B737-800 ALT INTV/ALT HOLD Question...

[Finals19]

Just a quick question for those more in the know...

Studying some ATPL notes on the B737-800 APFDS/MCP and reading about ALT INTV and ALT HLD functions. As I understand it, ALT INTV is used to delete an altitude currently held in the FMC and on to a lower / higher altitude. My question here is that if the a/c is in a VNAV descent, say to FL100, and one selects FL120 on ALT INTV, does this then disengage VNAV and descend the a/c to said altitude, at which point ALT HLD becomes the active mode? Does ALT INTV actually ammend the previous altitude in the FMC flight plan, or merely level the aircraft at demanded FL?

AIUI, selecting a value in the MCP ALT window whilst in VNAV descent / climb will cause the aircraft to level at that ALT/FL and for ALT HLD to become the active mode. I suppose re-engaging VNAV will cancel this active mode, and the aircraft will continue climbing / descending on profile to the next stipulated waypoint/altitude?

It just seems that ALT INTV and ALT HLD do the same thing in regards to deviating from VNAV?

[duckbelly]

Don't have ALT intervention on the 800's I fly so can't answer your first question.
If you have an alt in the ALT window and select VNAV, it won't go down until you select a lower altitude.
Eg. VNAV selected, cleared to FL 200, set in window. Aircraft will level off at FL200 regardless of what the VNAV thinks. What WILL happen is something called 'energy conservation'. The FMC will program a reducing IAS as you go above the VNAV PATH profile. When you get recleared to a lower level, the FMC will point the nose down to try and regain the VNAV PATH profile.
Usually what I do is select LVL CHG and crank up the speed to get back on profile, though, using speedbrakes as necessary.
seeya!:)

[omnidirectional737]

If you are in a descent aircraft will level off at VNAV constraint altitude in FMC or whatever is in the altitude window on MCP whichever is higher. So in your question it would level off at 120 as the higher level in the MCP. Butif you had FL120 as altitude constraint in FMC and ALT INTV selected and FL100 selected you will descend to FL100. It will remain in VNAV during the descent also then ALT HOLD when leveling off. That is my understanding but have never flown an aircraft with it installed so stand to be corrected by someone who has.