MIQUEL_7

FMC Question descending path

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Hello, I'm a flight sim enthousiast, 

 

I quit don't understand these parts of the auto LNAV and VNAV descent... 

On the first picture you see the FMC descending numbers you know what I mean... 

Can I have some explaining please what means the 'A' next to the 3000 and also why is after 3000 a 7000 and a 6500 feat? 

When I reached the T/D I put my final descend in the MCP which is at Ibiza 3000 and then I descend always with Vertical Speed above 2000 but that is not realistic? 

What do I have to do when I descend my plane to not use the Vertical Speed? 

Can the plane auto descend until I reached 3000 and then to put the Autopilot off and land it myself? 

Or is it normal that I have to use the VS that high always...? 

 

I'm sorry I can't explain this what I mean exactly... If someone can help me I'd be grateful, my English is not perfect but I tried to explain it. 

 

Please if someone knows the answer tell me ;)

 

Greetings, MIQUEL 

 

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Miguel,

 

The "A" means "at or above".  Altitude constraints may be at, at or above, at or below "B".   VNAV will fly a profile to the waypoint in a manner that will comply with the coded or manually entered constraint. 

VNAV can perform the descent one of two ways.  The first is a performance based descent where the TOD is selected based on descent winds, TAI on/off predictions, CI, descent speed, etc.  Black magic stuff in the box.  The objective is to obtain an idle thrust descent that meets the constraint.  Depending on the speed, CI, winds aloft, a descent vertical speed above 2000 FPM is not unrealistic for a B737. It might be a bit high for a much heavier airplane like the B777.  In designing STARS, one of the tricks is to place waypoints and constraints such that the majority of airplanes flying the descent profile don't have to use the speed brakes.  

The second way a descent can be made is geometric, which is a constant descent angle between to waypoints with altitude constraints.  The FMC figures out whether the points are close enough together for a straight descent path, or whether it is more economical to hold altitude after passing one constraint and then making a performance descent to the next.  Again, black magic within the FMC.  

There is an option for always doing s geometric descent between waypoints in the GE FMC used in the B737. If I recall correctly from Ryan, that is how PMDG programmed their FMC.  

Vertical Speed mode (VS) is not used much except for small altitude changes where the rate needs to be controlled or when necessary to limit the vertical rate at level off (ACAS/TCAS considerations).  After passin the last altitude constraint, you may need to use a different mode like FLC, but given that nearly all approaches have constraints, VNAV may continue to work well.  

VNAV can be thought of as a strategic mode, where it's use is planned well in advance.  FLC and VS are tactical modes where the pilot needs to respond to immediate changes, say in response to ATC changes. 

Hope this was helpful. 

Rich Boll

Wichita, KS

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