V3 DESCENT LOGIC AND LEVEL DECELERATION
V3's Thales descent modelling includes a decelerated approach with a level segment before final approach. A descent therefore need not be one continuous downward line. Part of the plan may provide room to slow before joining the final path. That can surprise a beginner who expected the aircraft to keep descending until it reached the runway. When it levels, first read the active mode and selected altitude and look at the planned profile. Those indications help establish whether we are seeing the intended deceleration segment, a cleared level being respected, or a different guidance state that needs attention. [11, Chapter LNAV/VNAV]
The computed deceleration segment can behave like an altitude constraint in the managed profile. It still works within the altitude authority given through the FCU; a calculated path does not provide a clearance. Keep three things connected: the level we are cleared to, the target selected in the FCU, and the path the active mode is trying to fly. A level aeroplane by itself tells us little about which of those is responsible. On the practice arrival, take a moment to identify the expected slowing and configuration before the final descent. The level segment then has a purpose in the approach we are building. [11, Chapter LNAV/VNAV] [13, PDF pp. 141-146]
Vectors and shortcuts change the distance left to do that work. Once the route changes, the earlier descent picture may no longer describe the aircraft's situation. Reassess height, speed, configuration and the distance to the intended intercept or approach point, then inspect the revised prediction. The horizontal route and the vertical plan are connected because every removed mile was also space in which the aircraft could descend or decelerate. Follow the guidance changes with that relationship in mind. The V3 presentation explains the modelled behaviour; the FMA, route and actual aircraft state let us recognise how it applies to this particular arrival. [11, Chapter LNAV/VNAV] [13, PDF pp. 141-146]