VNAV ALT HOLD - What is it?

[pcubine 143]

On a KTPA-KJFK flight this morning with a CAMRN STAR to 22L at KJFK the 738 would not descend below 10000 ASL even though the MCP had 4000 ASL dialed in and speed was at 206 kts. I then noticed VNAV ALT HOLD on the PFD. I waited as long as I could for the 738 to descend but at around 3000 feet above the VNAV path I used VS to finish the flight. I can find nothing in the FCOMs' about this mode using search and just browsing thru them. Does anyone know what would have caused this?Thank youMichael Cubine

[scandinavian13 5,192]

ALT HOLD means just that. It HOLDs the current ALT.

[pcubine 143]

KyleI know what it means. That is self-explanatory. My question is - what causes it and how do you resume a VNAV descent?ThanksMichael Cubine

[Spin737 102]

There's ALT HOLD and VNAV ALT. maybe you dropped out of VNAV with a late MCP change and into ALT HOLD.

[scandinavian13 5,192]

I know what it means. That is self-explanatory. My question is - what causes it and how do you resume a VNAV descent?

Sorry. I scratched my head on why exactly you were asking that (as I've seen you ask some more complex questions in the past), but figured sometimes people just need the plain stuff pointed out to them.Basically, it seems like what happens when you hit your initial alt restriction. I depart BWI and I get 4000 initial. I roll that into the ALT window on the MCP. After departing, when I get close to it, the buzzer goes off and the plane starts to level off. Even if I roll the alt higher (or in your case, lower), the plane will stay at 4000 feet. Basically, if I get cleared higher, I'd roll the next altitude in the window and then hit ALT INTV. At that point the climb would resume. Same goes for your descent. Hopefully that's what you're asking...I think the tutorial briefly mentions this when it's talking about the departure procedure. I'm not pointing it out to say RTFM (or T), rather so you can just go read through those few pages instead of the whole FM

[AirbusCG 52]

On a KTPA-KJFK flight this morning with a CAMRN STAR to 22L at KJFK the 738 would not descend below 10000 ASL even though the MCP had 4000 ASL dialed in and speed was at 206 kts. I then noticed VNAV ALT HOLD on the PFD. I waited as long as I could for the 738 to descend but at around 3000 feet above the VNAV path I used VS to finish the flight. I can find nothing in the FCOMs' about this mode using search and just browsing thru them. Does anyone know what would have caused this?Thank youMichael Cubine

Michael,was the aircraft in VNAV ALT or ALT HOLD mode? As far as I know there is no VNAV ALT HOLD mode.For the case that it was in VNAV ALT mode, which is the one I'd expect in your case), I think you have turned your alt selector on the MCP down to late.To get out of this mode you'd have to push the ALT INTV button which will cancel the alt restriction you've accidently entered and will resume the descent in VNAV PTH mode (or in VNAV SPD, not sure).When it was in ALT HOLD you'd have to push the VNAV button on the MCP again, or you select another descend mode like LVL CH or V/S.

[pcubine 143]

Michael,was the aircraft in VNAV ALT or ALT HOLD mode? As far as I know there is no VNAV ALT HOLD mode.For the case that it was in VNAV ALT mode, which is the one I'd expect in your case), I think you have turned your alt selector on the MCP down to late.To get out of this mode you'd have to push the ALT INTV button which will cancel the alt restriction you've accidently entered and will resume the descent in VNAV PTH mode (or in VNAV SPD, not sure).When it was in ALT HOLD you'd have to push the VNAV button on the MCP again, or you select another descend mode like LVL CH or V/S.

EmanuelIts possible that the airplane got to close to 10000 ASL before I dialed 4000 into the MCP atitude window. I don't like to go below 10000 on the MCP until I see that the plane is going to be under 250 kts and sometimes that can be a close run thing when in VNAV. If I remember correctly the VNAV button on the MCP was still lit and the altitude hold button on the MCP was not lit, but a least I know how to get out of it now if it happens again.ThanksMichael Cubine

[emvaos 13]

The system will not allow the plane in descend phase to cross the restriction altitude (usually 10000') at a speed greater by more than 10 knots than the restriction speed (usually 240). VNAV will temporarily level off the plane until active IAS bleeds off and then resume the descend (in which case you'll probably be in an over-the-path condition).Is that the case ? That is a real system feature (you could also check the manual....)

[Fabo 213]

MCP buttons are lit when you can cancel the respective mode by pressing them. This is not a representation of what the autopilot is set to (that you will find on FMA) but I guess you already know that.

[AirbusCG 52]

The system will not allow the plane in descend phase to cross the restriction altitude (usually 10000') at a speed greater by more than 10 knots than the restriction speed (usually 240). VNAV will temporarily level off the plane until active IAS bleeds off and then resume the descend (in which case you'll probably be in an over-the-path condition).Is that the case ? That is a real system feature (you could also check the manual....)

Most propably it went in the VNAV ALT mode. It can already engage about 900ft above the altitude in the window, when you only descend fast enough. The VNAV mode will remain engaged, as the airplane sets your 10.000ft as an altitude restriction. It will remain on this altitude until this restriction is cancelled. This is most easily done with the ALT INTV button.

[pcubine 143]

Thanks for all the help.Michael Cubine

[RYR738 9]

Michael !VNAV ALT was introduced by Boeing to stop VNAV from disengaging if the plane levelled off at an altitude not in the FMC.Before it was introduced, pilots would change altitude using the MCP and then engage VNAV again.With the VNAV ALT feature you just need to press ALT INTERVENTION to allow the plane to continue it's climb or descent.( a perfect example of this can be seen in the Tutorial Flight )** Not all pilots like VNAV ALT as it also goes into VNAV ALT at ANY MCP setting.........wheter in the FMC or not.Hope that helps.Fred.