Geometric Path confusion during VNAV PATH Descent.
Having gone through FCOM Chapter11 on MCNP SPD INTV, VNAV PATH, SPD descent modes, I still am not able to clear some of the materials.
This happened once during descent in actual flight and we were doing nominal descent from CRZ ALT of FL240. Here is the information for each waypoint.
CRZ: - FL240
Waypoint A (Start of STAR) : FL180 by ATC
Waypoint B : FL160A
Waypoint C : FL120A
During descent from FL240, having passed numerous waypoints on airways, waypoint A was the first restriction to cross AT FL180 assigned as such by ATC.
Then, ATC re-cleared us to delete FL180 and continue descend to FL150. I instantaneously pressed ALT INTV to delete FL180 from FMC then FMA changes from VNAV PATH to VNAV SPD.
Here is the confusion. Did the FMA change to VNAV SPD because the PATH no longer exists between present position to Waypoint A or we there were insufficient information to remain in VNAV PATH by TOD (as mentioned in FCOM chapter 11) ? FCOM states that the mode will change to VNAV SPD when insufficient info exists.
Next, the definition of phrase 'point-point(geometric path)' from FCOM, and its meaning in terms of altitude restriction being shown on FMC LEG page? There can Above, At or below, or hard restriction to be crossed at certain waypoint however FCOM doesn't specify the parameters.
Many Thanks,
John