the CLB mode, even though the DES NOW prompt is visible on the DES page while in CLB.

The reason for asking is to see if this would be a another option for getting out of CLB mode for a return to departure airport, prior to reaching your cruise altitude or, on a missed approach, prior to reaching the higher of your MCP altitude or the missed approach altitude. Of course, once you reach this altitude it will go to CRZ automatically and then transition to DES after crossing the T/D boundary.

I am aware that the option of using a non-VNAV mode (FLCH, VS) to descend while in CLB mode would be an alternative and probably preferred method, but I'm interested in whether DES NOW would force it into DES while still in CLB. Thanks.

[CCA]

It would only work if you had a runway for arrival active in the legs page, either entered earlier on the ground or during the climb, it can't descend now if it doesn't know where it's descending too. (In the vertical plane)

[cf6-80c2b5f]

Thanks for the reply, CCA. I was thinking more along the lines of setting a lower MCP altitude and using VNAV altitude intervention to descend, rather than FLCH or V/S. Would there be no interconnect between the FMC and a lower MCP altitude when you tried to descend?

For example, your cruise altitude is FL340. After departure you encounter a pressurization problem that requires your return. You level off at 10,000' with the MCP. You are in VNAV ALT and still in CLB mode. Without knowing which approach you will be doing (let's say you need to dump fuel first), you are cleared to 6,000'. When you set 6,000' in the MCP and altitude intervene, you won't descend because you are still in VNAV CLB mode. But if you press DES NOW at this point (and execute), will it let you descend in VNAV? Does this give the FMC an altitude to descend to?

If you install an approach for a return while still in CLB mode, when you bring any fix on that approach into 1L (extend from the CF or FF at 6R) it will shift to CRZ mode immediately and then DES mode when you cross the T/D boundary. But if you install the approach without extending at 6R, the first fix of the approach will end up on the last page of the LEGS page and it will not shift to CRZ. Will DES NOW allow a descent in this case?

ADDENDUM:

I think that in order for it to shift into CRZ when you install an approach and extend at 6R, your actual altitude must be at or above the altitude of the fix you are extending from. It's not as much an issue on a return for landing, but on a missed approach, if you level off prior to your missed approach altitude, and you install an approach and extend from the CF, if you are not at or above the CF altitude, it will not shift to CRZ mode.

[CCA]

Basically it needs an airport elevation (a runway wpt) to construct a VNAV path (normally 3°) from, until then the decent phase won't work as there's nothing to vertically navigate, there is no path and therefore no VNAV.

So trying to descend to 6000 using descend now won't work as it has no idea where the path is and therefore can't decide on VNAV SPD or PTH.

[mustafagander]

I suggest stop the climb with ALT HOLD. Then reset MCP to lower alt and descend using VS mode while you tidy things up with runway and approach selections. I assume you want a rapid return to an airport here.

[cf6-80c2b5f]

Thanks, CCA. So just putting a lower MCP altitude (6,000' in this case) isn't enough. You have to install an approach and put the first fix in 1L for it to descend. I think it would descend anyway in that case as soon as you rolled in 6,000 and altitude intervened. (Inserting an approach fix in 1L puts it into CRZ and altitude intervention puts it into DES.)

Mustafagander:

I am aware that the option of using a non-VNAV mode (FLCH, VS) to descend while in CLB mode would be an alternative and probably preferred method, but I'm interested in whether DES NOW would force it into DES while still in CLB.

[CCA]

It's better to think of DES NOW as activate descent phase now.

The VNAV decent phase however only works with an active end of descent point and you can only get that by selecting a runway from the data base or selecting a runway and rwy ext, thereby creating the end of descent point.

Once it knows where to finish it knows where to start and VNAV will work.

Edit auto correct

[cf6-80c2b5f]

Thanks. So, I would need to have a fix on an approach in 1L to use DES NOW? I'm not intentionally trying to complicate this, but we normally select the runway we anticipate we will be landing on at our destination, so there is always an approach in the LEGS page (albeit on page 20). Likewise, if I select an approach for the return and I didn't extend at 6R, that approach would be at the end of the LEGS page and I presume DES NOW would not work in that case either?

[CCA]

It only needs an end of descent point E/D associated with the active legs (it could be on pg20), once it has that it can calculate a descent path the descend now can be activated.

[cf6-80c2b5f]

Okay, Thanks for clarifying this for me, CCA. Cheers.

[felixthecat]

Is there not a diversions schedule like in the B777?

With the 777 if you have to make a fast return to the departure airfield and you have travelled less than 400nm or less than 1/2 the route if you do a rapid return the pressurisation uses the departure elevation to the nearest 100ft.

[cf6-80c2b5f]

It's not as much an issue of pressurization. It's just that the 744 will not descend in VNAV while in CLB mode. To get it out of CLB mode, your options are (1) install an approach for a return and have the first fix of the approach in 1L, (2) re-cruise (insert your present altitude on either the CLB or CRZ page and altitude intervene), or (3), according to CCA, select DES now, presuming you've already installed an approach at your destination airport (we normally do this prior to departure).

Installing an approach for a return should not only reschedule your pressurization, it should make moot options 2 and 3 for getting out of CLB. But there are times when you would want to descend and perhaps dump fuel in a hold for 40 minutes and you don't know what approach you would be using at that point. As Mustafgander pointed out, you could simply use FLCH or V/S to do the descent, but I was curious if DES NOW would get it out of CLB mode so you could descend in VNAV.

[felixthecat]

Not having flown the 744 I cant really comment further but interesting .... I have no idea if you can do it on the 777 either, but I think in the mist of battle I think the FLCH and V/S options are faster and less prone to messing up in a phase of flight when by the sound of things things are going south rapidly :)