GPS Holding Templates - 2

25.10.2004

Submission:

First, I agree with your point regarding the pilot reaction and bank establishment tolerances applied at the end of the outbound leg in the construction of the RNAV template. That would be consistent with the application of 11s tolerance to the DME distance tolerance for holding patterns with a DME limit on the outbound leg (PANS-OPS II, Part IV, 1.3.9.3). There is no need for any additional tolerances beside that, since the whole template is translated around ATT and XTT of the holding fix, together with point "a".

More importantly, though, I do not believe that the current RNAV holding template is appropriate for GPS holdings. I have spent some time studying this very issue last year, while preparing the new PANS-OPS GPS (GNSS) procedure design criteria for inclusion in our Quality Manual. In Part IV, Chapter 2 of the Vol II a few assumptions were made regarding the One way-point RNAV holding. Among them that:

a) RNAV system is able to compensate the effect of a wind coming from the outside of the outbound turn by a reduction of the bank angle.

c) RNAV system is able to correct the drift on straight segments

d) No heading tolerance is taken into account on the straight segments

All the above factors are taken into account in the construction of the template. However, none of these is justified in the case of so-called "basic GNSS receiver". (In real life - a receiver compliant with TSO-C129)

1. The basic receiver cannot compensate the effect of wind during the turn, which means the outbound turn may well be completed on the inside of the nominal outbound track, not only on the outside, as assumed in the template construction.

2. The outbound leg of the holding flown with the basic receiver is DR. While pilot can obtain more information about the drift angle than with the conventional navaids, he/she still has no indication of the aircraft position relative to the nominal outbound track.

In the light of the above, we in New Zealand adopted the conventional template construction methodology, as described in Appendix C, 3.3 for GPS holdings. Applicable GPS way-point tolerances are applied to the holding fix. The resulting basic holding pattern area is similar in shape to the conventional "overhead" holding pattern, with timed outbound leg.

Since the GPS receiver continuously provides the distance to the holding point during the holding manoeuvre, it can be used for defining the limit on the outbound leg. It is important to note that this distance is not a distance from the end of the outbound turn, as it is defined in the construction of the RNAV holding pattern. I have used the methodology described in Appendix C, paragraph 3.4 to reduce the holding area, constructed as described above, by limiting distance on the outbound leg. The holding way-point tolerance (SQRT(ATT^2+XTT^2)) has to be applied to the limiting distance. Unfortunately, this method of holding area reduction has not been accepted by NZ CAA. No valid reason, apart from the tendency to be over conservative with everything related to GPS procedure design, was given. As a consequence, only timed holding patterns are being used with the GPS procedures in NZ for now.

Obrad Puskarica

Senior Navigation Procedures Specialist - Airways Corporation of New Zealand

I have had second thoughts and would like to correct my statement. It is true that the 11s tolerance has to be applied at the end of the outbound leg. As a matter of fact, that's exactly what is done when you use the inverted conventional template to draw the protection of the inbound turn from points "i" and "i1", since the applicable tolerances (points "b" and "c") are built-in. So, the construction methodology of the RNAV template is consistent.

This still does not mean RNAV template is appropriate for the construction of the holding area for "basic GNSS receivers", as I explained in my previous message.

Obrad Puskarica

Senior Navigation Procedures Specialist - Airways Corporation of New Zealand

Answer or Commentary:

(I.W I agree that the 11s tolerance is included in the outbound leg if the inverted turning spiral from point "a" is used. However I think it should be made clearer that this is the case and not that a conventional wind spiral be used.)