VNAV

[jarmstro 2,406]

Is VNAV actually used by RL pilots? I mean descent is dictated by ATC so whats the point of VNAV?

Edited March 4, 2023 by jarmstro

[Gazzareth 848]

37 minutes ago, jarmstro said:

Is VNAV actually used by RL pilots? I mean descent is dictated by ATC so whats the point of VNAV?

If you watch P1D on YouTube he regularly uses it, pretty much every time he gets a crossing restriction. Although from what I've seen it's used less in the RW than I would use it on a flight.... ...

 

G

[carlanthony24 593]

VNAV will keep you in a steady descent and keep within restrictions on the sid/star. Airbus for instance start descent 10 miles before it says and it will start descending a 1000fpm when the VNAV catches up then it will start descending around 1800fpm pretty much same with Boeing. Only difference in an Airbus you need to push for Managed VNAV and in Boeing it would do it self and descend right on cue or go alt intervene to start descent early and wait for VNAV to catch up. 

It really helps in the US for instance where they just tell you to descend via the star so you can set the lowest altitude before they start vectoring you and it will obey all restrictions. 

Real world its more comfortable to use for passengers instead of immediately going in flight level change or open descent.

Edited March 4, 2023 by carlanthony24

[jarmstro 2,406]

10 minutes ago, carlanthony24 said:

VNAV will keep you in a steady descent and keep within restrictions on the sid/star. Airbus for instance start descent 10 miles before it says and it will start descending a 1000fpm when the VNAV catches up then it will start descending around 1800fpm pretty much same with Boeing. Only difference in an Airbus you need to push for Managed VNAV and in Boeing it would do it self and descend right on cue or go alt intervene to start descent early and wait for VNAV to catch up. 

It really helps in the US for instance where they just tell you to descend via the star so you can set the lowest altitude before they start vectoring you and it will obey all restrictions.

But what I mean is that you are told when to descend by ATC and to what altitude. Is this what happens in real life? I don't understand what use VNAV is if you are flying under the control of ATC? I'm not a pilot and am just trying to understand. EDIT Can you tell ATC you are going to use VNAV and that you are about to descend?

Edited March 4, 2023 by jarmstro

[carlanthony24 593]

4 minutes ago, jarmstro said:

But what I mean is that you are told when to descend by ATC and to what altitude. Is this what happens in real life? I don't understand what use VNAV is if you are flying under the control of ATC? I'm not a pilot and am just trying to understand. EDIT Can you tell ATC you are going to use VNAV and that you are about to descend?

You don't need to tell atc that you are using VNAV if they say descend FL250 and you are in the Airbus just push for Managed which is what they would do IRL or in the Boeing go ALT intervene to start with which still arms VNAV till the path catches back up. If they want want you to descend quickly then pull for open descent in the Airbus or Boeing go flight level change or you could even go vertical speed on both types.

[V1ROTA7E 732]

15 minutes ago, jarmstro said:

But what I mean is that you are told when to descend by ATC and to what altitude. Is this what happens in real life? I don't understand what use VNAV is if you are flying under the control of ATC? I'm not a pilot and am just trying to understand. EDIT Can you tell ATC you are going to use VNAV and that you are about to descend?

we use VNAV just about all the time. It's a fantastic tool with many use cases. If ATC said descent and maintain FL250, we could do so in VNAV. If ATC says Descend Via the arrival, VNAV will manage the entire descent, and adjust the descent path for winds/temp/engine and wing anti ice requirements. VNAV will manage all altitude restrictions, as well as do its best to manage speed restrictions as well. An example of a VNAV feature (in most airliners) is it creates a level off point to allow the plane to decelerate to 240-250 knots before 10,000' to comply with the speed restriction. Without VNAV, you can easily fly through 10,000 exceeding 250 knots, so VNAV will offer this protection. VNAV is cherished by airline pilots and is used as a standard vertical mode throughout the industry.

[martinboehme 586]

1 hour ago, jarmstro said:

But what I mean is that you are told when to descend by ATC and to what altitude. Is this what happens in real life?

It depends. They might dictate when they want you to start descending, but if there isn't any conflicting traffic, they might also tell you "when ready descend" or "descend at pilot's discretion". This means they're leaving it up to you when to descend.

[jarmstro 2,406]

12 minutes ago, martinboehme said:

It depends. They might dictate when they want you to start descending, but if there isn't any conflicting traffic, they might also tell you "when ready descend" or "descend at pilot's discretion". This means they're leaving it up to you when to descend.

But what if TOD on the FMC is before the instruction from ATC?

[vgbaron 3,731]

37 minutes ago, jarmstro said:

But what if TOD on the FMC is before the instruction from ATC?

Depends on last ATC instruction. If you got descend and maintain FL15 and you are at 15 when FMS reached TOD you either contact ATC and request descent or stay at 15. If you got a proceed as filed then ATC would be expecting you to follow your filed flightplan with expected descents. There are a lot of variations

 

[Bobsk8 12,960]

1 hour ago, V1ROTA7E said:

we use VNAV just about all the time. It's a fantastic tool with many use cases. If ATC said descent and maintain FL250, we could do so in VNAV. If ATC says Descend Via the arrival, VNAV will manage the entire descent, and adjust the descent path for winds/temp/engine and wing anti ice requirements. VNAV will manage all altitude restrictions, as well as do its best to manage speed restrictions as well. An example of a VNAV feature (in most airliners) is it creates a level off point to allow the plane to decelerate to 240-250 knots before 10,000' to comply with the speed restriction. Without VNAV, you can easily fly through 10,000 exceeding 250 knots, so VNAV will offer this protection. VNAV is cherished by airline pilots and is used as a standard vertical mode throughout the industry.

Stands to reason , if it were not useful, it wouldn't be on airliners. 

[regis9 4,051]

VNAV generally will do a lot of the work for you to ensure to aircraft is climbing/descending appropriately, adhering to speed and altitude constraints etc.

A cool feature as it relates to ATC is that if they tell you to be at a certain altitude (and or speed) at a particular waypoint you can enter that into your FMC and the aircraft will attempt to fly accordingly (the pilot still needs to monitor to ensure you’ll comply with the instructions).

Think of VNAV as more of how you get to a speed/altitude rather that what speed or altitude.

[DavidP 1,426]

1 hour ago, jarmstro said:

But what if TOD on the FMC is before the instruction from ATC?

I think it very much depends on the country. In my country, it is very common to receive the instruction "when ready, descend to XXX". This means that the aircraft can descend from their own ToD point - I do it in my own aircraft using VNAV on my GTN750. I would imagine the airliner would simply activate VNAV until they receive instructions from ATC that precludes them from using VNAV.

So much depends on the circumstances - traffic, weather etc. 

[Dave_YVR 1,000]

7 hours ago, jarmstro said:

But what if TOD on the FMC is before the instruction from ATC?

You request decent prior to tod.

[jon b 2,083]

15 hours ago, jarmstro said:

But what if TOD on the FMC is before the instruction from ATC?

Top tip on this one if ATC delay your descent……”if you can’t go down, slow down”

Wind the speed right back, then when you finally get descent clearance you can crank the speed right back up and the aircraft when in VNAV SPD (Boeing) use pitch to obtain selected speed and so dive at idle power to regain VNAV PTH.

The 1980s technology 747-400 had a very solid and trustworthy VNAV as I believe do the Airbus aircraft, the one on the super hi-tech 787 is utter rubbish in my opinion.

[JaneRachel 531]

17 minutes ago, jon b said:

Top tip on this one if ATC delay your descent……”if you can’t go down, slow down”

Wind the speed right back, then when you finally get descent clearance you can crank the speed right back up and the aircraft when in VNAV SPD (Boeing) use pitch to obtain selected speed and so dive at idle power to regain VNAV PTH.

The 1980s technology 747-400 had a very solid and trustworthy VNAV as I believe do the Airbus aircraft, the one on the super hi-tech 787 is utter rubbish in my opinion.

and Jon's advice is really always good! He is one of the most experienced "real-world" pilots here for a major airline I will not name 🙂