RNAV approaches

[Matt2218 4]

On 2/17/2019 at 12:11 PM, berts said:

Simon has already mentioned the Final Approach Fix (FAF) in his earlier post. This position is particularly important on a non-ILS (i.e. non-precision) approach, because it is often the last opportunity a pilot has of ensuring their altimeter subscale reading is set correctly.  Assuming it is set correctly then the aircraft's altitude should agree with that shown on the published procedure.  However, if the altimeter reading disagrees by a significant amount then it could be potentially very dangerous to continue the approach, because you could be starting the final descent at a much lower (or higher) altitude than you think. 

Sorry if I'm a little dense, but I know nothing about RNAV approaches yet. Are you talking about comparing the Radar Altimeter to the baro Altimeter here? How does one "ensur[e] their altimeter subscale reading is set correctly."?

[Copper. 78]

Matt, by comparing your altimeter reading to the charted altitude on say, a Jeppesen chart.

The faf (final approach fix) is shown as a maltese cross. At the faf, you can compare pressure altitude or radio altitude to the chart as usually both are given on charts.

Basically making sure you’re at the right place, at the right height, at the right speed by comparing your lateral position, height and speed to the requirements on the chart.

[richjb2 195]

On ‎2‎/‎17‎/‎2019 at 12:25 PM, downscc said:

Excellent question, and one that I had to have a look at the KELM RNAV 24 plate and do some searching.  An article that explains this well is https://www.boldmethod.com/learn-to-fly/navigation/when-lnav-minimums-are-lower-than-vnav-which-should-you-use/

The TERPS experts can explain this better perhaps, but it boils down to the differences between determining a DH vs MDA and in this case how obstacles will influence the outcome.  As mentioned by Bertie and discussed in the article the choice of which one to choose is part of good piloting, and one must consider the weather, equipment available and ones own piloting experience.  The LNAV MDA will have more pilot workload and if the weather is close to minimums I probably would not select the non precision LNAV over the LNAV/VNAV to get that extra 130 foot of ceiling if there were other options, unless I was familiar with that approach and the area.

2

Hi Dan,

The question of why LNAV minima are lower than LNAV/VNAV minima comes up quite often, so much so that FAA decided they need to explain it in the AIM:

 

3. Vertical guidance (LNAV/VNAV). A line is

drawn horizontal at obstacle height until reaching the

obstacle clearance surface (OCS). At the OCS, a

vertical line is drawn until reaching the glide path.

This is the DA for the approach. This method places

the offending obstacle in front of the LNAV/VNAV

DA so it can be seen and avoided. In some situations,

this may result in the LNAV/VNAV DA being higher

                           than the LNAV and/or Circling MDA

The AIM has nice picture in it describing the difference, but you are correct.  LNAV/VNAV uses a sloping obstacle clearance surface (OCS), while LNAV uses a level, flat surface OCS.  As described above, this can result in LNAV/VNAV DA and visibility minima being higher than just LNAV only minima.  Nothing prevents the pilot from using VNAV to continue the descent to the LNAV MDA; however, the pilot cannot descend below LNAV MDA without the runway environment in sight. This requires the pilot to sight the runway environment prior to reaching the LNAV MDA to either initiate the level off at MDA or a missed approach some height above MDA to avoid going below, a practice the airlines call "derived decision altitude".  In Europe, the application of a heigh loss additive is mandatory when CDFA is used to an MDA. Leveling at MDA will cause you to destabilize an approach, not a good thing in a jet. 

The decision to use the lower LNAV MDA or the higher LNAV/VNAV DA depends a lot on the type of airplane involved and the runway.  For jets, most pilots and operators would elect to use the LNAV/VNAV DA because of the greater risk of destabilizing the approach.  In a light single engine aircraft landing a longer runway, it might be beneficial to continue down the extra 100 feet, level at the MDA and perhaps sight the runway. 

A few years back, we looked at raising LNAV only MDA to that of the LNAV/VNAV DA in these cases, but decided it was not in everyone's best interest. We did; however, decide to ensure that Circling MDA was always no lower than the straight-in LNAV only MDA, which can happen in very rare occasions when the controlling obstacle is just outside of the circling protected airspace but penetrates the LNAV only OCS.  There's an airport in Texas where this can be found.  It creates a strange case where the circling MDA for one approach is higher than the other approach and the LNAV MDA for that same approach, which could lead a pilot to try to circle from a different approach to achieve a lower landing minima.  Perfectly legal, but we're trusting pilot common sense not to do it. 

One of these days, we need to have lunch at Waterstreet!

Rich Boll

 

[richjb2 195]

On ‎2‎/‎15‎/‎2019 at 8:07 AM, Richard McDonald Woods said:

I am trying to get to grips with using RNAV approaches, so will appreciate any corrections/feedback to my current understanding:

1. Selecting an RNAV approach provides only GPS quality (equivalent to category 1 ILS) LNAV.
2. As opposed to ILS VNAV/LNAV, current PMDG aircraft have no knowledge of WAAS signal receivers. The PMDG  GPS system is 'perfect' enough anyway!
3. RNAV approaches currently do not support vertical guidance, so no LPV (localizer  performance with vertical guidance) approaches are supported.
4. Vertical guidance whilst flying an RNAV approach is given only by setting QNH for the airport. So autoland is not supported.
5. The pilot must use the decision altitiude to determine the aircraft's descent to the runway. So accurate setting of local QNH for the approach is essential.

This leaves me with one main question - what criteria should I use to choose between an ILS and an RNAV approach to an airport? 

 

3

Richard, 

Please allow me to take a hack at your questions.

1.  An RNAV approach (RNP APCH in ICAO parlance - and soon to be FAA PBN parlance - and not to be confused with RNP AR approaches) is based on a Global Navigation Satelite System (GNSS). In the US, only GPS is allowed for approaches, although, that's about to change.  FAA will soon begin accepting other systems, like Europe's Gallielo.  Category 1 approaches cover everything from ILS, to RNP APCH, to VOR and NDB approaches. CAT II and CAT III approaches cover only ILS and GLS down below CAT I landing minima.

An approved GNSS (in the US - GPS), provides the position solution for the RNAV system when flying an RNP APCH and provides accuracy for basic containment within the initial, intermediate, final, and missed approach segments. 

2. ILS is technically not LNAV/VNAV.  It's localizer and glideslope, two ground-based navigation systems providing lateral and vertical guidance to the runway threshold. The localizer is a VHF transmitter, while the glideslope is UHF, but all that is transparent to the pilot as the ILS receiver in the airplane handles it. ILS is more accurate than any un-augmented RNAV system.  WAAS provides a ground and satellite augmentation system that removes errors inherent with GPS by providing correction signals to the aircraft.  WAAS approach capable RNAV systems can use the LPV line of minima, which are based on the lateral and vertical obstacle clearance surfaces and containment areas as an ILS approach.  Provided the airport lighting system supports it, LPV minima can be as low as 200' and 1800 RVR. 

WAAS is a four letter word for US airlines.  Airbus offers an option for their A350 line,  Boeing is offing an option for their B737MAX line. The airlines are looking to have three types of approaches: ILS, GLS (GBAS or ground-based augmentation where the satellite corrections signals are sent directly to the airplane over VHF ground stations), and RNP AR.  They grudgingly accept RNAV approaches to LNAV or LNAV/VNAV minima until something better comes along. 

3. Not correct.  RNAV approaches do support vertical guidance, either in the form of Baro-VNAV or a WAAS generated glidepath. These approaches are classified as "APV" or Approaches with Vertical Guidance.  These do not meet the ICAO definition of "precision approaches", so they had to invent a new term. The LPV line of minima is based on using a WAAS generated glidepath that is not subject to barometric or temperature altimetry errors since it is electronicly generated by the WAAS system and provides equivalent to an ILS glideslope without inherent distortions with the later. That allows use to put RNAV approches with LPV minima where we might not be able to put an ILS approach. 

4.  Autoland is a function only of an ILS approach.  No RNAV approach supports autoland.  GLS will in the future, assuming we get past the GPS jamming issue.  

5.  accurate QNH is essential.  For an ILS, it does not affect the electronic glideslope.  It does however affect the height above the touchdown zone where you will reach DA.  For Baro-VNAV on an RNAV approache to LNAV/VNAV minima, a wrong QNH can fly you into the ground. A nasty simulator instructor would sometimes give a transposed altimeter setting of 29.96 when he actually set the airport's QNH to 29.69.  The way low RA indication on final is the only clue that something was wrong and something very bad was about to happen.  BTW, PMDG's VNAV Will simulate this very nicely. 🙂

Last Queston is easy.  ATC will usually assign the approach to use.  Don't upset their apple cart until there is a a real reason to do so.  For example, at Teterboro NJ (KTEB), the RNAV 19 approach has lower LPV minima than the ILS 19 approaches (quirk because these two approaches were designed at two different times to differing sets of criteria).  NY only uses the ILS 19 because the missed approach supports overhead KEWR traffic.  However, if the weather drops low enough, then you may need to request the RNAV 19.  ATC will provide alternate missed approach instructions. 

Hope this helps,

Rich Boll

 

[Richard McDonald Woods 168]

Hi Rich,

Most helpful. Many thanks.😊

[Copper. 78]

On 2/25/2019 at 12:03 AM, richjb2 said:

Nothing prevents the pilot from using VNAV to continue the descent to the LNAV MDA

 Hello Rich

Hate to nit pick at what was a good post, but -

are you sure this statement is right? It sounds a bit off because I think LNAV only procedures are predicated on another vertical mode being used (ie. not vnav).

The lnav/vnav vertical minimum being more restrictive (higher) because of the possibility of a miss-set altimeter in VNAV could cause collision with obstacle/s in the approach splay.

All other vertical modes are not susceptible to a miss-set qnh, so the minima can be lowered.

At least, the above is my understanding between lnav/vnav and lnav only minima in the vertical control context during the respective procedures.

[skelsey 765]

1 hour ago, Copper. said:

are you sure this statement is right? It sounds a bit off because I think LNAV only procedures are predicated on another vertical mode being used (ie. not vnav).

I'm not Rich but yes, it is correct. 

The article Dan linked explains it well - the difference is that the LNAV is designed as a '2D' approach to an MDA in the same way as a VOR or NDB, whilst the LNAV/VNAV is designed as a '3D' approach to a DA.

As you will be using your barometric altimeter to descend along the published vertical profile then either is equally at risk from a mis-set QNH, regardless of whether you V/S in to the ground or let the autopilot do it for you in VNAV :).

In practice, however, as I mentioned, it may not be possible to use VNAV because you're probably using the LNAV minima because the temperature is below that required for the VNAV and an airline probably won't let you use VNAV after the FAF if you've messed with the altitudes on the FMC.

[Copper. 78]

Aah yes, thanks Simon. That was an embarrassing mistake.

thanks for the corrections

[richjb2 195]

On ‎2‎/‎25‎/‎2019 at 8:34 PM, Copper. said:

 Hello Rich

Hate to nit pick at what was a good post, but -

are you sure this statement is right? It sounds a bit off because I think LNAV only procedures are predicated on another vertical mode being used (ie. not vnav).

The lnav/vnav vertical minimum being more restrictive (higher) because of the possibility of a miss-set altimeter in VNAV could cause collision with obstacle/s in the approach splay.

All other vertical modes are not susceptible to a miss-set qnh, so the minima can be lowered.

At least, the above is my understanding between lnav/vnav and lnav only minima in the vertical control context during the respective procedures.

11

It is perfectly acceptable to use VNAV down to LNAV only MDA.  

By US TERPS, any MDA is based on a level obstacle clearance surface. That surface provides between 250 to 300 of required obstacle clearance (ROC) above the controlling obstacle (determined by the navigation source, e.g., RNAV, LOC, VOR, NDB, etc.). As long as you don't descend below the published MDA, you are guaranteed the ROC for the type of approach.  That's why when using Continous Descent Final Approach (CDFA) techniques to an "MDA" using any vertical mode (typically VNAV based on Baro-VNAV, VS, etc.), the pilot must add an additive to the MDA so that they do not penetrate through MDA while executing the missed approach if the runway environment is not in sight.  How the pilots descends to MDA doesn't matter. 

RNAV approaches in the US that only have LNAV minima usually have a VNAV angle provided on the IAP chart as well. In this U.S. this angle is provided on the source document by the FAA, the 8260-3 Form.  The database providers then code this angle with the procedure to support the use of VNAV (Baro VNAV or WAAS generated, doesn't matter).  This VNAV angle is advisory only!!!  It provides NO OBSTACLE PROTECTION BELOW MDA.  Recently, we have had to beat that message into the real pilots once again! 

The LNAV/VNAV line of minima is based on a sloping OCS.  That OCS is based on a slope of 102/GPA.  For a typical 3 degree GPA, that is a 34:1 surface,   The 3 degree path itself is a 20:1 surface. The difference between the two surfaces is the ROC for the LNAV/VNAV line of minima.   An obstacle may penetrate the LNAV/VNAV OCS, and if so, the procedure designer draws a horizontal line back from the top of the penetration to the OCS.  He/She then draws a vertical line from that intersection upwards until it reaches the GPA.  That position on the GPA establishes the DA for the LNAV/VNAV line of minima.  The visibility associated with the DA that would allow the pilot to also see the runway environment from the DA, will also allow the pilot to see the obstacle.  

That same obstacle may not necessarily penetrate the level OCS used by the LNAV line of minima, or if it penetrates, it may allow for a lower LNAV MDA than the LNAV/VNAV DA.  Obstacle protection is still assured if the pilot descends on the VNAV path but does not go below the LNAV MDA.

Let's see if this works.  Here's an illustration from US AIM of what I just described:

This situation doesn't happen often but happens enough that the FAA decided they needed to explain it to pilots. These quirks in TERPS can sometimes lead to strange things. A while back, there was a quirk in the LPV criteria on the initial missed approach lateral obstacle clearance area and OCS that would occasionally result in LNAV/VNAV minima being lower than LPV minima.  That made no sense at all since LPV is inherently more accurate. That glitch has been fixed.

We are currently working on a fix to the RNP AR criteria that results in the RNP AR DA being higher than the RNAV (GPS) LNAV/VNAV minima even though RNP AR is a higher performance standard than just plain old RNAV.  

Rich