Inside the FMS

This article, Part 1 in a three-part series, provides a deeper understanding of Honeywell FMS logic by looking at approach minima. By understanding the logic behind how the FMS operates, pilots will get a better idea of how FMS pages are formatted and populated, as well as see how the aircraft executes instructions and complies with settings. This series begins with approach minima since there has been some recent confusion regarding the available minima selectable on the FMS ARRIVAL page versus what is charted on an approach plate.

Note that examples in this article use FMS pages and line select keys (LSKs) for the Honeywell NZ FMS, while this logical design is not restricted to NZ FMS.

For RNAV (GNSS) approaches or RNP approaches of the non-AR variety, Honeywell FMS selects the “LPV” RNAV MIN type by default. The reason for this is that the FMS is using the Final Approach Segment (FAS) data block when the LPV minimum type is selected.

However, if the desired minimum type is either LNAV or LNAV/VNAV, the pilot must manually change the minimums type selection by using the RNAV MIN prompt shown in Figure 1. If the approach has published LPV minimums, pilots will find the LNAV/VNAV option to be selectable regardless of whether the approach plate actually contains published LNAV/VNAV minimums. This option is simply a toggle for the FMS to do the approach as an LPV, using angular deviation rather than performing it as a traditional GPS approach with linear deviation.

When LNAV/VNAV minimums are not published, selecting the “LNAV/VNAV” RNAV MIN type can be useful in providing advisory lateral and vertical guidance to the runway during a visual approach. Operators should be aware, however, that no obstacle protection is provided in this scenario; the LNAV/VNAV guidance is not authorized as primary navigation because LNAV/VNAV minimums are not published on the approach chart.

An example of an approach that only provides LPV minimums is the RNAV (GNSS) Y RWY 05 at LIRQ (Figure 2).

When the pilot views the ARRIVAL page for this approach (Figure 3), the RNAV MIN prompt (LSK 2R) shows LPV as the default selection.

Pushing LSK 2R displays the APPROACH MINIMA TYPE page (Figure 4). On this page, there is a selectable LNAV/VNAV option (LSK 3L) despite the fact that LNAV/VNAV minimums are not published on the approach plate.

To summarize, for RNAV (GNSS) approaches or RNP approaches of the non-AR variety, the Honeywell FMS defaults to the LPV approach minima depicted on the approach chart. While other minima may be available for selection on the FMS, pilots must be aware that they are not authorized to fly to any approach minima not depicted on the chart.