….it is still possible that PRNAV will never actually arrive…
Knock….KNock…KNOck….KNOCk….KNOCK 
You missed a few. LNAV/VNAV, PBN, LNAV+V, and RNP.
LP approaches are a WAAS equivalent to a localizer only approach. In the US, an LPV approach is usually the default approach for a runway. If obstacles deem the approach path unsuitable for vertical guidance (LPV), then a LP is considered. Because the area evaluated for obstacles for the LP approach is considerably narrower than the area for a LNAV procedure, there are some cases where an obstacle will lie in the LNAV evaluation area and not in the LP evaluation area. This can permit a lower MDA for the LP approach. If the LP approach has the same MDA as the associated LNAV approach, the LP is not published. The LP gains its smaller evaluation footprint because of the tighter navigation accuracy and integrity requirements. For an LP, the integrity requirement is 40 meters laterally. For a LNAV, it is 556 meters. Anytime a LP is published as a minimum line on a RNAV (GPS) approach, there will always be a LNAV minimum line. Both of these approaches are considered NPA (Non Precision Approaches), but the LP is a WAAS approach and may only be flown by a qualified WAAS GPS, whereas the LNAV may be flown by a non WAAS GPS. Advisory vertical guidance (+V) may be provided on either a LP or LNAV approach, although currently the Garmin WAAS GPS units don’t support it with the LP approach. Garmin is expected to provide this as an upgrade sometime in 2014.
Note: Just because one owns a WAAS GPS does not mean it supports the LP approach type. Some pilots have erroneously used unqualified WAAS GPS units to fly the LP procedure MDA. One may only use the LP minimums if and only if the GPS annunciates LP.
Advisory vertical guidance (+V) is undergoing serious scrutiny in the US. Up until now, it was strictly a manufacturer’s option and the FAA didn’t get involved. Now they are finding that many approaches are simply not suited for use of +V and are indicating so on the RNAV (GPS) approach charts by removing the VDA (Visual Descent Angle) and TCH from the affected charts and adding a note “Descent Angle NA”.
For an LP, the integrity requirement is 40 meters laterally. For a LNAV, it is 556 meters
Isn’t an autopilot mandatory for that sort of accuracy? Also, is the CDI full scale deviation much smaller for the LP approach? On a LOC or LNAV you are supposed to go around at half scale deflection; how does it work on the LP approach?
Advisory vertical guidance (+V) is undergoing serious scrutiny in the US. Up until now, it was strictly a manufacturer’s option and the FAA didn’t get involved. Now they are finding that many approaches are simply not suited for use of +V and are indicating so on the RNAV (GPS) approach charts by removing the VDA (Visual Descent Angle) and TCH from the affected charts and adding a note “Descent Angle NA”.
LNAV+V has been a big thing here in the UK, where LPV doesn’t exist (except on a tiny island called Alderney; I expect they stuck one there as a due diligence “LPV test done” box ticking exercise for the CAA and if somebody crashes off it, nobody will notice). The great thing about LNAV+V is that you had a synthetic glideslope for every GPS approach. But then stories started to spread that sometimes the procedure didn’t work (the glideslope didn’t appear). I am not sure whether anybody got to the bottom of exactly what criteria Jepp used to publish this, or not, in the database they supplied to Garmin. Not many people had much experience because not many have “W” boxes and even fewer knew how they work, and how the glideslope availability is annunciated. Is there some clear criteria? Without LPV (which isn’t really coming, for all sorts of reasons to do with funding the procedure design and then recovering the money) the LNAV+V approach is really important.
LNAV/VNAV, PBN, LNAV+V, and RNP
I know #3 (see above) but the others …… AAAAARRRRGGGHHHHH. Is LNAV/VNAV applicable to light GA (having no baro input on the glideslope guidance)?
An autopilot is not required for LP. Full scale Deflection (FSD) on the CDI is angular, +/- 2 degrees. This is similar to a localizer and identical to the lateral for an LPV. At the runway threshold, full scale is equivalent to +/- 350 feet which is also identical to a localizer at this point. Unlike a localizer whose angular displacement will vary depending on the length of the runway, it is fixed for the LP or LPV. On a localizer, a short runway uses a larger angle FSD than a longer runway since both are set to have the FSD at the threshold to be +/- 350 feet.
On all approaches, one is considered to be established on the course as long as the CDI is less than 1/2 FSD. In actuality, the primary protected area is equal to 2 X FSD. The integrity value HPL (Horizontal Protection Limit) may not exceed HAL (Horizontal Alarm Limit) of 40 meters. Integrity is not an accuracy figure. It is the 99.99999% probability that the actual position error will not be greater than the HAL without generating an alarm.
LNAV/VNAV is an RNAV approach procedure that can use GPS for the lateral guidance. It is not a WAAS approach type, but found on most approaches that have LPV and LNAV minimums. It is considered along with the LPV as an APV (AProach with Vertical), sort of in between a NPA or a PA. Like an ILS and a LPV, it has a DA. It was originally developed to support high end FMS equipped aircraft that have a Baro-VNAV system installed. When the WAAS TSO was introduced, the WAAS GPS units were authorized to fly these procedures using WAAS for vertical guidance in lieu of the Baro-VNAV guidance. Essentially a LNAV/VNAV approach is one that requires regular GPS LNAV for the lateral guidance and either a Baro-VNAV or WAAS vertical for the vertical guidance. Now a days, there are very few stand alone LNAV/VNAV procedures and most have LPV which has better accuracy and lower minimums, I tell pilots that for all practical purposes, they can ignore LNAV/VNAV.
The advisory vertical guidance +V is a creature of the manufacturer and you can’t always determine from looking at the approach chart if it will be available on a given approach. The LNAV or LP NPA uses a MDA and here in the US, that is a hard floor. A DA is a decision altitude and the pilot makes the decision while in a descent at this point. Provisions are made for obstacle clearance below the DA as the aircraft sinks thru the DA because of its momentum as the pilot initiates the missed approach and the MAP is located at the DA. In the case of a NPA, the MAP is often at the threshold or another waypoint well beyond the point that an advisory glidepath reaches the MDA. The pilot is expected to level off at or above the MDA and to continue to the MAP before initiating a climb. The approach designer does not take any obstacles into consideration if the aircraft flies below the MDA while following the advisory glidepath. The FAA has made it clear that pilots are not to follow the +V below the MDA, but the law of primacy that we do what we are trained to do comes into play. On an ILS, we have been taught that for safety to follow the GS even when below the DA and we are visual. On a NPA with +V, this can ruin your day. I can point to several approaches that following the +V below the MDA will end badly. One in particular, the +V takes you on a path that runs at least 100 feet below the ridge line on final. The FAA has started to take action to require the +V be removed from approaches that are obstacle challenged, but there are a whole lot of them they haven’t yet gotten to, maybe as many as several thousand.
In Europe the situation is different. As usual, the infrastructure is built for CAT, with GA picking up the scraps. There are relatively few LPV approaches, as IATA doesn’t really support SBAS (WAAS/EGNOS), and the majority of APV are LNAV/VNAV intended for BaroVNAV and its linear vertical guidance indication. When they were designed, they weren’t checked for obstacle clearance based in angular guidance, which means the flag/character that indicates that was not set.
Unfortunately that has an unforeseen side effect if an LNAV approach is upgraded to LNAV/VNAV — the Garmin navigators would offer LNAV+V on the LNAV approach, but when they find the LNAV/VNAV without the ‘A’, the vertical guidance is not presented.
Bookworm – what are the criteria in Europe for Garmin (or Jepp) deciding whether to offer (or not) the LNAV+V mode?
From what you say, the advisory glideslope may be suppresssed only at the large airports which have had the LNAV/VNAV approach designed.
IMHO, LNAV+V is the biggest current driver in Europe for spending money on a “W” box. For a few people, flying some regular and unusual routes on mainland N. Europe, LPV may be a significant driver.
I can tell you what WTF means, not sure about the rest (:-)))
Have a Happy New Year
Ben
Well, if pretty experienced pilots can’t even figure it out, what hope is there for the likes of me or Jo Blo? Sounds like an abbreviation soup and a safety hazard. There should be only two approaches – one with vertical guidance and one without, no? Just dump the rest and clean up in the soup.
That hasn’t been the case since IFR was invented decades ago, so it isn’t going to happen now.
