VNAV will not engage

[rgamurot]

I know this topic was already discussed because I saw it a while ago but I can't find it anymore. Anyway, whenever I attempt to activate the VNAV, I get two FMC messages. One saying "Unable to crz alt" or something similar. After I press CLR, a second message pops up saying "VNAV Unavailible. I looked through the manual but I usually look through the wrong areas when I look for things anyway. I checked the FMC section.Does anyone know what I'm doing wrong or where I can look? Thanks

[Guest Delta07]

Hi:I get that sometimes...in my case I've found that I've entered something wrong in the FMC and it provides for a T/D (Top of Descent) before you can reach your planned CRZ ALT. Therefore you can get an "Unable CRZ ALT". Check through your flight plan using the legs page and see if there are any "DICONTINUITY" or if the green T/D on the display is before your planned CRZ ALT.I don't know if that helps you or not, but it was something I thought of off the top of my head.Fletcher R.

[Guest s_L_Y_F_a]

did you enter a value in EVERY box in the varios FMC sections? I mean flightlevel, reserves, ZWF and so on

[rgamurot]

"So on" doesnt include Cost Index does it? I have a feeling it does though. I Usually don't touch that because I don't really know what to put so I leave it blank.

[Tabs]

Really need the exact details of the flight to be able to judge what could be happening... route, attempted cruise, fuel load and payload etc...

[tigerred53]

You leave the cost index blank?Try inserting 100 into the cost index, that's what I usually do. See if that helps.Sanjeev Mundluru

[rgamurot]

I tried entering 0050 into the cost index and that did the trick. I've encountered another minor problem with having the VNAV holding my speed but have since fixed it. Thank you everyone for your help.

[crvm]

Please sign your name to your posts sir. CI is never as low as 50. You need to input in the range of 100-150 and even higher in some cases. 100 will do fine for you here. Remember that all BOXES in the FMC are required, these must be filled in, dashes are optiononal.Randy Smith

[rgamurot]

> CI is never as low as 50. You need to input in the range of>100-150 and even higher in some cases. 100 will do fine for>you here.How do you calculate the CI? The manual's explanation doesn't make very much sense to me. Ryan Gamurothttp://www.precisionmanuals.com/images/forum/supporter.jpg

[crvm]

CI is airline specific although it does not mean they all always use the exact number each flight. You simply use one that you like. Remember that real pilots do not calculate or choose a CI, they are provided one from their flight dispatch center along with the rest of the flight details. Personally I believe using a CI on the 744 around 150 is a very good default... Randy Smith