RNP holdings

28.10.2004

Answer or Commentary:

I have an observation that might be of interest to some of the specialists out there. Recently I have been doing a lot of RNAV procedure design work. A small part of this work was the design of RNP holding procedures. As usual, prior to commencing work I went over the appropriate section in Pans-Ops (Part IV chapter 5). This section gave me the formulas for the calculation of d1, d2, and d4 that are used in the construction of the obstacle protection areas and on to this area was to be added the buffer area (paragraph 3.4.2).

The areas were easy to construct compared to conventional holdings but did not look correct to me, especially the primary area. It struck me as strange that no tolerance for the inbound track to the waypoint was allowed for, even though the holding can have an RNP of 5 nm. Meaning that an aircraft could be within 5 nm of the nominal inbound track. Logically I thought that the area should be designed as it is stated in Pans-Ops and then the primary area should be increased by at least the RNP value. I had the feeling that this offset (RNP) of the designed area has been inadvertently left out of this section of the Pans-Ops criteria. Then I also looked at chapter 2 Area Navigation (RNAV) holding procedures based on VOR/DME, which leads one to Attachment C to part III. Tucked away on the very last page of this attachment is the information on construction of RNP holdings. In this section however I did not see a reference stating that the tolerance of the waypoint should be taken into account.

I think that at least a reference to attachment C should be made in Pans-Ops Chapter 3 RNP holding procedures.

I. Whitworth (Procedures specialist)