RNAV Tolerances 2

28.10.2004

Submission:

In the Boeing Flight Crew Training Manual for the B737-800 we have a table listing the tolerances for different approaches. They seem to distinguish between RNAV (0.5) and GPS (0.3). The database in our FMC defaults automatically to 0.5 when we do a RNAV (GNSS) approach. We do not have any GPS approaches but these RNAV approaches are based on the availability of GPS information. I also understand your argument that 0.3 is safer than 0.5, but what should the default RNP be for a RNAV (GPS / GNSS) approach?

Jaco de Beer, South Africa

Answer or Commentary:

(I.W.

In ICAO Doc. 8168 Pans-Ops chapter 33 (AREA NAVIGATION (RNAV) APPROACH PROCEDURES FOR BASIC GNSS RECEIVERS) it states:-

33.2.3.1 The values of the space elements (including control element) and the airborne system tolerances (including system computation tolerance) are taken into account within the integrity monitoring alarm limits for basic GNSS systems. They are:

En route: 3.7 km (2.0 NM)

Terminal: 1.9 km (1.0 NM)

Approach: 0.6 km (0.3 NM)

This seems to indicate that 0.3nm is the value for a GNSS approach.)