RNAV tolerances

28.10.2004

Submission:

What are the lateral limits on RNAV approaches from the FAF to the MAP? Is it .3 or .5 or what? The reason I ask is that when we fly RNAV approaches in our A320s, some with and without GPS, we are instructed to go to the PROG page in the FMS and type .5 in the "required" accuracy column. Therefore, if the EPE exceeds .37 which is the default value, we won't get a "Nav Accuracy Downgrade" message necessitating a missed approach. However, when I look at Jepp material in an advertisement for their Instrument Ground School on CD, I see the picture showing full CDI deflection represents .3 nm during an RNAV approach from within 2nm of the FAF. Normally, one would execute a missed approach whenever he had full scale needle deflection because he would not know how far he was laterally from the approach course. By resetting the FMS to .5 from the default value of .37, we are resetting the limits, i. e. where full scale CDI deflection if we had CDIs. Is the protected airspace .5 or .37 or .30? I need to know for both non precision and precision like (baro VNAV).

Stephen G. Erickson

Answer or Commentary:

(I.W.

For Basic GNSS approaches the obstacle protection areas start at the FAWP with +- 1 nm for the primary area and +- 1 nm for the secondary areas. From this point the areas linearly narrow down to +- 0.5 nm for all areas. Full obstacle protection is provided in the primary area while limited protection is provided in the secondary areas. So one should remain within the primary obstacle protection area for maximum safety.

Fairly simple one would think. The answer to the question is 0.5 nm but lets move on.

When one starts dealing with RNAV approach procedures based on VOR/DME or DME/DME a slightly more complex construction procedure comes into place. One now must calculate the along track tolerance (ATT), the cross track tolerance (XTT) and the semi area width. As you can see in the following diagram the important one is the semi area width.

This semi area width is divided into 4 equal segments to determine the primary and secondary obstacle protection areas. The value of ATT, XTT and the semi area width depends (basically) on the distance from the facilities that your FMS system is using. The semi area width can vary between 1.6 nm to 4.06 nm depending on altitude and distance. When dealing with VOR/DME RNAV procedures ICAO states that the minimum ATT of either the FAF or the MAPt is +- 2 nm (3.7 km), which is the same value as a GNSS procedure. A lot of these problems should be taken care of automatically by the FMS system. In the future you will see important notes on approach plates that will ensure that the FMS system is capable of remaining within the design parameters that were used by the procedure specialist when he design the approach. For example: -

On an RNAV DME/DME approach:-

"Only available to aircraft capable of automatic reversion to updated IRS navigation with positive course guidance"

On a Baro VNAV approach

"Only available to aircraft equipped with RNAV systems capable of RNP 0.3 nm approach operations"

So the definitive answer to your question is: - all of the above depending on the procedure and the design parameters used.

However as I fly a lot keep it set on 0.3 )