Marenostrum

VNAV computing during vectoring LNAV

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Posted (edited)

To ask the question I write what I did then I ask the question (be patience with me I cannot find a shorter clear way to explain it!).

1. I am cleared to Take off from EGCC rwy 05L.

2. My first wpt is on the other direction (230° toward KUXEM).

3. No SID here, ATC will give me vectors: after the take off I am expecting a left turn 90° and a little later another turn 90° left direct to KOXUM. (so not a 180° continuous left turn).

4. my NAV display shows the same but with a small difference: the left  turn is quite immediately, continuous for 180° directing to KOXUM. 

5. I plan to take off with  only the HDG SEL armed on the autopilot. 

6. After the take off I climb straight ahead heading 053. Once at 800 ft I engage the autopilot with the HDG SEL active, proceeding 053 and I engage the VNAV too (so HDG + VNAV but LNAV is off).

7. I see on my NAV display that I am flying 053 whilst the FMC planned route was telling me that the 180° left turn was before my actual position: Now my real course and the planned FMC course are diverging  because I am following the ATC instructions.

8. when I am about 3 miles after the planned turning point in the NAV display, ATC gives me the first 90° left turn heading 330°. I do it. Now I am about 3 miles out of my planned course.

9. Now I am heading 330°  whilst the FMC planned course wanted me to direct to KUXEM heading 230° so I am still diverging some miles out of  the FMC route.

10. Now ATC tells me to turn left 230° to resume my planned navigation (to KUXEM).

11. At this stage I turn a little more left than 230 directing to KUXEM and, mile after mile, I am narrowing  my real route with the FMC route on the NAV display. The divergence of some miles becomes less and less until KUXEM where it becomes zero and I engage LNAV on the autopilot.

Now the question.
As soon as my real course and the FMC course were diverging, I noticed that my climbing angle was less and less.....once I turned in the direction to KOXUM narrowing the difference between my real course and the FMC course the climbing angle increased (at least  so it seemed ).
Is this happening because the FMC is calculating that the vertical navigation points (real and planned) are diverging so it slows down the climb (because my real course was longer that the FMC course) and once I was narrowing the planned course the FMC understood my increasing gradient of narrowing and then increased my rate of climb?

in other words: how the LNAV computes my vertical navigation path if I force my aircraft not to follow the planned course?

And what if it was the contrary: real route shorter than planned FMC route...have  I to expect a steeper climb ordered by the VNAV function? 

Is it allowed?

Thank you for your patience before than your answer!

Edited by Marenostrum

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Posted (edited)
27 minutes ago, Marenostrum said:

how the LNAV computes my vertical navigation path if I force my aircraft not to follow the planned course?

LNAV computes nothing vertical. This is VNAVs job.

What you experienced is as expected. VNAV calculates around the lateral route. If VNAV plans let's say reaching FL100 in 30 NM and you divert from that route by making it longer you will have a lower climb rate.

When you don't have a SID then you normally can use VECTOR so LNAV is flying rwy heading (in most cases). 

You can see this behavior during descent as well because, when deviating from the FMC route you change your rate of climb.

When flying ATC guided I use FLCH normally then the route doesn't matter anymore and you automatically get a good rate of climb or use V/S because ATC can sometimes give you a rate of climb.

Edited by 30K

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Posted (edited)

Thank you!

But what if my real route is shorter...does VNAV compute a steeper  climb?...is there a limit and a consequent error that I could receive in my FMC?

 

(the LNAV at the end of my post was a mistake..I was intended to write VNAV as you say)

Edited by Marenostrum

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6 hours ago, Marenostrum said:

VNAV compute a steeper  climb?

I may not understand the question but I will try.  The VNAV does not computer off-route climbs or descents.  It will adjust once you return to LNAV/VNAV modes and execute the direct or new route.

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King of tough to explain but something to take in consideration in a short explanation without getting involved into much details.

VNAV will be active and compute a climb/descend within 20 NM left/right of magenta line (adjusting constantly to stay on PATH or fulfill crossing restrictions adjusting ROC/ROD).

And for that reason, yes there is a relationship with LNAV based on this criteria. 

Outside of 20 NM L/R of course (magenta line) cannot compute a PATH, will just go on SPEED.

It's very difficult to understand what actually is doing without proper reading and training, and quite impossible to explain in a few words.

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1 hour ago, downscc said:

I may not understand the question but I will try.  The VNAV does not computer off-route climbs or descents.  It will adjust once you return to LNAV/VNAV modes and execute the direct or new route.

mmhhh. if it so...I wonder why the B738 flew in the way I described: when I was off-course compared to FMC route (following the ATC instructions) with the VNAV engaged, the aircraft continued to climb (I had no alert messages in the FMC) but simply I was climbing at a very small rate of climb.
As soon as I narrowed my off-course route approaching the planned FMC route the rate of climb increased.

For this reason I have supposed that the VNAV (I repeat, engaged) was trying to follow its vertical planned path, but because it was off-course (for a wider turn than the planned one),  it was reading the approaching of the subsequent interpolated points of the route (between the two waypoints I was flying), at a lower speed...and consequently it calculated a smaller rate of climb. As soon as I narrowed the real course respect the FMC course, the computer was reading the sequence of points from the first to the second waypoint coming fastly...and the ROC increased.

But this is only my "dislexis" reading of the computer calculation. 

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55 minutes ago, killthespam said:

King of tough to explain but something to take in consideration in a short explanation without getting involved into much details.

VNAV will be active and compute a climb/descend within 20 NM left/right of magenta line (adjusting constantly to stay on PATH or fulfill crossing restrictions adjusting ROC/ROD).

And for that reason, yes there is a relationship with LNAV based on this criteria. 

Outside of 20 NM L/R of course (magenta line) cannot compute a PATH, will just go on SPEED.

It's very difficult to understand what actually is doing without proper reading and training, and quite impossible to explain in a few words.

I was replying to downscc and, as soon as I have posted I have seen your answer.
I think that your post answers my question.

If the course is within 20 NM from the magenta line, the VNAV function does calculate the adjusted ROC.

Now it is interesting to know (and I can test it) what does it  happen if I do an off-course turn left (following the example) that makes the distance between the two waypoints in the FMC shorter. the VNAV should ask for a steeper ROC...because it will read the sequence of points in the route, faster than the planned route... and the adjusted ROC will steeper than the planned one.

Is there a limit after that the VNAV gives an alert and eventually disengages because a too steep ROC?

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1 hour ago, killthespam said:

It's very difficult to understand what actually is doing without proper reading and training, and quite impossible to explain in a few words.

Can you point out some rading on this topic? Curiuos case. 

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Posted (edited)

Ahhh, again I will try to shorten the explanation without any details.

The computer is looking along the route at the waypoints to see if is a wpt with altitude or speed restriction, if one is found now the computations take place to fulfill the restrictions.

As an example you have wpt1, wpt2 and only wpt3 has an altitude restriction, the computer will predict a altitude crossing for wpt1 and wpt2 but for wpt3 it will adjust ROC/ROD as necessary to respect the restriction even you do a course change between wpr1 and wpt2 within that 20 NM criteria. If you cannot make it you will get unable next altitude (usually when distance is shorter than the initial one will trigger unable next alt) , otherwise outside the 20 NM of magenta line VNAV will stays on SPEED, basically is just descending x climbing without any concerned to a wpt.

 

Edited by killthespam

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11 minutes ago, rsvit said:

Can you point out some rading on this topic? Curiuos case. 

Yes, you can try to get the FMS Honeywell pilot guide, aircraft specific.

There are also some guys that compiled some info but they got some wrong stuff , unfortunately nobody is perfect, that's why I suggest the real "FMS Honeywell pilot guide, aircraft specific".

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VNAV does not change climb rate. It only commands an airspeed at CLB, CLB-1 or CLB-2. It will level you at the lower of any LEGS restrictions or the MCP.

It's possible I'm missing some option, but in my experience, it's basically full thrust until you get to the next restriction.

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14 minutes ago, Spin737 said:

VNAV does not change climb rate. It only commands an airspeed at CLB, CLB-1 or CLB-2. It will level you at the lower of any LEGS restrictions or the MCP.

 It's possible I'm missing some option, but in my experience, it's basically full thrust until you get to the next restriction.

You see, unfortunately you mixing thrust settings, climb rates, altitude restrictions on MCP versus legs page restrictions. We where not talking about any restrictions on MCP in our case. That's another scenario.

It's not full thrust setting until you get to the next restriction, it's actually what you select.

And yes, VNAV is changing climb rate based on changing the distance between wpts or getting a direct , OAT, wt, wind and also is a cruise climb or cruise descend rate, not to be confused with max rate x max angle speeds that also will change your rate of climb. 

VNAV in ground-schools is a hot topic and it takes time to be understood, later, after flying the line things will get clear.

I will stop right here, I don't want to get involved into unproductive discussions and misunderstandings.

 

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30 minutes ago, killthespam said:

VNAV in ground-schools is a hot topic and it takes time to be understood, later, after flying the line things will get clear.

Hopefully, someday, I'll get that experience.

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26 minutes ago, Spin737 said:

Hopefully, someday, I'll get that experience.

I almost spit my coffee out.... hahahah

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1 hour ago, Spin737 said:

Hopefully, someday, I'll get that experience.

Fantastic. 

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