Question on RNAV approaches

[gibbons]

I'm trying to understand RNAV approaches better and was looking at a couple of charts this morning. Hattiesburg, MS (KHBG) has two RNAV (GPS) approaches for runway 13, RNAV Y and RNAV Z. I understand that the Y/Z suffix is used when more than one RNAV approach is in place for the same runway.

RNAV (GPS) Y RWY 13
RNAV (GPS) Z RWY 13

It appears that the RNAV Y approach has been in place but the procedure has changed, and that the Z approach is new.

Here are my questions:

Why design a new approach for LPV instead of using the existing Y approach and just adding an LPV minimum to it? Does the fact that the Y approach has circle to land minimums and the Z approach is straight in only have anything to do with this?

Why would the approaches have different missed approach procedures?

Is it unusual to find an airport with an LPV approach and an LNAV approach but no LNAV+VNAV approach?

 

[AirBaker]

Looks like they didn't quite trust the LPV approach yet as it has higher minimums than the LNAV. Strange.

 

[poadeleted20]

Can't say for sure without talking to the procedure designers, but it might be due to the existence of the circling mins, or more likely involving the location of the MAP and the missed approach routing. The LPV MAP is almost a mile and a half short of the runway (intersection of 3-degree glide path and 503 AGL HAT), allowing the right turn southwest to UTOVE, but the LNAV and circling MAP is at the end of the runway, where penetration of the DeSoto 1 MOA and R-4401 might result before you got turned southwest. They'll always prefer a missed approach routing that does not go back up the final approach course since that closes the approach until you leave the MA holding fix even if the weather is good.

 

[Everskyward]

I understand that the Y/Z suffix is used when more than one RNAV approach is in place for the same runway.

I discovered recently that the Y/Z suffix does not just apply to RNAV approaches. Check out these two:

http://naco.faa.gov/d-tpp/0803/00504ILY19.PDF
http://naco.faa.gov/d-tpp/0803/00504ILZ19.PDF

The Z approach has lower minimums because it has a non-standard (steeper) missed approach climb gradient. For years Jackson Hole has only had one ILS approach which I have shot quite a number of times so I was pretty surprised a couple months ago to discover two.

 

[gismo]

Yep, AFaIK the XYZ naming thing was first implemented to allow different missed approach procedures and was shanghi'd for RNAV approaches.

BTW, wouldn't it make a lot more sense to put the GS angle at something like 4 degrees on the LPV if that would allow better mins?

I discovered recently that the Y/Z suffix does not just apply to RNAV approaches. Check out these two:

http://naco.faa.gov/d-tpp/0803/00504ILY19.PDF
http://naco.faa.gov/d-tpp/0803/00504ILZ19.PDF

The Z approach has lower minimums because it has a non-standard (steeper) missed approach climb gradient. For years Jackson Hole has only had one ILS approach which I have shot quite a number of times so I was pretty surprised a couple months ago to discover two.

 

[midlifeflyer]

I discovered recently that the Y/Z suffix does not just apply to RNAV approaches.

Yep. All it really means is that there are multiple straight in approaches that use the same primary navaid but have differences in the procedure.

 

[Brad Z]

I'm trying to understand RNAV approaches better and was looking at a couple of charts this morning. Hattiesburg, MS (KHBG) has two RNAV (GPS) approaches for runway 13, RNAV Y and RNAV Z. I understand that the Y/Z suffix is used when more than one RNAV approach is in place for the same runway.

RNAV (GPS) Y RWY 13
RNAV (GPS) Z RWY 13

It appears that the RNAV Y approach has been in place but the procedure has changed, and that the Z approach is new.

Here are my questions:

Why design a new approach for LPV instead of using the existing Y approach and just adding an LPV minimum to it? Does the fact that the Y approach has circle to land minimums and the Z approach is straight in only have anything to do with this?

Why would the approaches have different missed approach procedures?

Is it unusual to find an airport with an LPV approach and an LNAV approach but no LNAV+VNAV approach?

There's an easy answer to this...it's the missed approach. TERPS allows an LNAV to have an immediate turn on the missed. LPVs are required to have a straight segment of about 2 miles before turning. For the LPV, the proximity of restricted airspace meant pushing the MAP closer to the FAF to keep planes out of R4401, raising the DA and the viz requirement.

They seperated these two approaches onto seperate pages so that the LNAV would not be penalized by having to follow the same missed approach procedure.

There are many airports with LPV but no LNAV/VNAV. Generally they are non-aircarrier airports.

 

[TangoWhiskey]

There's an easy answer to this...it's the missed approach. TERPS allows an LNAV to have an immediate turn on the missed. LPVs are required to have a straight segment of about 2 miles before turning. For the LPV, the proximity of restricted airspace meant pushing the MAP closer to the FAF to keep planes out of R4401, raising the DA and the viz requirement.

They seperated these two approaches onto seperate pages so that the LNAV would not be penalized by having to follow the same missed approach procedure.

There are many airports with LPV but no LNAV/VNAV. Generally they are non-aircarrier airports.

Good answer, Brad. My question is "why" for the missed to be "required to have a straight segment of about 2 miles before turning". Any idea? What makes an LPV require that, but not a LNAV+VNAV or other approach?

 

[Brad Z]

Good answer, Brad. My question is "why" for the missed to be "required to have a straight segment of about 2 miles before turning". Any idea? What makes an LPV require that, but not a LNAV+VNAV or other approach?

Terps for LPV is similar to ILS. The idea is that at the missed you are 200 (or lower, it is a DA after all) so years of safety analysis and good common sense says that you shouldn't immediately begin a turn until you are clear of airport obstables and maintaining a positive rate of climb. At the DA, you are attempting to make a descending airplane climb. At the m issed approach point of an LNAV, which is usually at the threshold, it is assumed that you are leveled out at that point so you are just initiating a climb. Also, you are generally a few hunder feet higher AGL than an airplane at the DA on an ILS. Thus an LNAV (or LOC, VOR, NDB, etc.) can turn right away.

An LNAV/VNAV, being a vertically guided approach, generally requires the same straight segment, although the TERPS obstacle evaluation areas are larger than an ILS or LPV due to the lower accuracy and integrity of standalone GPS.