Using VNAV

[Guest monkey]

How do you guys incorporate this in your flights with live atc? It seems to me that it's great for your hardwired "programmed" route/alt/speeds, but as soon as atc get involved your nicely programmed route in the FMC would go to the pack! Okay, try to follow me here!For example, say I'm at FL330 ATC comes in before the estimated T/D as tells me to descend to FL210, okay I change ALT the MCP to FL210 and hit "descend now" on the FMC. 5-6mins later I'm at FL210, the FMC has caught up with projected altitudes and I'm hanging around at this alt longer than expected until I lose VNAV because it decides that "desc path not available"!I know you can change values in the FMC to start her up again but isn't there a general time during the flight we all use VNAV? It seems to me that it would be easier to do everything through the MCP, at least you wont be conflicting with settings as much...I think I must be missing something...

[crvm 0]

Use a CRUISE DES instead of DES NOW ***IF*** you are further than 50 TOD. It's all about knowing how to use VNAV to effectivly and about it's limitations and features.[h4]Randy J. Smith[/h4]

[Guest tmetzinger]

>How do you guys incorporate this in your flights with live>atc? >>It seems to me that it's great for your hardwired "programmed">route/alt/speeds, but as soon as atc get involved your nicely>programmed route in the FMC would go to the pack! >>Okay, try to follow me here!>>For example, say I'm at FL330 ATC comes in before the>estimated T/D as tells me to descend to FL210, okay I change>ALT the MCP to FL210 and hit "descend now" on the FMC. 5-6mins>later I'm at FL210, the FMC has caught up with projected>altitudes and I'm hanging around at this alt longer than>expected until I lose VNAV because it decides that "desc path>not available"!>>I know you can change values in the FMC to start her up again>but isn't there a general time during the flight we all use>VNAV? It seems to me that it would be easier to do everything>through the MCP, at least you wont be conflicting with>settings as much...>>I think I must be missing something... You're not missing anything at all. You can generally get away with using VNAV on the climb and cruise portion of the flight, but with ATC it's quite rare to use it on descent and arrival, unless you get a controller who's very sharp and gives you the arrival clearance early. Sometimes they have too much traffic and just can't do it.For this reason, become proficient with using LVL CHG and VS modes in the Descent. LVL CHG is perfect for those times when you're cleared to a new altitude with no specific waypoint, and VS is your friend (along with the green arc on the ND) when you are told to cross a certain waypoint at a certain altitude.Also, you should pay attention to the nuances of your instructions from ATC. A controller might say:Descend to XXXX, Fly the arrival as published - this means you fly the descent and comply with all crossing and speed restrictions and don't go any lower than xxxx. This is about the only time that VNAV may work for you.Descend to XXXX, cross YYYY at ZZZZ - this means you have to cross YYYY at ZZZZ, but you can continue to descend after that point to XXXX. Cross YYYY at ZZZZ - this is the clearance that leaves you "hanging", since you can't descend below ZZZZ without further instructions. Don't be shy about querying the controller for a new altitude.Best Wishes,

[Guest dwinters]

I agree with Tmetzinger. I usually use VNAV to fly the climb and cruise but have found that it is very unlikely that ATC will give you a descent clearance at the same time as the FMC predicted TOD. I then usually swap to Level change entering the cleared altitude into the MCP.David