Wanted to check my understanding on RNAV / LNAV

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Hi everyone! I hope all of you are enjoying the new MSFS! Coming from X-Plane 11 I am so impressed with all of this. Yes, its not perfect and I know they still need to do some updates… but wow! Really love this sim so far :slight_smile:

I have always wanted to be a pilot, but it’s just not practical in real life for me at this time. So, the sim it is lol.

I just quickly wanted to check my basic understanding on a few things. With the sim, I generally tackle one thing at a time and try to move on. For now, I feel like I understand the ILS landing approach / system pretty well. Frankly, I would imagine in the real world this would be the preferred method of landing for any pilot. I would never understand (if available) why a pilot would never want to use an ILS approach and have as much of the decent / approach automated as possible.

I generally activate “approach mode” when I have turned on final and have the speed and flaps all set for touch down. Plane does the rest and around 1K - 500ft ill disengage and hand land it.

Now, (especially with the new MSFS) I am wanting to venture out and “site see” more and most of the smaller places I want to see and experience don’t have an ILS approach. So, this brings me to RNAV / VNAV. So, first I have tried to google this and watch videos… and frankly they make me more confused. I would prefer another simmer to chime in and help.

As I understand… RNAV is just GPS mode in the FMS flying to different points. Really the same thing as “NAV” autopilot mode in the plane. “RNAV” is basically just another name for GPS.

VNAV, is where the AP system will “look” at the FMS way points / legs and will attempt to control ALT based on the flight level assigned to them. Some planes have this mode and some do not, just as in the real world.

The main difference between an ILS approach / landing and an “RNAV” approach / landing… is that ILS of course uses local frequency to “talk” to the plane to offer vertical guidance down. RNAV (by itself) offers none of this at all. This (again) is just GPS going to point to point. BUT, VNAV can offer a similar result to ILS, what I will call “automated manual” vertical guidance. It will automatically fly the lower ALT assigned, based on each incoming waypoint. So basically it takes care of the RATE of descent automatically and trys to ensure that you hit the target ALT for each point for a more “stepped” controlled approach.

So far, is that about all correct?

Now, lastly… a Visual Approach is none of the above and would “start” at the IAF. So, it would be common to fly most of the flight plan (assuming an IFR flight) in GPS / RNAV and them once in the approach legs, the pilot would disengage AP and hand fly the plane to the runway, calling out each leg of that part. This is 100% manual flight all the way down.

If this is all correct (again, keeping to basic terms) then I don’t understand the difference with LNAV and RNAV. Seems like they both would use GPS to go from point to point.? Obviously VNAV is for up / down.

Before I ask anything else, am I correct on all of this?

Thanks guys!

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LNAV is Lateral Navigation and does not provide vertical guidance unless it’s an LNAV/VNAV approach. VNAV is Vertical Navigation.

Also, technically, you have to maintain your last assigned altitude until you are “Cleared for the approach” And on a published segment of the approach. You should not enter an approach mode on the AP unless you have been cleared and are on a published segment.

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It’s been a while, so I’m not 100% on all this:

RNAV - Random Navigation - essentially just implies GPS navigation. Enables you to plug in any waypoint anywhere and fly directly to it. It is used in the approach plates to indicate a working GPS is required.

LNAV - Lateral Navigation - the lateral, 2 dimensional navigation that specifies points in latitude and longitude. Altitude is controlled by the pilot. Simplest form of RNAV.

VNAV - Vertical Navigation - the altitude component of an RNAV approach that allows for GPS altitude control, similar to an ILS but much less precise. Does not imply LNAV as well, and often you’ll see LNAV/VNAV together.

LPV - Localizer Performance with Vertical guidance - this is a GPS approach with nearly identical performance to that of an ILS. As you get closer to the runway threshold, the precision increases as you fly down the cone of the approach.

ILS - Instrument Landing System - the combination of a runway localizer (LOC) and glideslope (GS) that is broadcast over VOR frequencies. Like LPV, the signals form a cone to the runway threshold for precision approach guidance.

Visual - pretty much what it says on the tin. Look at the runway and put yourself on it. You can still use the GPS or ILS to assist your approach, but you will not be required to stick to minimums. See and avoid. Obviously this can’t be done in IMC and the airport will usually have favorable VFR conditions for this approach to be issued.

EDIT: Spelling

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I see I made a typo in my opening post. Everywhere I said “LNAV” I meant to say “VNAV”.

Which, then opens up a new question of what is “LNAV”. lol.

So, going back to a very basic understanding… RNAV + VNAV will give the results like what I was asking above? This is basically just a GPS FMS plan where it “looks” at the FMS data and will descend based on the pre programmed ALT. etc…

I think this is correct. Again, lets just keep it basic here in terms of how it works.

So, then yes what would the difference be with LNAV? Seems like RNAV (GPS) is lateral navigation as it is.?

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Thanks for this wonderful response. This is what I was looking for :slight_smile:

So, it looks like I was pretty much correct on everything I was thinking. (I was looking for that confirmation). I made a typo in my first post and have fixed it now. The whole time I meant to say VNAV lol. and you have added in the other methods. Thank you!

So, RNAV is really GPS and VNAV is really the “up / down” version of that, (tho I do know its based on the FMS data and then uses GPS to direct etc). So, then LNAV is now the confusion. How, is this any different to GPS / RNAV?

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Essentially GPS, RNAV, and LNAV are the same thing. The terms are generally synonymous but it’s important to know the distinctions between all the acronyms.

GPS refers to the satellite tracking system, whereas LNAV is a type of RNAV, which are how the airplane uses GPS data.

As many flight instructors have told me; words have meaning.

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Ah, I think I see what you mean. Kinda like people think that the “=” sign means the sum of a math problem.,… when in fact, it means both the left and right side of a problem are the same.

Okay then they really all are fundamentally the same. Curious, as to why there is a dedicated LNAV button in some planes when there is also a NAV button. So, when I select FMS from the CDI and then click the NAV button on the control panel, plane will fly to all points. Perfect. So… when in practical use would LNAV be pressed / used?

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For that you’ll have to reference the POH or the manual for the AP system itself. It’ll be different for almost every situation and there’s no telling if the game has it implemented correctly in the first place.

Kinda like real life, if you can get it to work the way you want it to, that’s great!

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Haha. Yes, good point. Well thank you so much for taking the time and helping out a fellow enthusiast! I appreciate it very much. Have a wonderful day, and happy landings!

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Here’s the way to think about them. LPV vs LNAV/VNAV vs LNAV are not types of approaches they are minimums within RNAV approaches .

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RNAV is actually a navigation type/specification, and stands for Area Navigation. It means being able to navigate from one point in space to another, independent of ground based navaids. Originally it included LORAN, INS, but these have now obviously been superseded by GPS.

LNAV is an aircraft Autopilot/Flight Director mode, usually selected by the pilot. If your navigation system is RNAV capable, and you have a valid route loaded, activating LNAV will make the autopilot follow this route.

Edit:
Some aircraft have both LNAV & NAV modes avaliable. LNAV works as stated above, whereas NAV allows tracking of raw radio signals from ground based navaids. For example, if your GPS died, tune a VOR and NAV mode will attempt to track the radio on the selected radial.

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Thank you for this. This makes sense to me. So, RNAV is just referring to a method of flight, which now is generally all GPS. This is not some “mode” or anything. Got it.

Yes, I can see both the LNAV and NAV buttons in some of the planes I fly. I use the NAV to follow the FMS (gps)… but, I can’t seem to understand why then is there an LNAV button? What actions will the plane take differently vs. GPS / NAV?

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Aircraft with LNAV & NAV buttons, most likely won’t have a NAV/GPS selector. A lot of older GA aircraft only have NAV modes avaliable on the autopilot system, so an additional switching option is provided to change the lateral navigation source from radio aids (VOR/LOC) to GPS

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Thank you! Makes sense!

I wonder when MSFS 2020 will support the use of VNAV? That will be nice to use. As of now when I try to do an RNAV approach, I am of course having to manually descend and try to fit the final fixes accurately. I have been using the (loss of altitude, divide by 1000 times 3) to determine TOD to IAF and then taking the current / projected GS and x5 to get rate. It’s been a lot of fun. I would assume in the same plane, in the real world the VNAV would really help in this sense?

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Vertical guidance for RNAV(GPS) approaches, especially for GA aircraft is a relatively recent development. Think approx last 10 years. Before that, RNAV approaches were flown exactly as you describe, calculating descent points and rates manually. In GA this is still extremely common, unless you’ve updated your equipment to current models.

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First off, awesome to see you pursuing your passion. LNAV is typically used as a IFR (Instrument Flight Rules) environment tool. As you mentioned before, it is based on lateral guidance between fixed waypoints. It can of course also be used to fly waypoints that a pilot enters into an FMS or Navigation unit like the G1000/3000.

The NAV button existed before LNAV. This was on the most basic of autopilots and was designed to fly VOR to VOR. There were no waypoints to punch in. It took its steering guidance from either a VOR receiver or the more handy HSI which is still used today on many aircraft. This evolved into early GPS models where you could also enter in waypoint s like the Garmin 430/530, but aircraft would often have the older VOR system still installed. Hence the NAV mode button.

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Perfect! Thank you so much! All of this can be a little confusing lol. But, this helps a lot!

Yeah, I am a wedding photographer by trade. All my business has been postponed ./ cancelled due to COVID… so, Flight sim it is for now. It’s been a wonder sense of adventure and travel. Which, of course I cant do now. I always had interest in aviation… but, I just dont want to be traveling all the time and away from my family. So, never “went for it”. If I were a single guy… I think I might do it. Maybe one day if I can ever make any money again, would be fun for private lessons.

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It sounds like you’re doing it all correctly, as I have fond memories of calculating my desired descent rate while spinning my whiz wheel to figure out my exact ground speed, all while in the midst of flying the approach itself.

Don’t forget that the aircraft manufacturer is rarely the AP manufacturer, and, especially in GA aircraft, the AP will have buttons and functions that are either redundant or incompatible with the rest of the avionics in the stack.

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Yep. A MSFS aircraft where NAV mode simply doesn’t work, and you have to fly around in HDG mode, constantly changing it by hand, is not a bug. It happens all the time in GA aircraft haha.

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Ah! Good points! Yes, and I bet some planes get “upgraded” avionics etc… so thats a good point!

Yeah, the “busy” work and math that pilots need to be doing to keep one step ahead of the plane is really impressive. For me, this is the fun. I literally print out the charts and sit here and try to fly them correctly. I get frustrated when I get to the IAF and I am way too high :-/ Then, there is terrain to deal with.

I just attempted an RNAV approach on runway 15 at KASE… the IAF ALT was 12,900’ hugging the mountains… then, drops to 7,800’ for the runway. The strange thing that I can’t figure out (about this specific location) [and I know I am getting off topic here]. is that when I flew this approach plate (and I have it printed here in front of me) the runway heading is 166, and sure enough in the sim everything seemed great… I came in on the RNAV (GPS)-F approach entered at “DBL” at 12,900 and then proceeded to the GS “ALLIX” for 12,200 and from there had 6.5nm till touch down for 7,800. However, when I flew this… and I broke the clouds… Once I had the field in sight… I could see that I was not lined up AT ALL with the runway heading, even though I was on a track of 166.

So I either:

  1. Have NO idea what I am doing (possible, lol).
  2. The approach plate is wrong.
  3. The KASE airport is not correct in the sim and this is throwing me off.

On the flip side, last night I did a very beautiful flight into KJAC and had no issues. Everything lined up, and I ran about a -500fpm descend from the IAF right down like an ILS and then manually landed. it was nice. This is why I do think I am doing it correctly. So, why in the world would Aspen’s approach not place me on the correct path?

It sucks not having a nice landing after a full flight from somewhere. :-/

EDIT - I just noticed there is a LOC/DME chart… and it looks different. So, maybe I need to use this and re-fly this to see the difference. So, I would use the FMS / GPS to fly the points just the same, even tho it says “LOC/DME”?

I also see the approach heading is 155 this time. So, this could be the key.