T/D - VNAV - Alt Hold /Question

[Guest J_Niny]

Hi all,lets say that I`m at the T/D and I don`t want to start the descent. In the NG, the pilot should change from the VNAV to the ALT Hold or the AP change the mode automatically like in the 767/757?thanks,Jorge Niny

[Guest rcarlson123]

If you still have your cruise altitude set in the MCP, then the FMC will display a "Check MCP Alt" error and disconnect VNAV mode, leaving you in ALT HOLD mode, I think.Otherwise, you would have to disengage VNAV manually by setting some other pitch mode such as ALT HOLD or VS.

[Guest J_Niny]

>If you still have your cruise altitude set in the MCP, then the FMC >will display a "Check MCP Alt" error and disconnect VNAV mode, >leaving you in ALT HOLD mode, I think.Hi Ross,thanks for the reply.Yes you`re right, but in this case the speed is decreased from cruise speed to descent speed.In the 767s, if the MCP altitude is not reset prior to the T/D, the AFDS removes VNAV mode and enters ALT HOLD mode, but it will maintain the current cruise speed, I think.That`s the reason of my question. I

[Guest rcarlson123]

I would say certainly the "right" thing to do if you don't want to descend is to disengage VNAV before the top of descent. If you don't want to descend, then you want to hold the current altitude, right? So the right mode would be Alt Hold.

[Guest J_Niny]

Yes, actually that

[Guest Iz]

Nah, in theory, you should just be able to keep on flying. VNAV will just not descend below the set MCP altitude. There is such a thing as energy compensation where the aircraft may slow down, even below descent speed, but I think that only happens when you're already in a VNAV descent.In any case, the aircraft should automatically enter ALT HOLD. I believe the 757/767 would enter VNAV ALT, but since 95% of 737NG APFDs don't have altitude/speed intervention, this mode is not available and it leaves VNAV, for ALT HOLD.I remember 767PIC was also without altitude intervention, so yes, it probably enters ALT HOLD as well. But all 757/767s delivered after 1990 (and possibly even earlier) should have altitude intervention installed.(by the way, altitude intervention allows you to push the ALT selector on the MCP, giving you all kinds of extra VNAV functionality, like VNAV Level Change, Descend Now, Delete Altitude Constraints and such)

[Guest J_Niny]

Hi IZ,firstly thank you for the prompt response and the tips.>In any case, the aircraft should automatically enter ALT HOLD.Well, I did some more investigations and I noticed that it even enter the ALT HOLD mode. In a low level flight (BOS-JFK FL240) it did start do decelerate before the T/D and didn`t enter the ALT HOLD after having passed the T/D. Please take a look on the attachments below.Kind Regards,Jorge Niny

[Guest J_Niny]

>I noticed that it even enter the ALT HOLD mode.Just to correct my post above.Please read:I noticed that it even NOT enter the ALT HOLD mode.Jorge Niny

[zubart 5]

VNAV mode is terminated by any one of the following:- selecting another pitch mode- glideslope capture- reaching end of LNAV route- transition of glideslpoe intercept waypoint if G/S is armed- crosstrack deviation exceeds twice the RNP value during PTH descent for an active leg with a database verticle angle and LNAV not engaged- if the airplane altitude is more than 200 feet below MCP altitude and the autoflight system is not in altitude acquire.Floyd