Help,help:"unable Crz Alt"advisory Messages Display
 

BOEING 737-300 AIRCRAFT FLIGHT DECK EFFECT REPORTS "UNABLE CRZ ALT"ADVISORY MESSAGES DISPLAY ON FMC CDU,IT IS HAVE BEEN DESCRIBED "
PERFORMANCE PREDICTS A ZERO CRUISE TIME AT THE ENTERED CRUISE ALTITUDE OR IT WILL
BE UNABLE TO REACH CRUISE ALTITUDE DUE TO PERFORMANCE LIMITATIONS " BY BOEING MAINTENANCE MANUAL .CHECK FMC OPS UPDATE "U5.O",FMC-CDU PERF FACTORS PAGE MIN CRZ TIME ONE MINUTES. FMC OPS UPDATE 5.0 HAVE BEEN Revised the computation of maximum altitude so that it will now be cruise mode dependent. BUT "UNABLE CRZ ALT"ADVISORY MESSAGES SOMETIMES DISPLAY ON CDU. FMC CAN'T COMPLETE ETA Algorithm AND REMIAN FUEL Algorithm.WHY?

MANY THANKS.

If the FMC predicts you will be unable to reach an altitude, it cannot then caculate performance if you have been able to get up to that altitude. You have to wait until the FMC is happy for you to maintain that altitude. Sometimes it helps to input a slightly faster or lower cruise speed as this may affect the associated cruise altitude. It is not unusual in the 737 to find the upper speed buffet margin and the low speed buffet margins meeting in the middle.

THANK YOU VERY MUCH!
 

:O NOW , I SEE ABOUT THIS .

If you see that message one of the first things you should do is go to the cruise page (CRZ) and amend the level. Computer says no!

Never flown the 737, but have seen same message on other aircraft.

On ALL occasions its because i forgot to put the minus sign in front of cruise level temp when i amended the isa temp( no airlane will reach 35000 feet when temp there is +60 C )

The 737 is quite critical at high altitudes. The upper and lower buffet sticks come together very quickly. WHen they are going to meet, the cruise altitude becomes unavailable in the FMS and it will give this message. It is unable to compute any performance for a level it doesn't think it can make.

yueyueju - can you give us some idea of the flight/flights in question when this message appears? To me it sounds more like the message you get when you put too high a cruise altitude in for the distance involved. I have also seen it after landing with a g/a altitude in the legs page. I assume that is a software 'glitch'.

Maybe you should hit it with a bigger hammer?