Modifying FMC crz level can cause actual CLB&DES while in VNAV PATH mode in 747 ???

[FlywithPark]

I just experienced an unfamiliar thing during yesterday flight.
While i was in VNAV PATH Cruise at FL350, i modified FMC Flight level to FL349 with expecting "VNAV ALT". However what i've really seen was the A/C did actual descent to FL349 without changing FMA(was Still VNAV PATH). Furthermore, after i modified to FL350 again, A/C started to actual climb to resume FL350.(with almost 500~700 fpm)
I tried to figure out why this thing happened from FCOM but couldn't find it.
it has been my common sense that FMA should have to change to VNAV ALT if there is any difference MCP target alt with FMC VNAV crz alt.(in VNAV mode)

any idea?

p.s: "TD"was not even close at that point.(it was around 200NM away)
and this thing did not happen when i tried to modify to FL340.(FMA changed to VNAV ALT)

[nitpicker330]

Yep, 777 did the same thing. Something about being within 100' ??

[FlightPathOBN]

While i was in VNAV PATH Cruise at FL350

Wouldnt you be in VNAV PATH while getting to FL350 cruise altitude of VNAV ALT?

If your preselected ALT was FL350, and you were crusing at that level, wouldnt that be then VNAV ALT? not PATH?

[Wizofoz]

No, you re in VNAV Path if maintaining the altitude in the FMC Cruise page.

[SMOC]

it has been my common sense that FMA should have to change to VNAV ALT if there is any difference MCP target alt with FMC VNAV crz alt.(in VNAV mode)


If that were the case then any change in the MCP altitude would cause an FMA change to VNAV ALT.

The MCP has very little to do with "smart" concepts its an ENTER/DELETE button effectivly, VNAV ALT results when the MCP prevents the FMC/VNAV from doing what it wants, seeing you stepped down to FL349 the MCP didnt inhibit the change so it stayed in PTH, try stepping to FL351 and it'll go VNAV ALT as the "dumb" MCP is preventing it from going there.

[mustafagander]

AFAIK within 200ft the FMC is happy that you are at the selected alt in VnavPath. It is merely trimming the alt to agree with the MCP selection.

Try it again with, say, 400ft difference.

[SMOC]

VNAV PTH captures within 150' (so thats why 100' results in no mode change) enter a 200' or more change (climb) and it will go into THR REF - VNAV SPD unless as I said above the MCP altitude stops it, resulting in VNAV ALT.

[FlywithPark]

Thank you, you made me clear.