RNAV tolerances 1

28.10.2004

Submission:

What does it mean, explicitly, for Baro VNAV approaches to meet RNP 0.3 requirements. Does that imply that the accuracy of the FMS is within .3nm. In that case, our default value on our Airbus A320 is .37. This would not qualify as it exceeds my interpretation of RNP .3 by .07. WE are flying baro VNAV approaches using FMS with and WITHOUT GPS. Besides that, as previously stated, we go to progress page and type in .5 in the required navigation accuracy. From your previous post, it appears that would be okay for an LNAV approach, but I'm not sure for a Baro VNAV approach. Could you please explain.

Stephen G. Erickson

Answer or Commentary:

(I.W.

If the approach has an RNP of 0.3 then yes the FMS system has to be accurate to 0.3 nm. This does seem to suggest that your default value of 0.37 is too high and you may have to manually set it to 0.3.

Baro VNAV criteria has only just come out in the latest Pans-Ops amendment and I don't think there has been time enough to develop any actual Baro VNAV approaches. What I think you may be doing is flying an LNAV designed approach using the FMS system to give you vertical guidance. This is similar to flying a localiser only approach and still using the glide path signal for vertical guidance. Such an approach is fine as your lateral tolerance and guidance will be within limits. The only thing to careful about is not to descend below the minimum non-precision altitudes for the segments.)