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Richard McDonald Woods

RNAV approaches

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7 minutes ago, Driverab330 said:

Basically the 777 VNAV system uses something called Baro-VNAV. The FMC computes a path from a point 50ft over the runway waypoint (which is the threshold) backwards to the FAF assuming the ISA weather condition (15 deg C on the ground with a standard lapse rate), and VNAV solely depends on the altimeter reading. (unlike the G/S of and ILS which defined a fix geographical path independent of the altitmeter, hence it is called a precision approach)  

Thank you for the explanation.  Why on earth doesn't the aircraft in VNAV on approaches use radio altimeter once it becomes available (I know this is not your problem -- you didn't design the 777!)?

 

1 hour ago, scandinavian13 said:

Yep. Note the verbiage in the mins section for an LNAV approach. DA or DH? Why?

This is a non-precision, visual approach at last stage.  If I can't see the runway from that altitude soon after passing FAF I must go around.  This would be true even with good VNAV guidance (although maybe the minimums are different?).

Mike

 


 

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4 minutes ago, Mike777 said:

Why on earth doesn't the aircraft in VNAV on approaches use radio altimeter once it becomes available

I guess one reason could be that the use of the radio altimeter is reliable only if the terrain ahead of the runway is prepared for that (flat, surveyed?). It is the case for the cat III approaches but in an airport with bumpy and slopy terrains before the runway, the height measured by the radio altimeter would vary constantly making difficult to follow a constant slope down to the runway.

But I could be wrong, this is just a logical explanation out of real knowledge.


Romain Roux

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Avec l'avion, nous avons inventé la ligne droite.

St Exupéry, Terre des hommes.

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24 minutes ago, Budbud said:

I guess one reason could be that the use of the radio altimeter is reliable only if the terrain ahead of the runway is prepared for that (flat, surveyed?). It is the case for the cat III approaches but in an airport with bumpy and slopy terrains before the runway, the height measured by the radio altimeter would vary constantly making difficult to follow a constant slope down to the runway.

Hi Romain,

This sounds right to me.  I was going to say that the FMC could compare the radio altitude info with the baro altitude info, but that would just reintroduce the baro altitude temperature uncertainty.  I suppose the radio altimeter method could work if the FMC data contained a very accurate topological map of the approach terrain, but that is probably very impractical, and the terrain itself might be subject to frequent change anyway.

Mike


 

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40 minutes ago, Mike777 said:

Why on earth doesn't the aircraft in VNAV on approaches use radio altimeter once it becomes available (I know this is not your problem -- you didn't design the 777!)?

Same reason you don't use RA for ILS approaches, too. You use the altimeter (DA), unless the plate specifically states that you should use height (DH based on RA).

Design spec.


Kyle Rodgers

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Richard

You may perform the RNAV (GNSS) Rwy 34 in the 744 and most likely the 777 aswell.

In the real world, each aircraft and operator would need approval and likely do have approval to perform RNP APCH’s, which is what RNAV (GNSS) Rwy 34 is.

You should read the:

1 FCTM “Non-ILS Instrument Approaches”

2 FCOM Normal Procedures “Landing Procedure - Instrument Approach using VNAV”

Additional reading:

https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_90-105A.pdf#page36

 

Edited by Copper.

Brian Nellis

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13 hours ago, downscc said:

There is a wealth of information in the NGX FCTM on how to fly a non-ILS approach, including RNP using LNAV/VNAV beginning on page 5.42.  I suggest it  is required reading for students of this aircraft, and a good review for weathered veterans.

(I tend to avoid RNAV approaches in the 777 because it lacks IAN, and the 747 because it is not qualified for RNP0.5 ((to my knowledge)), so basically my focus for RNAV approaches in restricted to the NGX, which unfortunately cannot do RNP0.30 (RNP0.50 only).

I’ve never flown a 737, but in my time at Horizon I’m almost certain the 737 at Alaska can go to RNP .15 or at the very least, RNP .3

Amusingly enough, we were approved for RNP .10 on the Q400 at Horizon, which is as low as you can go with RNP at this point. Pretty impressive what RNP can do. After leaving Horizon, I’m flying the 757/767 and my airline didn’t even have RNAV approach OpSpecs when I got hired (we have them now, but no RNP). Quite a change!


Sean Wood

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10 hours ago, swood721 said:

I’ve never flown a 737, but in my time at Horizon I’m almost certain the 737 at Alaska can go to RNP .15 or at the very least, RNP .3

Of course the B737 can, but I am referencing the PMDG NGX product and its limits.  Sorry, the TLA NGX refers to a PMDG simulator and not the real aircraft, which is a NG.


Dan Downs KCRP

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1 hour ago, downscc said:

Of course the B737 can, but I am referencing the PMDG NGX product and its limits.  Sorry, the TLA NGX refers to a PMDG simulator and not the real aircraft, which is a NG.

Hello Dan

Im genuinely interested in knowing why the ngx is only capable to the rnp that you say it is.

It has the sensory necessary to do rnp down to rnp 0.1.... just doesnt have the supplemental rf capability... off the top of my head


Brian Nellis

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Well, the NGX simulation lateral navigation is based on a database which format allows a good approximation of what is done with the magenta linein the real aircraft. But the format of the simulated databank (that you download from aerosoft or navigraph) on our computer is not the real format of the actual aircraft. It's an approximation.
To link two waypoints you have several types of legs in the real norm. In the NGX, some are not implemented. It was not a problem some years ago because the number of approach designed with the few advanced legs types (like radius-to-fix) is only rising currently.
Even in the real aviation, some aircraft are not able to perform those approaches, because they have not the mandatory systems required (a monitor system to enforce precision of the position, a FMS able to follow RF - legs, and other systems depending of the kind of vertical guidance and kind of system to enforce vertical precision). Or the aircraft is not certified,or the crew is not.

So you have to deal with a combination of real limitations of aircraft equipment and simulated aircraft limitations and simulated navigation databank limitations (that varies from Navigraph to Aerosoft due to different providers and parsers).. Have a look at my paper for a deeper insight on RNP operations :ha: in the sim.

What I would be interested in, by PMDG, is a radio panel with GBAS and SBAS channel.

Also, you can pretend that the NGX is RF-capable, because the work around by PMDG is to create pseudo-waypoints along the constant arc leg to simulate it. I do.

Edited by daguerreotype

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Simulator: Prepar3D 4.2 | A2A Comanche, MJC8, Leonardo MD-82, FSL320-X, NGX

Simulator: XP11 | FFA320 U, IXEG 737 Classic, JF Hawk T1, JF C152

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Forgive me if this is a "n00b" comment, but......am I right in thinking that a normal descent using LNAV and VNAV is effectively an RNAV approach until APP locks on to the ILS localiser beam?

Edited by Christopher Low

Christopher Low

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25 minutes ago, Christopher Low said:

Forgive me if this is a "n00b" comment, but......am I right in thinking that a normal descent using LNAV and VNAV is effectively an RNAV approach until APP locks on to the ILS localiser beam?

Terminology in aviation is precise.  You may be in a normal VNAV descent on an arrival (STAR) but an arrival is not an approach.  The approach is defined from an IAF through the FAF to the runway, and is designed with all manner of variances for equipment and pilotage to ensure safe distance from obstructions.  The APP doesn't lock onto the ILS localizer, rather you might have a system that provides for an APP button that might be used for both ILS and RNP approaches. Depends on equipment.  The APP button means that an automatic flight system is armed to acquire approach modes.  So in the case of the 737, the APP button arms the AFDS with an IAN option for either an ILS or RNP approach.

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Dan Downs KCRP

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Interesting paper. It highlighted the limitation between real and simulated. This is the primary limitation in that the differences will never be reconciled for obvious reasons. So I don’t think strict adherence to the literal is appropriate for if we did no sim airplane should ever be flown.

Evidence of aircraft eligibility to perform RNP APCH or any other spec cannot be produced for ANY simulated product. So then the user has to decide, based on what is available in the product and what isn’t, whether that OPSpec can be flown or not without reliance on system statements of compliance which are supplied by OEMs and considered by the airplane manufacturer with an associated statement that the airplane is capable of RNAV ops which is stipulated either in the AFM or similar.

Other things like establishment of operating procedures for the RNP system, control of those procedures through the operations manual (as required), identification of flight crew training requirements and control of process for updating the navigation database require similar certification and approval processes which, again, cannot be produced for ANY sim product available to us - and if we are to adhere to the literal, then they shan’t be flown.

So in essence what I’m trying to say it is impossible to determine what spec the respective sim airplanes can perform. The best that can be done is proximation based on what is shown to be available and to work with that, and, where possible, manipulate the simulated product to reconcile the differences for the spec sought after I.e. pseudo waypoints to build an RF leg.

From what I am able to see considering what is available on the NGX, PMDG777 and PMDG747 - they are all capable of performing RNP APCH, which was an initial point of confusion.

As to my question to Dan - out of genuine curiosity, I’d like to know why it is thought that the ngx is capable down to an RNP of 0.50 only where, depending on options selected, the ngx looks to have the necessary avionics (monitoring and alerting) to support RNP 0.10 operations. NOTE: I don’t believe RF is an absolute requirement. I believe some operators are still able to fly RNP AR procedures without RF, so long as they fly RNP AR procedures that exclude their use, obviously.

Perhaps someone should run through the approval processes with the NGX and see what is found.

Edited by Copper.
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Brian Nellis

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@Copper. that's a good post I can only agree with ! With your conclusion on reconciliation of sim and reality in mind, yes I support your view about the NGX being OK to go down to RNP.10 !

 

@Christopher Low Sir, you said, .am I right in thinking that a normal descent using LNAV and VNAV is effectively an RNAV approach until APP locks on to the ILS localiser beam?

No, you are definitely wrong. an RNAV approach (should use the ICAO denomination "RNP APCH") is an approach not relying on traditionnal navaids. It relies on internal means of position assessment, reinforced by augmentation systems like GNSS (Egnos, Waas) or eventually new kinds of ground based stations on airport.

What you are saying,if I understand your position, is closer of an RNAV arrival (RNAV STAR) followed by a classical approach relying on ground navaids.

RNAV arrival is not RNP approach. Please see the typology on page 1. (edit : or the pictures page 4 and 6 of the 2012 paper linked below)

The main difference with RNAV is that RNP requires a performance monitoring system.  The aircraft must be equipped with a system that checks the integrity of input signals (satellites, navaids) and alert the crew if the integrity or the required performance is lost. Because RNP operation allows to design paths which should be flown only with a great accuracy by the aircraft (otherwise you hit the mountain for instance, in LOWI !). Those patheswere not possible to be designed before, when we relied on less efficient RNAV operations solely.

SIDE NOTE : In the simulation, in the old days of FS9 for instance,you could try an RNAV approach down to a runway end, but not always end aligned with the runway (missalignemnt, try and see the Wilco/Feelthere Airbus for example, see the picture on page 22 of my 2012 paper on RNP), unlike if you were using an ILS. That's in essence is a justification of what Cooper said :"The best that can be done is proximation based on what is shown to be available and to work with that". However, nowadays,  our simulators have strongly become more accurate and you can use LNAV down to the runway end without too much thinking. After all, the basic RNP APCH in LNAV or LNAV/VNAV is the easiest approach available (because you just stay in LNAV/VNAV !) as Kyle R. said.

Edited by daguerreotype
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Simulator: Prepar3D 4.2 | A2A Comanche, MJC8, Leonardo MD-82, FSL320-X, NGX

Simulator: XP11 | FFA320 U, IXEG 737 Classic, JF Hawk T1, JF C152

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Thanks for the clarification Dan/Geoffrey. I was quite prepared to stand by my "n00b" status, but at least I learned something in the process :biggrin:


Christopher Low

UK2000 Beta Tester

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Christopher, you may want to watch this video: 

 

 

Hans van Wijhe

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Kind regards,
Hans van WIjhe

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