VNAV - Altitude calculation in FMC

[vnangli]

I was doing a flight from KORD to KMSP yesterday.

STAR : EAU CLAIRE 9 ARR
Runway : 30L
IAF : HASTI (7000A)
Speed at T/D : 283K

Past the T/D (after EAU) FMC had computed the altitude at HERMI at a certain value (9500). Just after crossing the T/D, I applied flaps 1 to observe the flight path and vertical deviation in ND.
I noticed the Altitude values in 1R of FMC CDU (ACT RTE page) was continuously being revised (it was reducing) after I applied flaps and changed the speed for flaps 1 in MCP. And the VERTICAL DEVIATION scale was indicating that I was below the path.

This was a new learning for me. I had an understanding that the FMC CDU Altitude values at 1R would remain "AS-IS computed" before applying flaps 1, and any altitude deviation during DESCENT due to flaps would be indicated as deviation in the Vertical deviation scale...In my example above both were happening, meaning the FMC CDU 1R values were continuously being recomputed as well as the Vertical deviation scale in the ND was showing a progressively increasing deviation....Just to clarify, I hadnt set any hard altitude at HERMI. Any explanation would be helpful.

[Hardy Heinlin]

The predicted altitudes (small font) are just as dynamic as your true weather along the flight path and your manually controlled flap and speed schedule, anti-ice activations, QNH/STD differences etc. It's not a static path like an ILS. If you use the NG FMC, the FMC will provide a more stable path prediction due to the NG's off-idle thrust path.


|-|ardy

[vnangli]

The predicted altitudes (small font) are just as dynamic as your true weather along the flight path and your manually controlled flap and speed schedule, anti-ice activations, QNH/STD differences etc. It's not a static path like an ILS. If you use the NG FMC, the FMC will provide a more stable path prediction due to the NG's off-idle thrust path.


|-|ardy

Thank you for the explanation...
NG FMC is in my shopping cart already..