RNP AR IAPs can have a line of minima as high as 0.30 and as low as 0.10. Also, the missed approach can be conventional TERPS or PANS-OPS missed approach criteria or it can be an RNP missed approach. An RNP missed approach is "telescopimg" RNP segments, which presumes at least one IRU to maintain positioning throughout the missed approach in the event of loss of GPS, albeit degrading position information, thus the reason for the "telescoping" expansion.
Also, if a line of minima is used that is less than RNO 0.30 then at least one IRU is required for that purpose, independent of the missed approach construction.
So, only on an RNP 0.30 procedure with a conventional missed approach could position be lost unless a dual failure occurs (loss of GPS and IRU(s) ). Because most airplanes have 3 IRUs, this event is highly unlikely.
Interesting tale, no doubt, but certainly a tale none the less.
First off, how RNP and GPS work, and how GPS and the IRU connect together. The GPS signal is not directly fed to the FMS/FMS. The GPS feed is connected to the IRU. The IRU manages the GPS signal, with
the GPS signal telling the aircraft where it was, and the
IRU calculating using the Kalman filter, and telling the aircraft where it is. You cannot have a direct feed from the GPS to the FMC. Without an IRU, you have no GPS navigation.
The IRU is receiving the GPS signals from all of the antennas and balancing the results, feeding the results through the Kalman filter, and providing the update to the FMC, as to the estimate of where the ac is. This is where you see the actual vs required numbers on the FMC.
When the GPS signals degrade, or are lost, the IRU will still balance where the aircraft is, according to the other feeds it is receiving. This is where the IRS update values come from.
In the past, you had IRU systems that were not fed by GPS, but now the update value that used to be fed by the pilot, is now fed by the GPS system.
RNP procedures per the criteria, transition to RNP 1.0 from the MAP. This has absolutely nothing to do with # of systems online. It does not matter if you are RNP 0.1, 0.3, or 0.5..you still transition to RNP 1.0...this is the criteria. This transition has to happen before you start an RF leg.
With approved custom RNP, there is no transition and the missed can be at the FAS level. (the power of custom criteria!, and in reality, the only way to avoid many obstacles and terrain.)
You appear to be mixing in ANP growth due to lack of GPS signal, where the IRU is providing sole guidance. In reality, the ANP growth rate must still be below the containment area for the RNP level. As an example, for 0.3RNP the loss of GPS signal must correspond to an ANP growth rate of 0.5RNP, less than the 2x containment of 0.6.
Here is an RNP AR procedure that does not require any IRU(s), so the loss of GPS could cause the wings to level during the final segment RF leg, thus pointing you at very high terrain.
This statement has no basis in reality.
