RNAV-RNP Questions looking for someone well versed with PMDG 737 and RNP approaches.

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I’m looking for some help to try and understand RNP approach’s better.

For my questions let’s look at San Diego international (KSAN) runway 27.

Runway 27 has two different RNP approaches it has a “yankee” & “Zulu” I understand that each one has different minima. I also understand that RNP “Zulu” is going to be the one used in the worst weather conditions because it’s decision height is the lowest to ground level makeing it the most precise.

My question is, what factors of weather are absolutely needed to decide what RNP is the most appropriate for your approach?

Part 2: Second question.

When looking at the RNP “Zulu” there is two separate minima or decision heights with 0.11 being the lowest and there is also a 0.30 being the highest. From my research I have gathered these numbers have something to do with how currently precise your aircraft is in relation to GPS.

My question is can you pick what precision level you want or is that automatically calculated by the FMC.

Second part to that question.


When I’m about 40 miles out from runway 27 I sometimes see my flight data say my current RNP preciseness is 1.0 or 2.0 and obviously those are huge numbers compared to 0.11-0.30. But as I start to get closer to runway 27 I notice that the preciseness starts to drop closer to 0.30 when I’m about a 5-10 mile final. Is this a normal behavior in real life? Does it actually drop like that the closer you get? Because to me that kind of seems a little sketchy that it constantly moves. It’s almost as if you don’t really know what your going to have until you are almost to your FAC.


So much goes into RNAV approach’s I find it very interesting.

Thanks.
Jordan W.

[davidbarrocasr]

I could be saying this wrong, but the PMDG does not simulate an RNP app.

[Murf7413]

I’m looking for some help to try and understand RNP approach’s better.

For my questions let’s look at San Diego international (KSAN) runway 27.

Runway 27 has two different RNP approaches it has a “yankee” & “Zulu” I understand that each one has different minima. I also understand that RNP “Zulu” is going to be the one used in the worst weather conditions because it’s decision height is the lowest to ground level makeing it the most precise.

My question is, what factors of weather are absolutely needed to decide what RNP is the most appropriate for your approach?

Part 2: Second question.

When looking at the RNP “Zulu” there is two separate minima or decision heights with 0.11 being the lowest and there is also a 0.30 being the highest. From my research I have gathered these numbers have something to do with how currently precise your aircraft is in relation to GPS.

My question is can you pick what precision level you want or is that automatically calculated by the FMC.

Second part to that question.


When I’m about 40 miles out from runway 27 I sometimes see my flight data say my current RNP preciseness is 1.0 or 2.0 and obviously those are huge numbers compared to 0.11-0.30. But as I start to get closer to runway 27 I notice that the preciseness starts to drop closer to 0.30 when I’m about a 5-10 mile final. Is this a normal behavior in real life? Does it actually drop like that the closer you get? Because to me that kind of seems a little sketchy that it constantly moves. It’s almost as if you don’t really know what your going to have until you are almost to your FAC.


So much goes into RNAV approach’s I find it very interesting.

Thanks.
Jordan W.

In short it depends what your airline is approved for... for example mine is approved for an RNP RNAV approach down to 0.3. We are not allowed to do an approach down to say 0.2. So yes, weather has a lot to do with it.

Here is a good depiction of RNP...

image.png
The big difference between RNP and RNAV is the equipment. RNP equipment constantly monitors the accuracy of the signals and equipment and lets the crew know what the accuracy is.

Murf

[Stearmandriver]

I’m looking for some help to try and understand RNP approach’s better.

For my questions let’s look at San Diego international (KSAN) runway 27.

Runway 27 has two different RNP approaches it has a “yankee” & “Zulu” I understand that each one has different minima. I also understand that RNP “Zulu” is going to be the one used in the worst weather conditions because it’s decision height is the lowest to ground level makeing it the most precise.

My question is, what factors of weather are absolutely needed to decide what RNP is the most appropriate for your approach?

Weather isn't the factor in choosing between these approaches. What RNP you're currently capable of meeting, and which approach has a transition from your arrival direction, have more to do with it. Assuming you're an aircraft that can do any of the approaches, of course you'll choose the lowest approach.

Part 2: Second question.

When looking at the RNP “Zulu” there is two separate minima or decision heights with 0.11 being the lowest and there is also a 0.30 being the highest. From my research I have gathered these numbers have something to do with how currently precise your aircraft is in relation to GPS.

My question is can you pick what precision level you want or is that automatically calculated by the FMC.

Second part to that question.


When I’m about 40 miles out from runway 27 I sometimes see my flight data say my current RNP preciseness is 1.0 or 2.0 and obviously those are huge numbers compared to 0.11-0.30. But as I start to get closer to runway 27 I notice that the preciseness starts to drop closer to 0.30 when I’m about a 5-10 mile final. Is this a normal behavior in real life? Does it actually drop like that the closer you get? Because to me that kind of seems a little sketchy that it constantly moves. It’s almost as if you don’t really know what your going to have until you are almost to your FAC.


So much goes into RNAV approach’s I find it very interesting.

Thanks.
Jordan W.

The changing numbers you're seeing in the FMC are RNP vs ANP numbers. Every phase of flight has different levels of RNP - Required Navigation Performance. Simplifying, this is the value in NM that the aircraft is required to be able to determine its position within. You see that number decrease as you transition from the enroute to terminal phase of flight because your required navigational performance is getting tighter - you need to know where you are with a higher degree of precision.

The other number, next to your RNP value, is ANP - Actual Navigation Performance. This is the value in NM that your aircraft is currently able to determine its position within. This value is usually around .06.

As long as your ANP is smaller than your RNP, your aircraft is determining its position with a higher level of precision than is required - thus you're ok to fly that procedure.

If your ANP ever exceeds your RNP, you need to change to a procedure with a less stringent RNP, or revert to conventional nav.

This is the entire reason we're required to manually set the RNP value on an RNP approach: it allows the Nav Performance Scales (NPS) on the PFD to graphically show you the relationship between RNP, ANP, and cross track error in real time. Setting RNP does not change in any way the manner or precision with which the aircraft tracks an LNAV course - the plane will fly that course the exact same way whether you've got an RNP set of .10 or of 10. It just changed the display of the NPS.

You can (or should be able to, I've not tried it in the PMDG) be able to watch the relationship between RNP and ANP change by failing both GPSs. You'll see the ANP value start to creep up as minutes go by, as position uncertainty grows (the FMC is still able to keep position without GPS using IRS and nav triangulation, but it's not as precise.). When ANP exceeds RNP, you'll see it clearly on the NPS.

EDIT: Oh, and yes, you can choose which RNP value (and subsequent minimums) you'll use for the approach, based on your aircraft's capability and certification. The 737 is certified for the lowest RNP you'll find: .10. Other aircraft can't get down that low, so they'll need to choose higher RNP values. Also, based on the visible GPS satellites at a given time, you may or may not be able to achieve an ANP of less than .10. if you can't, you'll need to choose a higher RNP value and mins.

[Stearmandriver]

I could be saying this wrong, but the PMDG does not simulate an RNP app.

It does; I've built many of my company's custom approaches in the PMDG, and there are many public ones in the navdata by default (like the SAN approaches the OP mentions.)

What the PMDG can't really do is RF leg segments, but it approximates those with a series of waypoints to define the arc. It's sloppy, but works well enough to get you in until PMDG updates their navdata format (hopefully soon!)