Wing From Turkey, joined Oct 2000, 1552 posts, RR: 25 Reply 4, posted (8 years 5 days 9 hours ago) and read 1951 times:
The VNAV SPD and LVL CHG both descent with the given speed at IDLE thrust(assuming the speed for VNAV SPD descent will be around 280-300 kts),uses pitch up and down to control the airspeed.The only difference is VNAV will obey any speed restrictions previously written on FMC,(ex:250 kts below 10000 or any speed restriction required by the arrival) while level change doesnt care about any restrictions continue with dialed speed.
And you must have an lateral flight plan to follow to do a VNAV descent,any time the ATC gives you a different radar vector,you can not use VNAV,You have to calculate your descent path yourself and descent with LVL CHG or VS depending on your need.
CosmicCruiser From United States of America, joined Feb 2005, 2212 posts, RR: 16 Reply 5, posted (8 years 5 days 4 hours ago) and read 1951 times:
Quoting Wing (Reply 4): The VNAV SPD and LVL CHG both descent with the given speed at IDLE thrust(assuming the speed for VNAV SPD descent will be around 280-300 kts),uses pitch up and down to control the airspeed.The only difference is VNAV will obey any speed restrictions previously written on FMC,(ex:250 kts below 10000 or any speed restriction required by the arrival) while level change doesnt care about any restrictions continue with dialed speed.
Reconsider what you wrote here. VNAV (prof) does NOT use pitch for a/s control. If it did you would not stay on "profile". It uses thrust for a/s. I rarely see "idle" in PROF unless the winds are different than input or we have a late descent that has us above profile already. This being the case pushing "PROF" will give "idle" and maybe "ADD DRAG" until you are on profile and then you have "PROF" and spd. on thrust.
Quoting Wing (Reply 4): And you must have an lateral flight plan to follow to do a VNAV descent,any time the ATC gives you a different radar vector,you can not use VNAV,You have to calculate your descent path yourself and descent with LVL CHG or VS depending on your need.
True to a point but a little trick some guys use is, when on a heading for a vector for the app. enter direct to the FAF (final app. fix) and don't clear the T/P (turn point). Now you're still on a heading but the FMS is still calculating the descent to cross the FAF at the app. alt. Honestly, I don't use this much myself but some guys do.
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