Hi,

 

lately I updated my Airac to 1513. I was doing approach to EPKK 07 runway in Vnav/Lnav mode using RNAV non precision approach. When I used it before it seemed to be pretty reliable, but this time the approach ended around 300 metres right to runway leaving me with no other option than a go around (I could switch to VOR localizer and do a standard VOR approach landing manually on runway, but I wanted to finish this to make sure that I'm really in wrong spot. 

 

Is it happening often or is it error within Airac? I know it's a non precision approach, but on the other hand LNAV and VNAV, if they operate correctly, should lead you to right place so I think the issue is Airac related? 

 

 

lately I updated my Airac to 1513

 

 

 

When I used it before it seemed to be pretty reliable, but this time the approach ended around 300 metres right

 

If the only thing that changed between working and not working was the nav data, then this would point to the nav data being the issue.

 

However, given that it's not an RNP approach, the use of LNAV (which does not use the narrowing path like an LPV approach would) means that the lateral limit for the approach is RNP 1 (at the MAP - as wide as 4nm at the FAF). That's 1NM of precision being acceptable (so, 300m would theoretically be within limits - even within RNP 0.3 limits, actually). This is why the mins are over 3 times what you'd see for an ILS approach, at 619' AGL for this basic RNAV approach.

 

This approach is to get you below the clouds, and then allow you to fly the rest of the approach by hand, down to the runway.

 

 

 

I could switch to VOR localizer and do a standard VOR approach landing manually on runway, but I wanted to finish this to make sure that I'm really in wrong spot

 

The RNAV approach would also mean that you would have to land manually, too. The only approach that autoland is certified for, at the moment, is an ILS approach.

Not sure about the equipment fit of the PMDG 737 (GPS etc etc) but my first thought would be -- did you do a navigation accuracy check before commencing the approach to ensure that the magenta line is actually in the right place?

 

In a non-GPS equipped aircraft, it is possible to experience position errors as a result of IRS/FMC position drift. Shouldn't really happen if you have been in an area with good radio updating, but it's not impossible. One should always check the position of the aircraft against a ground-based navaid to confirm the FMC position is correct: the easiest way to do this is to tune a known navaid manually on one of the NAV receivers, read the bearing and distance, enter the same VOR on the FMC FIX page, read the bearing and distance on the FMC, and compare the two. If there is a significant difference then one should treat the FMC position with caution and it may be inadvisable to carry out an RNAV procedure.

 

If the FMC position is wrong, the magenta line will be in the wrong place. However, this does sound like it could be a navdata error: if you are sure the FMC position is correct then it is worth bringing to the attention of the navdata provider.

Thank you both. I will retry the approach and see about result. I did no run this navigation check procedure and that just proves I still have lot to learn regarding NGX

Just a quick note: regular RNAV approaches have an RNP of .3, not 1.0. I agree though, sounds like a NavData issue. If for some reason a degradation of navigation precision was being simulated, you'd have gotten an indication.

Just a quick note: regular RNAV approaches have an RNP of .3, not 1.0. I agree though, sounds like a NavData issue. If for some reason a degradation of navigation precision was being simulated, you'd have gotten an indication.

 

You're right. I was confusing design versus RNP. When designing an RNAV approach, the normal template is 4.0 over the FAF and 2.0 (1.0 semi-width) over the MAP. This, in theory, gives you extra protection outside of the margin of error (RNP).

ICAO DOC 8168

 

Section 3

ARRIVAL AND NON-PRECISION APPROACH PROCEDURES

 

Everything you need to know

You're right. I was confusing design versus RNP. When designing an RNAV approach, the normal template is 4.0 over the FAF and 1.0 over the MAP. This, in theory, gives you extra protection outside of the margin of error (RNP).

FINAL APPROACH SEGMENT

 

The final approach segment for a GNSS approach will begin at a named waypoint normally located 9.3 km (5.0 NM) from the runway threshold.

ICAO DOC 8168

 

Section 3

ARRIVAL AND NON-PRECISION APPROACH PROCEDURES

 

Everything you need to know

 

FINAL APPROACH SEGMENT

 

The final approach segment for a GNSS approach will begin at a named waypoint normally located 9.3 km (5.0 NM) from the runway threshold.

 

Except what you just quoted is entirely irrelevant to the discussion here.

 

What you posted is to do with the waypoint's location, and not the segment RNP, nor the segment protected area.

Well in that case over the FAF or the final segment is < > 2.2km 1.2nm ?

Well in that case over the FAF or the final segment is < > 2.2km 1.2nm ?

 

Not sure what you're referring to here.

1.2nm boundary over the FAF

1.2nm boundary over the FAF

 

Already stated, above: 4nm.

You mentioned 4 over the FAF? Did you mean the boundary area

So what was 4nm meaning then? Because i can't find this in the ICAO DOC

So what was 4nm meaning then? Because i can't find this in the ICAO DOC

 

ICAO 8168 v 2 - Table III-1-2-2

 

Semi-widths (so, multiply by two):

FAF - 2NM

MAPt - 1NM

 

Page 3-13 shows this visually:

https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/library/media/8260-48.pdf

 

The above is an FAA link to an official document (that somehow uses Comic Sans - not even kidding), but it uses the same ICAO design concepts.