B747-400 Late Descent in VNAV SPD
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Join Date: Aug 2010
Location: Los Angeles
Age: 59
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B747-400 Late Descent in VNAV SPD
I am looking for some input on the following scenario:
You're at FL350 and you start a late descent, 15 miles past the FMC-calculated top of descent point. When you finally do reset the MCP to a lower altitude and push the knob in, it's going to go to IDLE/HOLD||VNAV SPD. At this point you are about 5000' above the descent path. For purposes of this scenario, let's presume that we are above the first speed or altitude constraint and that we have left whatever descent mach/speed on VNAV 3/3 that ECON computed based on our cost index.
What speed will the aircraft maintain with the speed window closed?
It is my understanding that it will seek to mainatin the speed displayed on VNAV 3/3 with no additives. Since you are more than 150' above the path and in VNAV SPD, VNAV isn't going to try to descend any faster to capture the path, so there is no attempt whatsoever made to increase speed past the FMC target descent speed (no additives to this speed).
You're at FL350 and you start a late descent, 15 miles past the FMC-calculated top of descent point. When you finally do reset the MCP to a lower altitude and push the knob in, it's going to go to IDLE/HOLD||VNAV SPD. At this point you are about 5000' above the descent path. For purposes of this scenario, let's presume that we are above the first speed or altitude constraint and that we have left whatever descent mach/speed on VNAV 3/3 that ECON computed based on our cost index.
What speed will the aircraft maintain with the speed window closed?
It is my understanding that it will seek to mainatin the speed displayed on VNAV 3/3 with no additives. Since you are more than 150' above the path and in VNAV SPD, VNAV isn't going to try to descend any faster to capture the path, so there is no attempt whatsoever made to increase speed past the FMC target descent speed (no additives to this speed).
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Join Date: Aug 2010
Location: Los Angeles
Age: 59
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Thanks for the quick reply. I had a friend propose to me that in my scenario VNAV SPD could go up to VMO-11. Apparently he read this in an LH manual. So, either LH had some modification to their VNAV SPD descent logic or the manual was not very clear and they were actually referring to a situation where the aircraft was in VNAV PTH -- not VNAV SPD -- and trying to maintain within 150' of path. The VNAV PTH scenario would be correct.
It has always been my understanding that a VNAV SPD descent (window closed) is flown at whatever speed is in the FMC target descent on VNAV 3/3 -- no additives to that speed -- because at that point VNAV SPD doesn't care about capturing the path.
It has always been my understanding that a VNAV SPD descent (window closed) is flown at whatever speed is in the FMC target descent on VNAV 3/3 -- no additives to that speed -- because at that point VNAV SPD doesn't care about capturing the path.
