Originally Posted by
Capn Bloggs
My head hurts.
Likewise!
You only need to use ALT INTV in the climb to set a new cruise alt in the FMC. For example: your FMC cruise ALT is FL380. You've leveled off at FL360 due to ATC. When they give you a climb to 380 you just set that in the MCP and the AP will go to ALT HOLD mode, when you re select VNAV it will resume the programmed CLB profile up to 380. If you later want to climb to 400 you simply set the MCP to 400 and press ALT INTV, it will automatically re cruise the FMC to 400 and begin a cruise climb at the calculated speed (Mach).
On descent INTV now has 2 functions depending on distance to TOD, at least on U11 and later software. If you're more than 50 miles from calculated TOD and set a lower MCP altitude then press INTV it will re cruise to the lower altitude and descend at 1000fpm at the lower alt cruise speed. You shouldn't do this for a large (4000 feet or more) change as the new cruise speed may be too slow for your current altitude. If you're less than 50 miles from TOD when you push INTV it will go to "early descent" mode, which means 1000fpm at the normal descent profile until you reach A: the new MCP altitude, B: a restriction in the FMC above MCP altitude, or C: the idle descent path at which point it will descend in the path until reaching A or B. Since the FMC was not re cruised in this case when starting a further descent one needs only set the new MCP altitude and re select VNAV if necessary, it will continue descending in early descent as above.
I didn't explain the use of intervention to delete FMC crossing altitudes as that seems to be understood already.
There are way too many techniques to get into, but if someone is fat fingering multiple modes in short time for no apparent reason it would seem to indicate a lack of confidence/familiarity with the automation. Which is understandable as it's not an intuitive system.