KSAN: Difference between LOC and RNAV approach

[Oddvar Tveito 1389576]

I need to learn the difference between LOC rw27 and RNAV (Y/Z) rw27 approaches. I have been practicing LOC27. That gives me a localizer beam to guide me laterally. Today I was cleared a RNAV Z rw27 app. It seemed RNAV was allmost the same approach but without a localizer. Is that correct? Any other differences? In my FMC I have both LOC27 and several RNAV 27, but what is the difference? I guess I should know that before arriving at KSANagain . I tried to read the docomeentation, but could not find anything.

[Andreas Fuchs]

Check this: https://www.vatsim.net/pilot-resource-centre/ifr-specific-lessons/instrument-approach-procedures-iap

 

I guess you will have further questions, bring them up here, but maybe make another more specific search and you'll find some good info on this. We are here to help as well.

[Josh Glottmann]

There are a lot of differences. Look at the charts. LOC RWY 27 and RNAV (RNP) Z RWY 27

Different: minimums, definitions of waypoints, missed approach procedures, initial approach fixes, the entire type of approach.

 

Always look at the charts, don't rely on your FMC.

[Oddvar Tveito 1389576]

Yes, both the Pilot Resource docomeent (thanks for the link Andreas) and the charts indicate LOC and RNAV as different things. LOC is about the two navigation aids, the Localizer at the runway and the VOR at POGGI. The RNAV chart, on the other hand, has nothing about the localizer, but it has a nice path of waypoints to follow using the planes' GPS. So far so good.

But I try to learn these things by flying. And when I look into the FMCs, it gets all mixed up. I have checked three of my planes that I trust the most. All of them include a LOC frequency also when I select an RNAV approach for KSAN. And they lock to it on final. And if I select the LOC27 approach, I still get the list of waypoints to follow – not only the Localizer. So, in practice, as seen from my programmed FMC, there is no difference between a LOC approach and an RNAV approach. Both have a LOC-frequency as a lateral guide and both have waypoints to follow until you lock at the localizer.

But at the end, one should be able to understand the clearances given by ATC. So, if KSAN_APP clears me for a LOC/VOR app to rw27, does it mean something like “I will vector you until you can lock on the localizer, then follow it and descend manually according to chart”. And if he clears me to an RNAV-approach, does it mean something like “follow the waypoints using the planes GPS until you get to the final leg, and descend manually according to chart. If there is a localizer and you use it to guide you on final is up to you.”

 

BTW, I use X-plane and

the B767-300 from FlightFactor

the IXEG B737

the Rotate MD-80

 

As far as I remember they all have the same behaviour concerning non ILS-approaches described above.

[Andreas Fuchs]

Hi Oddvar,

 

to make it short and to stop your confusion: if you follow an ILS or a LOC approach, you are using ground based navigation aids. This type of navaids is normally followed by using conventional modes, like "APP" or "LOC". If it is a LOC approach, your autopilot/flight director can only automatically follow the lateral part of that approach, the crew is responsible for initiating and controlling the vertical part of the approach procedure. Normally you would use "VS" (vertical speed) or "PATH" (if available) for this.

 

In the case of any RNAV approach you will rely on signals from satellites and the waypoints from your FMC. In this case you need to verify (against approach charts) and follow the list of waypoints by using "LNAV". With "VNAV" you should be able to arm and follow the vertical portion of this type of approach.

It is a bit more complex as to what minimum you can actually descend to, but for starters you can use LNAV/VNAV for these procedures.

 

Honestly, we are still scratching the surface here!

[Trent Hopkinson]

Hi Oddvar,

 

to make it short and to stop your confusion: if you follow an ILS or a LOC approach, you are using ground based navigation aids. This type of navaids is normally followed by using conventional modes, like "APP" or "LOC". If it is a LOC approach, your autopilot/flight director can only automatically follow the lateral part of that approach, the crew is responsible for initiating and controlling the vertical part of the approach procedure. Normally you would use "VS" (vertical speed) or "PATH" (if available) for this.

 

In the case of any RNAV approach you will rely on signals from satellites and the waypoints from your FMC. In this case you need to verify (against approach charts) and follow the list of waypoints by using "LNAV". With "VNAV" you should be able to arm and follow the vertical portion of this type of approach.

It is a bit more complex as to what minimum you can actually descend to, but for starters you can use LNAV/VNAV for these procedures.

 

Honestly, we are still scratching the surface here!

 

some aircraft can use an RNAV in "APP" mode, many with a waypoint crossing restrictions based constant rate descent available too (functionally a glideslope, though often based on barometric altitude and not an actual glidepath).

 

PMDG 737/777 and Aerosoft/FSlabs A320 can do this, along with the RealityXP GNS430/530 addons when installed into aircraft that are capable of flying an autopilot coupled an ILS approach with a flight director (ie: Realair Duke, A2A Cessna). The difference being that you're conducting an approach using the "APP" mode, and descending to an altitude below the selected MCP altitude on autopilot with a glidepath indicating on the instrumentation (sometimes in a different colour depending on the aircraft) to an airport or runway that does not have an ILS.

 

Examples of approaches I have used this at in the past include:

SBRJ in the PMDG 737 / FSlabs A320.

YKII in the Realair Beechcraft Duke BE60 with the RXP GNS530.

 

Same idea though, the ILS/LOC is recieving signals from a physical transmitter on the ground near the runway, and RNAV/RNP/GNSS approach is using signals from satellites, hundreds of kilometers away in space (GPS satellites are not in geostationary orbit by the way).

 

To be able to fly the procedure, your aircraft needs to be equipped for the approach, have the procedure in the FMS or GPS (precision requirements forbid entering waypoints and restrictions manually). They are not considered a precision approach, since the vertical component is derived from position along the procedure compared to barometric altitude, rather than following a radio beam down to a transmitter, however the chart will show the required minima.

Edited November 23, 2017 at 11:57 AM by Guest

[Andreas Fuchs]

My real plane does "APP" as well, but I was not aware that our addon aircraft have become that sophisticated already. So, thanks for the additional information.

 

When flying an approach with vertical guidance, the ASEL is usually set to the (initial) missed approach altitude.

[Oddvar Tveito 1389576]

Thanks to all of you for great answers. Think I've got it now (maybe not all the details ). I will stop using the plane to understand concepts,and rather read VATSIM articles only. Next time ATC clears me with an RNAV approach, I will know what it means. Thanks again! Great help!

[Andreas Fuchs]

Don't limit yourself to the VATSIM-articles. There is some more in-depth material on the internet and on YouTube. Just search for "RNAV approach guide" or similar terms. If you invest about 1 hour into reading, viewing and listening, you should be set with more advanced knowledge. And then it is all about practicing it! Why not request an RNAV-approach, although VATSIM-ATC is announcing ILS-approaches to be in use?

[Oddvar Tveito 1389576]

... And then it is all about practicing it! Why not request an RNAV-approach, although VATSIM-ATC is announcing ILS-approaches to be in use?

 

Very good idea! Haven't thought about that as possible - to ask for RNAV if ILS is available. I allways just fly what I am told.