Last post I looked at the situation where a non-standard missed approach gradient could cause two sets of minimums to be needed for the same approach, thus resulting in two versions of the approach, a “Y” and a “Z”. I mentioned there are three reasons two approaches of the same type could exist to the same runway, let’s look at the other two now- both involve RNAV approaches.
The second case is where two RNAV (GPS) approaches exist to the same runway. Typical of this example is Burlington, VT, with an RNAV (GPS) Y and Z approach to runway 15. The Z approach has published WAAS (or FMS) minimums of LNAV/VNAV to 660’ and LNAV-only mins of 940’. The Y approach only has LNAV mins published, but to 760’, lower than the LNAV mins for the Z approach. A quick view of the profile view shows why; the Y approach has a stepdown waypoint, JUNEL, abeam a 639’ high tower. So an aircraft not equipped with WAAS/ FMS would want to execute the Y approach, one capable of LNAV/VNAV approaches would want the Z approach.
BTV RNAV (GPS) Y RWY 15
BTV RNAV (GPS) Z RWY 15
Sometimes the approach course is completely different, as is the case with the RNAV (GPS) Y or Z --- at Half Moon Bay, CA. The Y approach has LNAV only minimums, and is flown mostly over the water, with two doglegs, or course changes. The Z approach takes advantage of LPV’s tighter guidance to get down twice as low as the Y, and is aligned with the runway from a 14 mile final.
HAF RNAV (GPS) Y RWY 30
HAF RNAV (GPS) Z RWY 30
The other case where multiple RNAV approaches are published to the same runway involves a new type of approach- the required navigation performance, or RNP approach. RNP approaches are the approach of the future, and represent a shift from navaid- based approaches to performance-based approaches. In other words, as opposed to a VOR approach, which can only be flown by reference to the designated VOR, an RNP approach only requires that an aircraft be able to maintain a specified level of accuracy and integrity. For all practical purposes, RNP approaches now require GPS, but the option in the future to rely on other sensors exists.
An RNP approach is an RNAV approach in that it depends on the ability to navigate to any arbitrary point in space. Yet its equipment requirements go beyond an IFR GPS unit, so it is classified RNAV (RNP), as opposed to RNAV (GPS). A way is needed to differentiate between these multiple RNAV approaches, so again X, Y, and Z come into play. An example is San Francisco, with an RNAV (GPS) Z RWY 28R, and an RNAV (RNP) Y RWY 28R approach.
We’ll look more at RNP approaches in the future, but for now there aren’t many GA aircraft capable of flying them. Only the flagship products from the largest business jet manufactures have received approval, and individual operators and pilots need authorization to fly even an approved aircraft on these approaches.
