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

[badderjet 181]

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,

 

[Spin737 102]

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.

[Driver170 42]

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).

[Spin737 102]

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.

[Driver170 42]

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?

[Driver170 42]

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

[Spin737 102]

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.

[Driver170 42]

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

[JoeDiamond 67]

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.

[Spin737 102]

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.

[Driver170 42]

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?

[Driver170 42]

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

[Spin737 102]

 

 

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.

[Driver170 42]

Ok i'll try it next time matt.

[Driver170 42]

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?