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VNAV descent B738

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can anyone please explain to me this paragraph :

 

Descent

 

VNAV can perform a descent in either of two modes- path descent or speed descent.

During a path descent , the FMC uses idle thrust and pitch control to maintain a vertical path, similar to a glideslope in three dimensions.

During a speed descent, the FMC uses idle thrust and pitch control to maintain a target descent speed, similar to a level change descent.

 

thank you for your help.

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First, the paragraph is not entirely correct. In VNAV PATH the FMC uses pitch control to maintain a vertical path, but it will also apply thrust if the speed drops below the target speed.

 

Now to your question, what exactly are you not understanding ?

 

PATH is like following a line drawn in the sky from cruise FL to the airport (or an intermediate waypoint)

 

SPEED is totally different as it is a descent which follows a constant IAS (well usually a constant MACH followed by a constant IAS)

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i don't understand the definition of : path descent and speed descent. i know what does  mean path , but i don't understand the maneuvers :

 

During a path descent , the FMC uses idle thrust and pitch control to maintain a vertical path

 

During a speed descent, the FMC uses idle thrust and pitch control to maintain a target descent speed, similar to a level change descent.

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i don't understand the definition of : path descent and speed descent. i know what does  mean path , but i don't understand the maneuvers :

 

 

 

 

Then google it. I asked what bit of the explanation you didnt understand and you tell me nothing.

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thank you for your help.

 

I would like to help further if you want to work with me?

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The only thing you really need to care about is:

 

If you're in a descent and it says 'VNAV PATH' then it's on the path and all is good.

 

If you're in a descent and it says 'VNAV SPD' then for some reason you're off the path and the aircraft needs some manual intervention from you (speedbrakes, SPD intervene etc) to regain it.

 

Also my 737 guy rarely uses VNAV below 10,000 feet anyhow. The usual modes are FLCH if high on the profile or VS if low.

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thank you for your help i finally understand the two modes of descent. But, actually i don't understand why the pilot, during VNAV descent and approach path,  must 

 

1/change to idle thrust

 

2/retract speedbrakes

 

3/descent wind speed decreasing with decreasing altitude

 

4/applicable target speed (even in descent path mode) 

 

are all those elements are used to compensate each other? i mean : reduce speed and at the same time keep a significant lift.

 

regards

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Simply speaking, sometimes the aircraft cannot automatically maintain the VNAV path without help. So it may be able to maintain the path but it can only do it by speeding up, which below 10,000 feet might be a problem because usually you're restricted to 250kts or less. Or it may be able to maintain the target speed, but doing so will mean it will not be able to maintain the target path. 

 

VNAV draws a line from the point at which you commence the descent (the TOD or Top of Descent) down to the ground. Well, there's a bit more to it than that but for the purposes of explanation, that's what it does. In an ideal world the autopilot, via the VNAV function, would simply fly down this line until you reach the runway. Of course in the real world there are winds, weather to avoid, other aircraft, ATC delays caused by a lot of traffic etc so in reality it's almost impossible to fly in a continuous path all the way to ground, as much as airlines and crews would like to. Many crews, especially of older aircraft, are also a bit distrustful of VNAV, and prefer to use FLCH or VS once closer to the ground. 

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