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Descent Methods  
User currently offlineAsteriskceo From United States of America, joined May 2004, 489 posts, RR: 0
Posted (9 years 7 months 1 week 3 days 12 hours ago) and read 2803 times:

What is the difference between a VNAV SPD descent and a LVL CHG descent in a Boeing aircraft.They seem similar to me.

5 replies: All unread, jump to last
 
User currently offlineMandala499 From Indonesia, joined Aug 2001, 6965 posts, RR: 76
Reply 1, posted (9 years 7 months 1 week 3 days 10 hours ago) and read 2700 times:

Basics...
VNAV Descent = Follow the most efficient path down.
LVL CHG = Hold the IAS/Mach at idle thrust.

VNAV SPD = the most efficient path down for a given speed.

LVL CHG can be uncomfortable if one's not careful...

However, the above are just the basic concept...

Mandala499



When losing situational awareness, pray Cumulus Granitus isn't nearby !
User currently offlineLemmy From United States of America, joined Dec 2004, 260 posts, RR: 0
Reply 2, posted (9 years 7 months 1 week 3 days 1 hour ago) and read 2634 times:

Let's throw VNAV PTH in here as well. What's the difference between PTH and SPD?

I understand that LVL CHG simply maintains a specified speed at idle thrust. Are the VNAV modes flown at idle as well?



I am a patient boy ...
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 3, posted (9 years 7 months 1 week 3 days ago) and read 2613 times:

Quoting Lemmy (Reply 2):
Are the VNAV modes flown at idle as well?

In VNAV the throttles will do what is required to maintain a given speed on the descent profile. That can be anywhere from idle up.


User currently offlineWing From Turkey, joined Oct 2000, 1575 posts, RR: 24
Reply 4, posted (9 years 7 months 1 week 2 days 7 hours ago) and read 2485 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.



Widen your world
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 5, posted (9 years 7 months 1 week 2 days 2 hours ago) and read 2485 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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