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Richard McDonald Woods

RNAV and vertical navigation

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Does RNAV provide vertical navigation?

 

If not, what is the source of vertical navigation when using RNAV?

 

Yesterday I flew into KSFO using GPS RNAV Y 10R successfully but received an RAAS warning of a long landing.

 

If I was flying into an airfield using RNAV but was vectored away from my planned route, what would be the meaning of a request to 'report established' when there is, therefore, no ILS being used?

 

Elucidation much appreciated.

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Does RNAV provide vertical navigation?

Yes and no. RNAV is simply a concept that allows for the determination of a position in 'space' without using old navigation radials. Back in the early days of RNAV, you couldn't easily determine your altitude, and most inertial nav systems that I'm aware of simply provided a reference in a 2D plane.

 

Today's methods of RNAV can provide a 3D reference, primarily through GPS (called "baro aiding"), though the primary source for those units is still the barometric pressure being fed into it.

 

Yesterday I flew into KSFO using GPS RNAV Y 10R successfully but received an RAAS warning of a long landing.

Not sure what relevance this has. RNAV is for an approach. A landing is a landing. It's a separate discussion. Did you try to autoland off of an RNAV approach? If so, this is not a certified (and therefore, at least in FAA-land, not a legal) method.

 

If I was flying into an airfield using RNAV but was vectored away from my planned route, what would be the meaning of a request to 'report established' when there is, therefore, no ILS being used?

Did someone ask you to report established? I guess you could simply report established on the approach in general, but they shouldn't ask you that for an RNAV. The reason for reporting established is partially outdated, and partially to remind a controller to clear you for something.

 

Initially, the "report established" was used in non-radar days where the controller couldn't see the fact that you were appearing to be on the LOC.

Later, this became used in mainly two ways (i.e. is currently used in mainly two ways):

  • It's a prompt to the controller, who may have gone off issue instructions to other aircraft, to clear you for the approach.
  • It's a method to sidestep issues where the minimum vectoring altitude (MVA) is well above the altitude of the approach - a control method called a T-PAC (variation of PTAC: position, turn, altitude, clearance, which is the order in which you're cleared for an approach). The MVAs around KROA are relatively high (near the charted MSA, but slightly lower, if I recall correctly). When vectoring for the LDA to Runway 6, a controller could vector (the turn or 'T' in T-PAC) the aircraft onto the LOC at the MVA (which is about 5400 in that area, off of memory - I could be wrong), and ask the pilot to report established. Because the aircraft is no longer subject to the MVA once established on a defined airway or procedure, and is then subject to the published mins thereon, the controller can then issue the rest of the clearance: "[Aircraft], 3 SW of EXUNE (P), maintain 5300 until EXUNE (A), ( C )leared LDA Y Runway 6 approach."

The main issue here, though, is that the LOC is still an older style nav technology. It's a radial that you have to intercept. You can't use the LOC alone, or even a VOR with it, to proceed from any location directly to a fix on that approach. You'd have to either join the LOC first and then use a cross radial or DME to identify that fix (e.g. RAMKE is identified by being on the LOC and at 10 DME), or you'd have to intercept the radial that brings you to a fix defined by centered indications on both the VOR radial and LOC (e.g. EXUNE can be identified by intercepting the PSK 077 radial, and then following the LOC - EXUNE is where both the PSK 077 and LOC are centered). In both cases, you need to establish yourself on a radio beam before you can do anything else.

 

RNAV, on the other hand, does not have this requirement. I can tell you "proceed direct [FIX], cleared [Name] approach." The controller doesn't need to re-establish on any sort of radio beam so that you can begin to 'see' where you are. You already likely have a moving map that has been showing you this entire time, and you possess the ability to navigate directly to that fix without any external help. With this in mind, reporting established really doesn't have a place in the discussion. If anything, it could be warped into "I'm on the approach path, inside the initial fix (IF)," but that's not exactly standard.

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Hi Kyle,

Many thanks for your lengthy explanation.

Yes, I was trying to use an RNAV approach to autoland. Naughty boy! This is why I was questioning vertical navigation during landing. In future it will have to be visual landings with RNAV? I was only using RNAV approach/landing on this flight as I chose to land at KSFO 10R because of the wind direction.

I also used the "report established" term to try to denote that I was on some sort of glide path. But of course with RNAV I wasn't tuned into an ILS frequency.

I have found this document re RNAV. But at present this is not adding to my flying abilities because the terms LP, LPV, advisory/approved vertical guidance, baro-aiding and baro-VNAV are so hard to remember when the great majority of my experiences are with ILS. Perhaps I should just remember that, if not ILS, then LPV is a good second best? (A third best might be Ctrl+Alt+Del !)

A quote from the GNS530W AFMS - it only applies to US airspace, but there are no restrictions on flying an RNAV (GPS) approach using WAAS for vertical guidance if it is in the database and the approach is inside the service volume of WAAS vertical guidance.

Also from the FAA:
As of January 8, 2015, there are 3,522 Wide Area Augmentation System (WAAS) Localizer Performance with Vertical guidance (LPV) approach procedures serving 1,730 airports. Currently, there are also 583 Localizer Performance (LP) approach procedures in the U.S. serving 424 airports.
How are PMDG aircraft equipped for LPV approaches and landings?

I shall keep trying to understand how to approach/land with RNAV. Anything that you choose to add will be most welcome.

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How are PMDG aircraft equipped for LPV approaches and landings?

 

They are not. Nor does the required data exist in the navdata files.

 

David Jones

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Yes, I was trying to use an RNAV approach to autoland. Naughty boy! This is why I was questioning vertical navigation during landing. In future it will have to be visual landings with RNAV? I was only using RNAV approach/landing on this flight as I chose to land at KSFO 10R because of the wind direction.

 

RNAV is not certified for autoland, so it'll be just like any other non-ILS approach: shoot the approach, get below the clouds, and land without the AP.

 

 

 


I also used the "report established" term to try to denote that I was on some sort of glide path. But of course with RNAV I wasn't tuned into an ILS frequency.

 

What I was getting at was: "who taught you to do this?"

 

 

 


I have found this document re RNAV. But at present this is not adding to my flying abilities because the terms LP, LPV, advisory/approved vertical guidance, baro-aiding and baro-VNAV are so hard to remember when the great majority of my experiences are with ILS. Perhaps I should just remember that, if not ILS, then LPV is a good second best?

 

LPV is the "ILS" of RNAV, essentially. Keep in mind, though, that autoland was a feature that came after the creation of the ILS. ILS was created to give you guidance down to the runway. Some clever individual later thought "hey, I can use these signals, an autopilot, and some extra brain work to get the plane to land itself," and autoland was born. One could theoretically use RNAV with equal precision on an LPV approach - probably with better precision than some of the early ILS installations - but as is usual with established industries, red tape will delay progress. Additionally, those developing the automation will also ensure that their system is not allowing pilots to break regs by allowing autolands off of anything other than the approved method: ILS, so even if the theoretical possibility is there, the plane won't do it without at least a software update.

 

 

 


Also from the FAA:
As of January 8, 2015, there are 3,522 Wide Area Augmentation System (WAAS) Localizer Performance with Vertical guidance (LPV) approach procedures serving 1,730 airports. Currently, there are also 583 Localizer Performance (LP) approach procedures in the U.S. serving 424 airports.
How are PMDG aircraft equipped for LPV approaches and landings?

 

Already equipped for LPV in the sense that WAAS isn't necessary in the sim. GPS is perfect in the sim because there are no signal anomalies, which means that there is no augmentation (the --A- in WAAS) of the GPS position that needs to be done.

 

It may be best to write the idea of landing out of the approach discussion entirely. Forget the idea that any approach allows the option to autoland. An approach brings you to the airport environment so that you can see it. That is its purpose. Note that a bunch of the approaches include circling minimums. This - again - is an allusion to the fact that it is there to get you to a point where you can see the airport, and then maneuver to land at it on your own. If an approach has straight in minimums, then consider it a coincidental factor that makes life a bit easier.

 

Once you understand the concept of the approach, then allow yourself the exception of autoland, which simply capitalized upon existing technology as a source of guidance. ILS was not developed to allow for landings using the signal. It was simply capitalized upon for that purpose. In a similar way, the VOR was designed to guide aircraft between airports, but was later capitalized upon to create approaches to airports based on them.

 

Similarly, RNAV approaches were made to capitalize on RNAV-based equipment (OMEGA, INS/IRS, GPS) that guided aircraft between airports, in order to guide aircraft to those airports. Later, the technology was modified - through the use of combining INS/IRS and GPS with ground-based augmentation (LAAS/WAAS, or GBAS/GRAS) - to create a more precise signal, augmented by programming that interprets that information as if it is a LOC beam, in that it narrows as you get closer to the runway. This is alluded to in the name of the approach: LPV means Localizer Performance with Vertical guidance.

 

The NGX is capable of these types of approaches, again, simply because the sim itself isn't dependent on LAAS/WAAS to correct for GPS inaccuracy.

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I have been guilty of using "established" without any authorisation, I'm sure.

 

Many thanks for your texts. Things are beginning to come clear.

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I have been guilty of using "established" without any authorisation, I'm sure.

 

Many thanks for your texts. Things are beginning to come clear.

 

That's the aim. If clarification is needed, lemme know! If I don't know it, I can get someone down the hall and have explained it to me.

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Hello Richard,

 

An RNAV approach is an NPA (non precision approach) like a localizer/glideslope out app, or a vor app or an ndb app. The difference is that you don't rely on radionavigation aids to follow your track. You can't do an autoland when performing an NPA, A/P must be disconnected at MDA/DA -50 ft the latest.

The preferential mode for lateral navigation in an RNAV approach is LNAV.

For the vertical navigation, you have multiple choice : doing it the old way with V/S, or using VNAV.

1) V/S : set your MCP altitude at MDA+50 ft. At the FAF or the final descent point, indicated on the vertical profile of the approach chart, set your V/S to a calculated value depending on your ground speed (may also be indicated on the charts), generally -700/800 ft/min. Adjust your rate of descent with the green arc on the ND. When at least at 300 ft below the missed approach altitude, reset the MCP altitude to the missed approach altitude. When visual with the runway, A/P off and land manually.

2) Using VNAV : different minima exist depending on the authorization delivered by the Civil Aviation Authority of your country. You may be authorized for an RNAV with LNAV only(a), or you may be authorized for RNAV with LNAV/VNAV( b ) (also named APV, Approach with Vertical guidance). Even if you are authorized for RNAV LNAV only, you can use VNAV for the approach but with higher minima.

(a) LNAV only : When approaching the FAF, assuming that your MCP altitude is set to the level altitude depucted on the chart, engage LNAV and VNAV, and check that the FMA indicates LNAV, VNAV PTH. Set the minimums to baro, and MDA + 50 ft. Set your MCP altitude to MDA + 50 ft in the 737 (rounded) or to the minima value in the 777. When intercepting the descent path, VNAV will do its job and the aircraft will start to descent on the calculated profile. At 300 ft below missed approach altitude, reset MCP altitude. Disconnect A/P when visual but no later than MDA-50 ft, or go around.

( b ) LNAV/VNAV is almost the same at (a) above except that the minimums (on the EFIS control panel) is set to the DA value indicated on the approach chart.

 

Stephane

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In FSX you can autoland on any ILS. Flight sim's ILS signals are perfect in that the localizer is perfectly straight and the glideslope signal is at a perfect angle without any irregularities. In the real world that is not the case.

 

The ILS signal can have a certain amount of scalloping on the centerline and/or the glidepath. To get smooth centerlines and glidepaths to touchdown and along the centerline of the runway for rollout requires upgraded ILS equipment so to speak. Siting of the glideslope can be especially critical since most use what is called an image array. The terrain in front of the antenna must meet tight grading tolerances to ensure no distortion in the signal. I've seen stuff that has been buried a mile away cause problems with the glideslope structure.  The standby transmitters must be in hot standby ready to fire up if the primary has a fault. Monitoring of the signals is at higher tolerances. The list goes on and on............ Basically a lot more money is involved to install and maintain the level of accuracy required.

 

Lets say your hand flying the ILS, any small irregularities in the signal usually will be dampened by the instrument itself and also by the fact that you wouldn't be chasing every little excurion in the glideslope signal.. Now imagine doing final at 140 kts and the autopilot is flying the raw ILS signals which are not dampened out. The closer you get to the touchdown the more critical the structure of the ILS signal needs to be. Any big excursions could cause the autopilot to plow you into the ground or at least unstabilize the approach at a critical time. Most of the tolerances the ILS is flight checked at won't even be noticed by the average pilot however the autopilot is a different beast!

 

Just some food for thought.

 

Steve Aull

Former USAF Navaids Technician

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