Is there a cruise descend procedure that doesn't mess up the vertical profile? Say, for turbulence. If the plane descends from, say from FL360 to FL340, the VNAV goes into descend mode and the T/D gets deleted.

If you change the CRZ ALT in the CRZ page to your new Flt Level it should adjust the profile.

1: in the FMS CDU, select "CRZ"

2: enter the new flight level in the Top left line select.

3: adjust the MCP altitude window to the new cruise level

4: Check pressurization cruise level on Overhead

5: press exe on the CDU to execute and initiate decent in VNAV mode.

1: in the FMS CDU, select "CRZ"

2: enter the new flight level in the Top left line select.

3: adjust the MCP altitude window to the new cruise level

4: Check pressurization cruise level on Overhead

5: press exe on the CDU to execute and initiate decent in VNAV mode.

Probably don't need to reset the Cruise Alt on the pressurization panel if you're going to a lower alt.

Probably don't need to reset the Cruise Alt on the pressurization panel if you're going to a lower alt.

Didn't say reset I said check.

 

If you have been cruising at FL360, are about to go down to FL340, but the pressurization panel is selected for FL370 and you'v never been that high during the flight, once you start decending to FL340 you will get a nice little yellow warning light come up on your glareshield.

you can press that little alt button on the MCP too. Change the altitude on the MCP, press that button, and it will enter that number as your new cruise in the FMS. Well, I do this for step climbs, I assume it works for step downs too......not sure why I assume that actually. Maybe that doesnt work for stepping down.

 

Something for me to check on my next NGX jump I guess.

 

 

Well, that post turned useless real quick....

Didn't say reset I said check.

 

If you have been cruising at FL360, are about to go down to FL340, but the pressurization panel is selected for FL370 and you'v never been that high during the flight, once you start decending to FL340 you will get a nice little yellow warning light come up on your glareshield.

yeah, I noticed you said check, but I thought I'd add for the others.

 

Alt Int will put you into DESC mode, IIRC. Haven't flown a jet with ALT INTV in a couple years.

Probably don't need to reset the Cruise Alt on the pressurization panel if you're going to a lower alt.

 

What if we descend from 370 to 350? Normal differential pressure at 370 is 8.35, at 350 - 7.8. If I don't set new FLT ALT, cabin variometer will show climb to get ΔP increase through the airplane descend and leave it at 8.35.

Shall I change the altitude in this case?

Change the altitude on the MCP, press that button, and it will enter that number as your new cruise in the FMS. Well, I do this for step climbs, I assume it works for step downs too......not sure why I assume that actually.

 

You have assumed... poorly.

 

As Matt stated also, pressing ALT INTV in that case will initiate an early descent (the equivalent of the DES NOW command on the FMC DES page), not a cruise descent to a new cruise altitude. That said, I've read around these parts that an early descent using ALT INTV will only be started when you're within 50 NM of your calculated top of descent, although I haven't tested that assertion myself. Yet.

If you have missed to enter your new cruise altitude before you get into the descent, and have lost your TOD and got into the descent mode, you can always go back to the CRZ page -which will be empty- and re-enter your cruise altitude, which will set you back on CRZ mode.

 

Regards,

Probably don't need to reset the Cruise Alt on the pressurization panel if you're going to a lower alt.

 

What if we descend from 370 to 350? Normal differential pressure at 370 is 8.35, at 350 - 7.8. If I don't set new FLT ALT, cabin variometer will show climb to get ΔP increase through the airplane descend and leave it at 8.35.

Shall I change the altitude in this case?

No need, because the pressure controller isn't trying to maintain that maximum differential. The cabin won't climb. It will descend slightly, just as with any normal descent.

 

If I don't set new FLT ALT, cabin variometer will show climb to get ΔP increase through the airplane descend and leave it at 8.35.
Shall I change the altitude in this case?

 

Actually the cabin needs to descend to keep the delta P high. That is not the problem anyway, the problem is with UNSCHED DES annunciation.

There's a Boeing publication out there (I haven't had enough coffee to find it yet) that says basically, Don't monkey with the Pressurization Controller. Set it for your anticipated final cruise and be done with it.

 

Maybe someone else know where it is.

 

Maybe someone else know where it is.

 

From my quick search with the Goggles, it seems to be a revision to the FCTM, so I can't post it here..

 

If you want to find it search "Setting the Cabin Pressure Control System" (leave the quotes in).

From my quick search with the Goggles, it seems to be a revision to the FCTM, so I can't post it here..

 

If you want to find it search "Setting the Cabin Pressure Control System" (leave the quotes in).

That's not the same source, but I think it's the same info. Good find.