Usage of VNAV in 737

[737Andi]

Hi,

I'm a flightsimmer and was flying on Vatsim this evening and the controller told me to level off at FL 110 at my next Waypoint GIVME which was approximately 30 nm away.

At that moment I was flying at cruise level at FL 210.

My plan was to use VNAV descent for that job. So I changed the altitude of GIVME in the FMC to FL 110 and pushed the "Des Now" Button on the DES page of the FMC.

The plane started to descend with a V/S of exactly 1000 fpm. The controller asked me if this will work. Since I was 10000 feet above GIVME and the distance was approx. 30nm I thought the FMC will do everything correctly.

I think the formula is 21000-11000 = 10000 feet

10000/300 = approx. 30nm

However the V/S of 1000 feet/min turned out to be too low.

Can anyone tell me what I did wrong?

Thanks!

Best regards Andi

[eckhard]

Hi Andi,

Great to see that you are trying to use the VNAV function.

As a quick rule of thumb, most airliners will descend at about 3 degrees at idle thrust and cruise speed. .
If you assume that, then the rate of descent needed will be approx 5 times your ground speed.
The distance needed will he approx 3 times the height difference (in thousands of feet).
So, to lose 10,000ft you will need about 30nm, as you already calculated.
Assuming your ground speed to be about 430kts, you would need a VS of 430x5 = 2150fpm to descend at 3degrees.
Hope this helps?

[737Andi]

Hi Eckhard, thanks for the great info about the x5 rule :)
That is very useful!

But why can the VNAV function not do the descent with the x5 rule on its own???

[eckhard]

VNAV is a bit complex and has different functionality, depending on FMC and phase of flight. For example, if your lower target altitude is at a waypoint that is still in the cruise portion of the FMC flight-plan, you may not be able to enter it on the legs page without an “S” after the numbers. For example 310S which means a step-change in the cruise altitude.

Another way of programming a VNAV descent is to put a lower cruise altitude in the VNAV cruise page and then push the MCP ALT selector.

Different models of 737 and different upgrade status of the FMC will have different capabilities, so it’s not easy to give a short answer. The main point is to make your inputs and then MONITOR the FMA to see what the aircraft is doing. For VNAV, one crucial aspect is to understand the relationship between the elevators and the auto throttle. Which of these is controlling speed? Are the autothrottles active or just armed, or in HOLD?

Good luck!

[FlightDetent]

Normally the VNAV descent mode will start with 1000 fpm, if below the optimal idle thrust profile. Some people prefer this way, 1000 fpm until intercepting the vertical guidance for idle descent, then dive with 2800 fpm. This will make the controllers nervous now and then, since they see you coming close and the original vector with 1000 fpm points way behind the restricted waypoint.

Sounds like a mis behaviour of the SIM sw, or improperly set up mode. On the real aircraft, the FMS guidance will work, that's why it is there in the first place.

Another approach to the task, using the x5 rule is this:

DES to FL 110 from 210 = 100 levels to go.
Distance is 30 nm, so times 3 you could conveniently lose 90 levels. (it's conservative)
Being slightly above the math already, there is no need to delay the descent.
Check GS, i.e. 456 kt, and "halv it" = 23 ..=> 2300 fpm is what you need to achieve 3°.
Set V/S 2300 fpm, put the seat-.belts on and fasten the shoulder harness. Re-attach the headset to your skull, start the sterile cockpit mode of thought. Smile and enjoy doing a good job, focus on what's ahead.

The advantage of true VNAV is the machine can cover for variances in wind in different layers, speed restrictions and ISA deviation. Provided you load the box correctly with accurate data, although satisfying both of these requirements is a bit of a drag now and then. The above "times 3" and "half GS" method never failed anyone. It is great because it provides you with 2 pieces of much-required information
- how far from the desired profile you are
- which V/S you need to make good in order to stay on it, or fly parallel.

I.e. when 2000 feet too high and GS showing 384 kt, if already descending on idle thrust and the resulting V/S is 1400 fpm:

a) the aircraft is deviating further above the profile
b) the aircraft is flying parallel to the profile
c) the aircraft is flying steeper than the profile angle, and wild eventually intercept it from above.

You know the answer.

(cross-posted)

EDIT: out of interest, what IAS would you normally fly in such an exercise?

[eckhard]

Good points made by FlightDetent.

Having read the original post again, I think that maybe the VNAV mode was a cruise descent? The title of the active VNAV page will tell you. It could be that your sim does not reflect all the functionalities of the aircraft?

Anyway, if VNAV doesn’t do what you want, you can always try VS or FLCH but keep an eye on the FMAs! You may need speed brake to stay on profile without accelerating too much. On the other hand, you (or the auto throttle) may need to add thrust. It takes knowledge of the system and lots of practice to predict confidently which modes will produce which results, given a requirement to reach a level by a waypoint.

One neat feature that your sim may have is on the VNAV descent page. Around the 4R Line Select Key you may see a Waypoint/Altitude line. Normally, this will display the next waypoint in the flight plan that has an altitude constraint associated with it. Below that, you will see some fields labelled FPA, VB and VS. The FPA shows your current Flight Path Angle, in degrees below the horizon. The VB shows the current Vertical Bearing between you and the waypoint/altitude (in a straight line, not via the flight plan waypoints). The VS shows you what Vertical Speed you need to achieve the VB at you current ground speed.

What is really useful is that you can enter any waypoint that is in the Nav Database, even if it is not in your flight plan. You can also enter any altitude against it (provided that it is lower than your current altitude) and the VB and VS fields will display what you need to do. Then you can either use VS mode to match the required VS and/or you can use any mode and see if your FPA matches the required VB.

Once you’ve finished with that waypoint, you can delete it and the next flightplan waypoint with an altitude constraint will once again be displayed.

Give it a try!

[Denti]

Well, in the 737 you always have the absolutely fabulous tool of the green altitude range arc, which shows where you will reach the intended level. Additionally of course the VNAV deviation scale and, if equipped with it, the vertical situation display. All those are great tools to use in vertical navigation, provided of course that the aircraft is equipped with it (VSD) and that VNAV works correctly. But it is always good to use a backup of the times 3 table and times 5 (or easier: halve it then times 10) in your mind. In a simulator the installed equipment is usually just an option button away.

The point Eckhard raised is actually a very useful tool as well, the VB/VS to a waypoint can help a lot with descent planning. Especially when expecting a large shortcut, where it can be used to adjust your profile to provide for that possibility.

[FlightDetent]

Last but not least, switching back to 737 for fun with VNAV was a good choice for Andi. The other most common type is seriously crappy with it

[737Andi]

For example, if your lower target altitude is at a waypoint that is still in the cruise portion of the FMC flight-plan, you may not be able to enter it on the legs page without an “S” after the numbers. For example 310S which means a step-change in the cruise....

I fear I did not get this point :(
My understanding of climb, cruise and descent phase with active VNAV was this.....

Climb phase is the phase from takeoff to the set cruise level.
As soon as I have reached the T/C (cruise level) the plane automatically switches to cruise phase and stays in this phase until it reaches the T/D where it automatically descends when a lower altitude is set on the MCP. If Des Now is pushed the plane starts the descent earlier with a moderate vertical speed until it reaches the planned descent path of the FMC. As soon as the plane has started to descend from cruise level to any lower altitude the plane is automatically in descent phase. How can the plane still stay in cruise phase when going down from cruise level to any lower altitude?? I understand the procedure of step climb to higher altitudes but not to lower ones? Would be great if someone can explain this to me. Thank you!

[737Andi]

Normally the VNAV descent mode will start with 1000 fpm, if below the optimal idle thrust profile. Some people prefer this way, 1000 fpm until intercepting the vertical guidance for idle descent, then dive with 2800 fpm.

So the VNAV will start with 1000ft/min if below the optimal profile. If it reaches the optimal profile point on the flight path it should have switched to a new V/S value that would have brought me to FL 110 exactly over GIVME, correct?


Normally
I.e. when 2000 feet too high and GS showing 384 kt, if already descending on idle thrust and the resulting V/S is 1400 fpm:

a) the aircraft is deviating further above the profile
b) the aircraft is flying parallel to the profile
c) the aircraft is flying steeper than the profile angle, and wild eventually intercept it from above.

You know the answer.

(cross-posted)

EDIT: out of interest, what IAS would you normally fly in such an exercise?

Answer should be a)
But I'm not sure about the right IAS.

[737Andi]

It could be that your sim does not reflect all the functionalities of the aircraft?

It was the PMDG 737 NGXu


One neat feature that your sim may have is on the VNAV descent page. Around the 4R Line Select Key you may see a Waypoint/Altitude line. Normally, this will display the next waypoint in the flight plan that has an altitude constraint associated with it. Below that, you will see some fields labelled FPA, VB and VS. The FPA shows your current Flight Path Angle, in degrees below the horizon. The VB shows the current Vertical Bearing between you and the waypoint/altitude (in a straight line, not via the flight plan waypoints). The VS shows you what Vertical Speed you need to achieve the VB at you current ground speed.

What is really useful is that you can enter any waypoint that is in the Nav Database, even if it is not in your flight plan. You can also enter any altitude against it (provided that it is lower than your current altitude) and the VB and VS fields will display what you need to do. Then you can either use VS mode to match the required VS and/or you can use any mode and see if your FPA matches the required VB.

Once you’ve finished with that waypoint, you can delete it and the next flightplan waypoint with an altitude constraint will once again be displayed.

Give it a try!

That sounds really cool. I will try that :)

[737Andi]

eckhard and FlightDetent

I suppose I'm a hopeless case?
Am my assumptions totally wrong?
Any hint is appreciated so that I can read further into the topic.

Thank you guys!

[eckhard]

Sorry Andi, I have been busy at home and also flying my small aircraft, so I didn’t get a chance to reply to your points. You are certainly not a hopeless case, as your questions show a good understanding already.

I am now relaxing in the garden with a beer or three, so it would not be a good time to try to respond!
If I can ask for your patience, I will respond as soon as I can give a sensible answer. Hopefully tomorrow, but no promises!

[Denti]

So the VNAV will start with 1000ft/min if below the optimal profile. If it reaches the optimal profile point on the flight path it should have switched to a new V/S value that would have brought me to FL 110 exactly over GIVME, correct?
.
Yes. Normally it will keep 1000 fpm until it reaches the profile (well, around 300 ft below the profile) and then reduces thrust to idle as the normal descent profile is flown in idle thrust for as long as possible. Now, it does get more complex if there are several constraints and depending on the options chosen when the plane was configured, as geometric descent is not the default equipment status.

[eckhard]

I fear I did not get this point :(
My understanding of climb, cruise and descent phase with active VNAV was this.....

Climb phase is the phase from takeoff to the set cruise level.
As soon as I have reached the T/C (cruise level) the plane automatically switches to cruise phase and stays in this phase until it reaches the T/D where it automatically descends when a lower altitude is set on the MCP. If Des Now is pushed the plane starts the descent earlier with a moderate vertical speed until it reaches the planned descent path of the FMC. As soon as the plane has started to descend from cruise level to any lower altitude the plane is automatically in descent phase. How can the plane still stay in cruise phase when going down from cruise level to any lower altitude?? I understand the procedure of step climb to higher altitudes but not to lower ones? Would be great if someone can explain this to me. Thank you!

Imagine that you have entered a single cruise level in the FMC RTE LEGS and VNAV cruise page. The MCP altitude is also set to this level.
Your weight allows a higher level and of course you could also cruise at a lower level, if desired.

To cruise at a higher level, you enter could enter the new level in the MCP altitude and the FMC VNAV cruise page and execute; however, if your 737 has “Altitude Intervention”, you can simply set the new altitude in the MCP and push the selector. The new altitude will be sent to the FMC VNAV cruise page as well and the page title will change to ACT ECON CRZ CLB.

To cruise at a lower level, the same technique can be used if you are more than 50nm before T/D. The page title will be ACT ECON CRZ DES.
If you are within 50nm of T/D this action will result in an “early descent” and the page title will be ACT ECON DES. The VNAV function will start a descent at about 1000fpm until intercepting the profile.

Step climbs are normally used for two main reasons:
1. to maintain recommended altitude as weight reduces; and
2. to comply with different cruise level systems in different parts of the world, e.g. China.

The recommended altitude will change as a function of weight, cost index and predicted winds. Normally, the recommended altitude increases as weight reduces but adverse winds could mean that a descent is recommended at a certain point. Adjusting to a metric flight level may also involve a climb or a descent. For these reasons, you can pre-load the RTE LEGS page with step climbs or descents against certain waypoints. The autopilot will not initiate the level change until the MCP altitude selector is also changed.

All of the above is based on the 787, so I’m not sure how much is applicable to the 737NG. I have about 5000hrs on the 737-300 and -400 and they had very similar functionality, so I think that the NG will have features broadly the same as I have described.

Hope this helps!

[737Andi]

Now everything is clear. Thank you all for the very useful information!
I will try it on my next sim flight.

[eckhard]

Ok, let us know how you get on. 👍