Unable CRZ ALT

[N55E008]

Hello.

very frequently I get the FMC-Message 

"unable CRZ ALT".

This happens even at moderate TOW ( 45tons ) and Cost-Index ( between 20-50 ) on trips with a trip-length of around 500NM.

According to FMC, a CRZ ALT of FL290 ( or higher ) for the above conditions is not possible, which I believe is not correct.

Due to that, I'm having troubles to engage VNAV after T/O. So I'm using LVL/CHG instead. And this mode shows that ( performancewise ) a cruise level of FL390 is easily possible, with a cruise segment of around 300NM...

Best regards,

Toto

 

Edited July 6, 2017 by N55E008

[GiantUnicorn]

Quote

Service Ceiling

11.278 m

37.000 ft

 

 

[mgeiss]

The unable crz alt warning usually doesn't mean that the plane isn't technically able to reach that altitude with the current W&B, CI etc., but that the FMC finds the route too short to make it to that altitude. If you are sure that you planned everything correctly, you can just ignore that message.

Edited July 7, 2017 by mgeiss

[Muchimi]

I too get this message on heavier zfw even on routes > 350nm (example KSLC-KDEN).

I think the primary cause for the unable crz msg is the VNAV computed step climb puts your top of climb after your top of descent, especially if you have a high FL for a shorter route.   Because for the life of me I can't see the computed TOC or TOD indicators on either the CDU or the plan map, it's hard to tell (perhaps my learning curve here).   I think the message is mostly correct as if you lower the ZFW or change the cost index, it goes away - I suggest you experiment with that.

This said, I still think VNAV gets confused sometimes especially if you start making changes mid-flight and use constraints (speed and/or altitude) or have more extreme weights / aggressive elevation changes in your route.

However, most of the VNAV issues I have relate to the VNAV descent profile.  

Then again, I find it's a way for me to use better planning tools and not rely so much on the automated course as the PIC I need to make sure what the computer thinks makes sense, and trust me, being in the computer field, computers usually don't make sense because I programmed them

 

Hope this helps

 

 

 

Edited July 7, 2017 by Muchimi

[jolmos]

You can see the Flight Crew Operations Manual (FCOM), page 11.60.18

ADVISORY MESSAGE: UNABLE CRZ ALT

CAUSE: FMC predicts that no cruise time is possible at the entered CRZ ALT.

CORRECTIVE ACTION: Clear the message and review the CRZ ALT selection

[Muchimi]

Another thing that occurred to me is that the VNAV computation is based on XP10, with the XP11 patch of the 733 not being quite out yet which may also account for some of the strange things experience with VNAV profiles running an XP10 version in XP11 (if you're doing that).

 

Edited July 7, 2017 by Muchimi