camrn
31st Mar 2012, 08:27
A fast question about the VNAV descent path. The plane descents in idle thrust, from Cruise to the first ALT constraint (performance path). From there, it follows the point-to-point calculated path (geometric path). Now the question. Let's say you have a ABCDE 240/9000 restriction on the active leg. And 50 miles ahead you have an FGHIJ 240/8000 restriction. The MCP alt is set to 8k.
1)The FMC will calculate and display a new T/D circle?
2)It will maintain 9000 until it reaches a point at which it will descent with IDLE thrust to 8000? (so it's going to use performance path again?)
3)Or it will remain on SPD and descent really slowly to capture the next constraint at 8000?
From the FCOM:
The top of descent (T/D) is the point where the cruise phase changes to the descent
phase. A green circle T/D symbol displays on the ND at the start descent phase
point. The FMC displays a T/D symbol at each point in the descent profile which
represents a transition from a level segment to a descent segment. The descent
path starts at the T/D and includes waypoint altitude constraints. The path to the
first constraint is based on:
• idle thrust
• speedbrakes retracted
• FMC cruise wind
• wind entries on the DESCENT
FORECAST page
• predicted use of anti-ice
• applicable target speed
Highlight FIRST waypoint please. But it also talks about multiple T/D circles on the ND.
Later on it says:
Descent path segments after the first altitude constraint waypoint are constructed
as straight line point-to-point segments. If the VNAV path segment is too shallow
to be flown satisfactorily at IDLE thrust, the FMC commands speed on thrust
levers (SPD). Elevators control the shallow descent path.
So in the example I used in the beginning this last abstract suggests that the plane will descent power on (SPD) with like 100fpm.
I feel that this is clear as mud. Any input?
1)The FMC will calculate and display a new T/D circle?
2)It will maintain 9000 until it reaches a point at which it will descent with IDLE thrust to 8000? (so it's going to use performance path again?)
3)Or it will remain on SPD and descent really slowly to capture the next constraint at 8000?
From the FCOM:
The top of descent (T/D) is the point where the cruise phase changes to the descent
phase. A green circle T/D symbol displays on the ND at the start descent phase
point. The FMC displays a T/D symbol at each point in the descent profile which
represents a transition from a level segment to a descent segment. The descent
path starts at the T/D and includes waypoint altitude constraints. The path to the
first constraint is based on:
• idle thrust
• speedbrakes retracted
• FMC cruise wind
• wind entries on the DESCENT
FORECAST page
• predicted use of anti-ice
• applicable target speed
Highlight FIRST waypoint please. But it also talks about multiple T/D circles on the ND.
Later on it says:
Descent path segments after the first altitude constraint waypoint are constructed
as straight line point-to-point segments. If the VNAV path segment is too shallow
to be flown satisfactorily at IDLE thrust, the FMC commands speed on thrust
levers (SPD). Elevators control the shallow descent path.
So in the example I used in the beginning this last abstract suggests that the plane will descent power on (SPD) with like 100fpm.
I feel that this is clear as mud. Any input?