B737 LVL CHNG and VNAV SPD difference

[ogorek]

Hey, is there anyone that can simply describe the difference between lvl chng and vnav speed descend?

[Amadis of Gaul]

I'm sure there's someone who can do it.

[B737C525]

Ogorek,

At the top of the page you'll read: 'Enthusiasts, please use the 'Spectators Balcony' forum'. I'm not sure if you're an enthusiast, but if you are, you might have more luck there.

Anyhow (and I'm afraid I need to assume you have sufficient background knowledge to understand what follows)...

From the cruise:

LEVEL CHANGE:

Select a lower altitude in the MCP, and then press level change. The thrust will drive to idle, the auto throttle will declutch, and the aircraft will pitch to descend at the speed in the MCP window. If doing this from VNAV path, the speed window will open at the present indicated Mach number (or speed if below approximately FL260). Mach number or speed is controlled by pitch, and thrust remains at idle until alt acquire unless manually adjusted. Changeover from Mach number to IAS will occur at the default altitude, approximately FL260.

VNAV SPEED:

Select a lower altitude in the MCP, make sure you are in VNAV, then line select VNAV SPEED descent and execute (or line select VNAV SPEED descent, execute, then select VNAV). The thrust will drive to idle, the autothrottle will declutch, and the aircraft will pitch to the speed in the DESC page. Speed is controlled by pitch and thrust remains at idle until alt acquire unless manually adjusted. Changeover from Mach number to IAS will occur at the altitude at which the selected Mach number and IAS intersect.

So, the two immediate differences are that in VNAV SPEED descent, you can pre-programme the speed values, and changeover occurs at Mach/IAS intersection. More importantly, by selecting the speeds in the VNAV page, the PATH calculated will be based on the selected speeds, rather than ECON speeds, enabling better appreciation of how energy management is going during the descent.