NG B737 VNAV question
 

Suppose you are climbing to FL350 in VNAV mode, and you have to level-off at FL280 on your way up, so you set 28000 in the MCP window.

What cruise speed will the FMC have the A/T maintain once in cruise, assuming no pilot intervention? The speed(s) entered in the CLIMB page of VNAV, the speed(s) entered in the CRUISE page, or something different?

Stamatis

The speed you entered on the climb page. The default for this is econ climb speed.
Hope that helps.

The speed you entered on the climb page

Thanks. And I suppose that if I am descending in VNAV and have to level-off at an intermediate altitude it will then use the speed(s) entered in the DESCENT page?

The speed(s) entered in the CRUISE page are only used when cruising at the FL entered in that very same page?

Stamatis

Stam,
Basically correct, however if you change level in the CRZ page (after selecting it also in the MCP window) and execute, you climb with the CRZ speed for your new level.

Static,

Thank you for the reply and apologies for coming back late on this.

I wonder what is the logic behind this.
I assume I can reset the desired climb/descent speed by changing the TGT SPD?

Also, if i use the access prompts to select the CLB/DES page, what will be displayed there? Will the initial climb/descent speeds have been automatically reset to those in the CRZ page when I select a new CRZ altitude, or what?

Stamatis

Correct on both counts, you can always change the TGT SPD in any VNAV climb mode.
If you select the climb or descent page, they will show ECON CRZ climb or descend.

Thanks!

What I fail to understand is why under these circumstances is the Climb conducted using the TGT SPD of the CRZ page and not the TGT SPD of the CLB page, as in a "normal" VNAV climb.
Any explanations for that?

Stamatis

Stamatis,
Well, I`m only guessing here. The object is a level change during the CRZ part of your flight. As far as a CRZ descent goes, it`s obvious, as the default TGT SPD in any descent mode but the CRZ descent seems to be a whopping 273 kts IAS. A CRZ descent will keep you at your M.78, which is what you really want.
As for the CRZ climb, I would say more or less the same, as the object here also, is the econ CRZ speed, and not so much the factor best climb speed, which is probably just left out of it. Most of the time it`s not more than 4 thousand feet anyway.
But, again, I`m only guessing.

If that is the object, I would use LVL CH, not VNAV, right? To me, the object of a VNAV climb or descent is to use the most appropriate speed profile, depending on the cost index involved, and the "restraints" in my flight plan, not so?

Thus, I would expect the FMC to use the CLB or DES TGT SPD whenever a VNAV climb or descent is executed.

But who said that FMCs are built to be rational? :)

Stamatis