RNAV GPS LPV approaches

Currently in Infinite Flight RNAV approaches are LNAV only, I think it would be great if we could get LPV approaches or at least LNAV/VNAV approaches. For those of you who don’t fly in the real world some of this may go over your heads a little, but I will try to explain what the basic concepts are and how they benefit you. I don’t intend for this to be a full technical explanation of, or guide to different RNAV approach types though. I will be ignoring many different types and considerations to boil it down to main types, and omitting some details for simplicity. If you really want to fully understand RNAV approaches, their requirements, pros/cons, limitations, etc then I encourage you to do your own research as there is a lot of information out there.

Let’s start broad though, what is an RNAV approach, and what type currently exists in the game. RNAV approaches are mainly GPS approaches (other kids do exist but will be omitted for this topic) and are considered a non precision approach, though some are flown much like a precision approach and can be just as effective depending on the airport and aircraft. The most basic kind of RNAV GPS approach is an LNAV only approach. LNAV meaning lateral navigation. This can be thought of as being much like just having the localizer element of an ILS. It will give you side to side info, but no vertical information. The key distinction for this type of approach as compared to a localizer approach is that the approach course does not narrow. The course width remains constant all the way from about two miles before the final approach fix (FAF) to the runway. It is usually about 0.3 miles, which means a full scale deflection on the CDI means you are 0.3 miles either side of course wether you are miles away from the runway, or at the missed approach point (MAP). This is currently (as far as I can tell) the extent of RNAV approaches in Infinite Flight. I am not entirely sure that Infinite Flight even actually scales down the course sensitivity to 0.3 miles of if it is the same sensitivity as anywhere else in your flight. These approaches have the highest minimums of the three (minimums are largely ignored in Infinite Flight though) and provide the least information to the pilot.

How could this be easily improved? Well the next step up that requires minimal change is for them to make the approach LNAV/VNAV. The LNAV component is the same, but now it adds vertical guidance as well, much the same as a glide slope on an ILS. This lowers the minimums and gives you more information for your approach. For the purposes of Infinite flight I think it would be perfectly fine if this was implemented as little more than the already existing glide-slope indicator but purple (GPS navigation information shows up as purple while radio based navigation like ILS systems and VORs show up as green, this is already how it works in Infinite flight too for the CDI). While the difference is somewhat more nuanced than that in the real world, that is more than acceptable for the level of simulation Infinite Flight has.

Could it be even better than that? Yes! The best type of RNAV GPS approach is an LPV approach, or Localizer Performance with Vertical guidance. So if an LNAV/VNAV approach already has vertical guidance what makes this one better? Well remember how I said the course with stays the same on an LNAV only approach and on an LNAV/VNAV? That isn’t the case here. Hence the “Localizer Performance” because it is made to behave identically to a localizer. (Worth noting that there are LP approaches that have no vertical component in the real world, but again I think including every possible type would make Infinite Flight too complex for the average user) Because of the increased accuracy these approaches are nearly as good or even better than a normal ILS, in fact they normally have the same minimums as a CAT I ILS and are generally more accurate. There is technically a significant distinction, RNAV approaches are not considered precision approaches because that only applies to ILS approaches really thanks to how ICAO classifies approaches, they are flown so similarly that I think you could essentially use infinite flights existing ILS system but make the indicators purple. There is really little difference beyond that from the pilot’s perspective. You may be asking though like if it’s so similar why bother implementing it? Good question. Since an RNAV approach requires nothing on the ground, just someone to set it up in the GPS system it is much easier for especially small airports to have. This means there are many airports already with RNAVs and no ILS approaches, at least to certain runways, and that number will only grow. This may not be a huge issue at major airports like JFK or London Heathrow where the infrastructure is at such a high level there are generally ILS approaches to every runway, but at a smaller airport, particularly general aviation airports an LPV approach may be the best way to get into that airport and could be the difference between being able to land and not being able to land. Infinite flight seems to already show many of these approaches as the R(runway number) but they are only LNAV approaches as far as I can tell.

I know this one is a bit of a more technical idea, and some users may not fully understand the difference, but LPV approaches are in many ways the future, particularly at smaller airports, so I think it is in the best interest of Infinite Flight to implement them sooner than later, particularly seeing as how it can be done so easily with a small adaptation to the ILS technology already in the game. At a minimum I would like to see GPS RNAV approaches improved and expanded upon because they are a growing part of how approaches are conducted in the real world but are relatively limited in Infinite Flight.

Me trying to read this at 11pm wasn’t a good idea…took me 10 minutes to understand…

This sounds great! Would increase the realism!

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Yeah, I know it is a little complex. I think an appropriate TL;DR would be that in many cases RNAV approaches should just be purple ILS approaches, everything else is just the technical reasons why.

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While this topic is rare, I’ll point out some shortcomings. RNAV is considered a navigation specification that only applies to SID STAR and ENROUTE. in terms of naming rules nowadays, there is no longer any such thing as an RNAV approach. But you may still see headings like RNAV(RNP) RNAV(GPS) RNAV(SAAAR) on FAA charts. They correspond to RNAV(GPS)=RNP RNAV(RNP/SAAAR)=RNP(AR) respectively.ICAO has asked all member countries to change the chart names as a way to harmonize standards (except for the US). So, we are now talking about RNP approaches. In reality, then, RNP operations involve several aspects, including but not limited to RAIM predictions, aircraft capability, crew capability, navigation performance… …RAIM predictions are complex and include, but are not limited to, satellite star clock calculations, aircraft turn shielding, and terrain masking. Aircraft capability should include L/VDEV and RF capability. L/VDEV indicates deviation by calculating a virtual path from the onboard database and comparing it to the current position. RNP(AR) is one of the few navigation specifications in RNP that falls under the category of precision approach because of its precision guidance capability. The crew capability… skipped. Navigation performance, I have tested and for now IF’s LNAV is capable of RNP 0.15 capability, but still needs improvement. In summary to achieve RNP approach in IF would require a complete overhaul of the AP as well as multiple aircraft UI adaptations.

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Ok so I get where you are going with that but a few points. Firstly overarching point is that Infinite flight has already simplified a lot of things so I wouldn’t be asking for a full simulation of RNP approaches of every type. To start with your first point you are right that I am off on my terms there a little, but as you mention the US is for whatever reason still not really doing the RNP approach thing and RNAV approaches are still very much still a thing here that you will find on the charts at almost any airport. I know terminology is a little different abroad across the board so I just stuck with what I knew since I am doing my instrument rating here so that is what I am familiar with and can speak to. So yes, these are technically all RNP approaches.

Secondly about the RAIM element, firstly RAIM isn’t always needed, or dare I even say often. At least here in the US WAAS often covers that, but I know WAAS is just our version of SBAS and I am not sure if other regions also allow you to forego RAIM if using the local equivalent. I would though say that is sort of a mute point though because I think we can just assume perfect GPS or more broadly GNSS in Infinite Flight because it is easy for the aircraft’s exact position to be know and you aren’t relying on any of the real world systems that introduce errors.

As for other RNP approaches they certainly exist, but I think for this we can limit it to just GPS there are several other kids but those all say special crew training required and special aircraft equipment required so I think it would be safe to limit this to standard GPS approaches that any instrument rated pilot can fly in any plane with an approved GPS receiver because both of those can be assumed fairly reasonably in the Infinite Flight environment.

Well, the RNP distribution is still the widest in the world. So let’s assume that every aircraft in IF has 100% reliability of the received signal, so now there are three things needed, L/VDEV, leg resolving capability of ARINC424 , reduced FTE.

In the current IF, an RNP approach can be accomplished via LNAV/VNAV if the navigation database applies. But we only know the XTT and not the vertical error. This needs to be calculated by FMC from NDB data. What is clear is that the IF flight plan is rather chickenshit and the VNAV doesn’t know which is VIP/FAP/FAF and MAPt. Then the vertical error is nowhere to be found, so I think this is an area that needs improvement.

The leg resolving capability of ARINC424 is very important, it is about the flight procedure that can be correctly resolved by the FMC. However, in the current IF, this capability is almost non-existent, just dots connected to dots in a line. This is fatal to RNP apch.

The reduction in FTE is also quite important, but this is based on the leg being parsed correctly. Then we assume that the program is being parsed correctly, but LNAV in IF is rather undesirable at low speeds, so this needs to be improved as well.

In addition, the RNP approach is affected by the altimeter, which is affected by barometric pressure and temperature. However, this amount of calculation undoubtedly burdens the system, so the altimeter manipulation can be ignored.

Yes, those are all good points, but I would retort by saying that this is more or less why I am asking for a quite rudimentary system. All airports that have LPV approaches would more or less just have the current ILS system but purple. Far from perfect but that is really the extent of what I am asking for. A full introduction of RNP approaches is a massive undertaking for reasons you point out.

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