Originally Posted by
Vendeeglobes
Interested in thoughts on conducting another (RNP, RNP AR) approach to the same/different runway after an UNABLE RNP message forced a missed approach. The issue could be an onboard equipment issue or a wider GPS coverage issue. Of course if subsequent aircraft were reporting the same thing then this would become evident quickly.
Any thoughts on considerations for flying another approach in this situ?
Thanks in advance.
My thoughts would be contingent on the circumstances under which the RNP approach is being flown. Do you have a backup of a non-RNP approach at destination or alternate(s) which is reasonably assured or are you in a bit of pickle as this RNP
was your backup?
In the first instance, SOP would undoubtedly be to discontinue and fly the alternative approach if the navigation issue remained present. In the second case, if you had a valid FMC position up to the moment the GPS dropped out, many airliners will revert to inertial unless you have enabled VOR/DME. Given that a modern IRS has a very low drift rate, in the time it takes to complete the approach you won’t have gone very far off track, if at all. You could go away and think about it but the more time you spend, the less accurate your forthcoming approach is going to be. This is, of course, if you have no other safer options left.
If you can get radio updating to work, you might even be able to get close to the RNP for the approach in question.