UNABLE RNP
 

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.

So, you've abandoned an approach due to a system degradation, and you want to fly another RNP approach despite the system degradation? No...you need to look for non RNP based approaches (which may be a visual approach).

If unable RNP its visual or ILS/VOR etc

If unable LPV its LNAV or LNAV/VNAV

Sure, but let’s say there is no other approach available and the weather is at the applicable minima. What are now the possible implications? What are the risks and benefits of another approach versus diverting?

I guess that’s what TS is aiming for in this discussion. Let’s say the RNP issue clears during the missed approach. Why not try a second approach? Obviously the RMP system worked as intended.

It’s an interesting question. I’m not sure I would like to go down into the INN valley with a NAV system that just, for whatever reason, lost its confidence in tracking. On the other hand, the system worked as intended. It warned you of its (transient?) failure.

If the weather at the airfield in question doesn't prevent another approach using a ILS, VOR or Visual procedure, that is likeky the simplest way to go. However, I do get a sense that you might also be querying what to do if options are pressured due to things like diminishing time (fuel!), changing weather, and/or lack of suitable alternative approach procedures. You then have to sift what your aircraft and company operations manuals expect you to do in the circumstances, along with any differing (more restrictive) regulations in the country you are attempting the approach and landing. It could have potential to quickly become administratively untidy!

Sure, ask on frequency if anyone else has reported issues due to UNABLE RNP. As you say, it might already be obvious if others are talking about it. But was it a relatively short aberration, that has now corrected itself, and can you now trust the equipment to perform as intended? Much will depend on the known rules, plus the external influences that may have painted you into a corner, and your obligation to get crew, passsengers and aircraft safely on the ground. The joys of Command!

If you want to further a hypothetical and create a scenario where the RNP resolved itself, sure, you can fly another RNP/RNP-AR. But if following the OP's scenario where it had not resolved, why/how could you fly a RNP/RNP-AR in IMC below MSA with 'UNABLE RNP'?

That’s a non-starter andmiz, you would not normally start an RNP approach when the equipment is not available or working!

Related discussion here - https://www.pprune.org/tech-log/6581...abilities.html

Depends.

Have you flown that approach before in the same aircraft with no problems?
.Did the warning clear by either by itself of after you recycled it or anointed it with Holy Water?
Check RAIM etc.
What are your other options?
Do you have lots of fuel?
Is your alternate Wx good. Is it close or hours away?
If needs be and the equipment is now working you could try a second approach unless SOP says no. Momentary glitches happen
However is it really necessary.
Be prepared to abort, recover to a sensible altitude and go somewhere else.
Avoid target fixation….do you really want or need to visit the thriving community of Little Moose Poop?

Wouldn't one of the the first steps in the decision process be to compare ANP with RNP?

An analogy: would You accept an ILS without a working glideslope needle or a visual in IMC? Flying an approach based on data Your systems cannot receive, digest and display appears a brave choice in most circumstances.

If other approaches to the same runway or airport based on different nav equipment up to and including a visual are available, pick them. If not, there may be ways to get Your onboard equipment working again. And if all this fails, consider a diversion.

I don't mean to pry into your personal business, but I can't help but think some more information and something resembling context might be helpful here. You know, minor things like maybe aircraft type, weather conditions, or even maybe you could share possible reasons that message can be triggered in your aircraft. Little things like that.

I might, if:

1. The approach is published with LOC only minimums.

and

2. I am authorized to fly such an approach.

This is why I think it would be helpful if the OP provided some context.

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.

Some approaches have different minima lines for different RNP (at least that's allowed according to AC 90-101A CHG 1).

https://cimg7.ibsrv.net/gimg/pprune....36ccc4ccf1.png

The unspecified hypothetical case could be that the approach was first attempted with the most restrictive RNP but ANP was still adequate for the same approach with higher minima.

If it remains as unable RNP, that’s quite clearly your answer. Unable RNP. If it clears due to a transient issue, yeah give it another go if you have the fuel and want to. Not sure why there is a huge discussion about this. If it’s unable, it’s unable, so do another type of approach or divert.