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badderjet

How are track miles/VNAV path calculated on a direct INTC?

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Hello,

 

I'd like to ask for a statement (with reference if available) about how PROG 1 track miles are calculated when you are e. g. intercepting the LOC with an extended centerline. Furthermore I'd like to have some information about the displayed VNAV path in that situation. Is some path calculated along the track of the aircraft up to the intercept point, or direct to the active waypoint irrespective of aircraft track...?

 

Thanks,

 

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Excellent question that is not understood by many. Heck, I'm not sure I get it. :huh:

 

Here's my take:

 

If you want an accurate PROG, then you need to be in LNAV with an intercept heading. If you're just pointed at the final in HDG SEL, I believe the FMC just projects you to a point  on final, perpendicular to that course.

 

If you want to test this, just flip back and forth between LNAV and HDG SEL and you'll see the VNAV move.

 

You can get it to work, but I just use the V/B on the DESC page and make sure I'm on about a 3.0 bearing to the FAF or RWY.

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I found this

 

Whenever direct to a waypoint is selected, whether that is with 'direct to' or 'intercept course', the distance calculated and displayed is always simply direct to the waypoint from present position and not direct to that part of the new leg that you're currently pointing at (it is always irrespective of the current track of the aircraft). The only difference with 'intercept leg to' is that it creates a specific LNAV course suitable for intercepting (perfect for a non-precision approach or re-intercepting the centreline of an airway).

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I found this

 

Whenever direct to a waypoint is selected, whether that is with 'direct to' or 'intercept course', the distance calculated and displayed is always simply direct to the waypoint from present position and not direct to that part of the new leg that you're currently pointing at (it is always irrespective of the current track of the aircraft). The only difference with 'intercept leg to' is that it creates a specific LNAV course suitable for intercepting (perfect for a non-precision approach or re-intercepting the centreline of an airway).

 

The problem with that info is that it doesn't show what typically happens - radar vectors. You can go direct in the FMC but remain in HDG SEL and the VNAV will be correct for a while, but as you move away from that course, it becomes less useful.

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If you extend the centreline outwards and getting vectored will the deviation scale show how high or low you are?

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http://www.pprune.org/tech-log/225183-737-fmc-intercept-course.html

 

 

Its something i'm not sure about, do you have to be on the magenta brick road when doing this procedure?

For the VNAV to be accurate, yes.

If you extend the centreline outwards and getting vectored will the deviation scale show how high or low you are?

The scale will be there, but won't be accurate unless you're on the course.

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Right well whats the need for extending the centre line? What accurate way can you check your height?

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Right well whats the need for extending the centre line? What accurate way can you check your height?

 

If you are being vectored to intercept a course in LNAV you need to have that course depicted.  It's hard to intercept a course that isn't there.  If you are usingVOR/LOC it's nice to have the magenta line for situational awareness.

 

There are several ways you can maintain vertical situational awareness during a descent, VNAV is only one of them.  You have the green arc on the nav display, as Spin mentioned you have the vertical bearing information on the VNAV descent page and nothing beats simple math.  Three miles out for every 1000' you need to lose puts you in a good position.

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There are many ways to plan the descent, but I really am a fan of the Vertical Bearing on the FAF. I don't need to worry about field elevation, just angles. If you've got 3.0 V/B to the FAF's crossing alt, you're in a good spot.

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Got a question! With the 737 on very light take off weights it happens to exceed 20° of pitch in order to fly V2+15 how can i control this?

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I'm speaking to a 737 ng pilot on pprune and hes giving me good usefull tips and help with my 60 dollar pmdg lol

 

This is what he said about the vertical profile you don't have to be on the magenta line to give you a reading of how high or low you are, you can be off it and still it will give you the correct height, here it is -

 

 

The subject of much discussion and I never in all my years had a definitive answer. The closest I can get is that it represents the vertical profile from where you are to the next 'fixed' height/speed waypoint on your extended c/l, allowing for turns. I never used the function and preferred a 10 mile range ring or DME and my 3 times table

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This is what he said about the vertical profile you don't have to be on the magenta line to give you a reading of how high or low you are, you can be off it and still it will give you the correct height, here it is -


The subject of much discussion and I never in all my years had a definitive answer. The closest I can get is that it represents the vertical profile from where you are to the next 'fixed' height/speed waypoint on your extended c/l, allowing for turns. I never used the function and preferred a 10 mile range ring or DME and my 3 times table

 

I wish that were true. The FMC isn't that smart. If I'm on base and I'm not on an LNAV course, it will not show the same results if I was on a base but with a track to the intercept. Try it yourself. I've done it before in the real world and toggling between LNAV and HDG SEL changes your VNAV deviation.

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How come selecting LNAV and HDG SEL change the vertical deviation i though VNAV would?

 

Also, when would you start the APU for engine start?

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How come selecting LNAV and HDG SEL change the vertical deviation i though VNAV would?

 

The FMC, like all computers is only as good as the programming and input. If you're in HDG SEL, the VNAV doesn't know what route you'll take to get on the glide slope. It assumes you're somewhere on final. I'm not 100% sure where that is. But, it isn't taking your heading into account as you can be on an intercept heading at a "good" altitude and the VNAV will think you are high.

 

If you then select LNAV while wings level on an intercept heading, the FMC can then figure out your routing and give you an accurate VNAV picture. The error between HDG SEL and VNAV decreases as you approach the FINAL APPROACH COURSE.

 

Basically, in HDG SEL the FMC thinks you are closer to the course than your actually are. LNAV does a better job of figuring our your route and therefore distance and therefore VNAV.

 


Also, when would you start the APU for engine start?

 

The APU takes about 50sec to spool up and then you need 1min to stabilize the APU before you can turn on the bleed. So, a minimum of 1min50sec. I'll usually start it about 5-10min prior to push unless there is a need for air conditioning. Then its when the FAs tell you they are hot or cold.

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Then its when the FAs tell you they are hot or cold.

 

Which is pretty much always the moment they set foot on the airplane.  :)

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Matt C

 

I'm doing an RNAV APP, i want to set RNP (0.30nm) on prog page 4 of the FMC how is it done?

 

also if i'm monotoring RAW DATA i tune the VOR but i don't inhibit the vor in the nav options is that correct?


Also for the RNP requirements RNAV 0.5nm and GPS  0.3nm 

 

what is the difference? i thought RNAV is GPS.....

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I'm doing an RNAV APP, i want to set RNP (0.30nm) on prog page 4 of the FMC how is it done?

 

LEGS 6L is where I do it. You might be able to do in on the PROG page.


also if i'm monotoring RAW DATA i tune the VOR but i don't inhibit the vor in the nav options is that correct?

 

We don't inhibit VOR normally. We inhibit DMU update by default. So I'd say you are correct.

 

 

 


what is the difference? i thought RNAV is GPS.....

 

GPS can be RNAV, but not all RNAV is GPS. RNAV could be LORAN, INS, IRS, OMEGA, etc.

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GPS can be RNAV, but not all RNAV is GPS. RNAV could be LORAN, INS, IRS, OMEGA, etc.

 

My first RNAV used VOR and VOR OFFSET.... it was very clumsy but you could draw a straight line on a sectional and then measure the radial and distance to each VOR you bypassed. Input that information into a box and voila you had Random Navigation!  They changed the name after that and the rest is history.

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I read the name GPS is not longer used but now its RNAV (GNSS) also LORAN and OMEGA are very old and not used.

 

I found this

 

Well from our newly updated operations manual :

 

 

RNAV (GNSS) APPROACHES

 

 

Definition : non-precision approaches with LNAV or LNAV/VNAV minima.

 

 

- "RNAV (GNSS) approaches" corresponds to "RNP APCH operations"

 

 

- "RNAV (RNP) approaches" corresponds to "RNP AR APCH operations"

 

 

RNP AR OPERATIONS

 

 

General

 

 

Required navigation performance instrument approach procedures with authorisation required (RNP AR) are charted as RNAV (RNP) RWY XX " and contain a special note on the chart to aircrew and operators. These procedures are commonly referred to as "RNP AR" or in the USA "RNP SAAAR".

 

 

Can someone tell me what RNP you require for both of these?

 

Thanks.

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My first RNAV used VOR and VOR OFFSET.... it was very clumsy but you could draw a straight line on a sectional and then measure the radial and distance to each VOR you bypassed. Input that information into a box and voila you had Random Navigation!  They changed the name after that and the rest is history.

In this case would you inhibit the GPS update on the 737? But RNAV VOR is also the thing of the past can't find much of those about.

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My first RNAV used VOR and VOR OFFSET.... it was very clumsy but you could draw a straight line on a sectional and then measure the radial and distance to each VOR you bypassed. Input that information into a box and voila you had Random Navigation! They changed the name after that and the rest is history.

Ah! I'll bet you are referring to the Bendix-King KLN 80. An amazing system for its era. I installed quite a few of them back in the early-mid 1980s. You probably are be familiar with the gridded RNAV enroute charts that both Jeppesen and NOS used to publish, making it easier to plot a linear course and find bearing/distance offsets for VOR/DME stations along the route. I don't know if those charts are still available - probably not.

 

That was about the time that the first aviation LORAN units came to market, which soon became extremely popular in the GA world.

 

Though few Loran models were ever certified for "official" IFR enroute use, they were so much easier to use than a rho/theta RNAV, that sales of the KLN 80 went into the tank, and the model was discontinued.

 

And, of course, Loran itself was rapidly supplanted by GPS in the 1990s onward, and since the U.S. Loran system was de-commissioned a few years ago, any old Loran navigators still installed in are now just "gap fillers" in an aircraft's radio stack.

 

At least a KLN 80 would still work today, assuming the internal circuitry has not failed with the passage of time. I still see them once in awhile in older GA aircraft. The only problem with making the national airspace system almost totally dependent on space-based GPS, is what happens if the GPS satellites are damaged by a massive solar flare? In that case, old KLN 80s that still work might become worth their weight in gold...

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what happens if the GPS satellites are damaged by a massive solar flare?

 

That keeps military planners up at night.  GPS must be the best civilian adoption of a military technology since interstate highways. I believe the planners plan survivability models based on the number of transponders in the constellation. You have to knock out quite a few to start having an operational impact.

 

Yes, I remember the KLN 80! And in fact we did migrate to the LORAN and after many years the TSO'd Garmins 530's became a necessity. I guess the next big thing is going to be ADS-B, which is already getting a lot of attention in parts of the world that don't have radar coverage.

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In this case would you inhibit the GPS update on the 737? But RNAV VOR is also the thing of the past can't find much of those about.

 

He's talking about an older system that the NG doesn't use, so it's N/A. The NG does use VOR for its position data, so you are in a way VOR RNAV. Of course there are other inputs that go into the MMR.

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