asteriskceo
Posts: 436
Joined: Fri May 21, 2004 12:42 pm

Descent Methods

Wed May 18, 2005 11:51 am

What is the difference between a VNAV SPD descent and a LVL CHG descent in a Boeing aircraft.They seem similar to me.
 
mandala499
Posts: 6459
Joined: Wed Aug 29, 2001 8:47 pm

RE: Descent Methods

Wed May 18, 2005 2:08 pm

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 !
 
Lemmy
Posts: 243
Joined: Fri Dec 10, 2004 8:40 am

RE: Descent Methods

Wed May 18, 2005 11:02 pm

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 ...
 
CosmicCruiser
Posts: 2049
Joined: Tue Feb 22, 2005 3:01 am

RE: Descent Methods

Thu May 19, 2005 12:05 am

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.
 
wing
Posts: 1357
Joined: Sun Oct 01, 2000 9:10 pm

RE: Descent Methods

Thu May 19, 2005 4:42 pm

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.
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CosmicCruiser
Posts: 2049
Joined: Tue Feb 22, 2005 3:01 am

RE: Descent Methods

Thu May 19, 2005 9:49 pm

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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