Originally Posted by
STBYRUD
Whats your point? Since the 737 in incapable of RAIM prediction itself the operator must at least use some kind of external RAIM prediction service, such as AUGUR, to dispatch aircraft in conditions that would require a certain RNP approach or procedure. If in flight RAIM causes the ANP to shoot up above RNP for the procedure, either because of signal unavailability or other reasons, you obviously can't shoot the approach...
Yes, but you must realize there are entire FIRs in EU and airlines (I believe DLH is one) in which RNP approach is banned during RAIM unavailability promulgated by NOTAM. On the other hand there are airlines who teach and examine their pilots that they
can attempt an approach even during RAIM unavailability. So there is a major inconsistency in the area. Someone has to be wrong.
If it is in fact true that a crew is only dependant on the ANP value than what is the point in banning the approach? In case of RAIM loss the ANP will rise, the G/A criteria will be met. But there is more to it than this. The B737 FMS is computing its position based on GNSS, DME/DME, INS and VOR/DME (this one is not even allowed any more on most SIDs and STARs). I can't confirm this but it sems like the latter three systems are considered in the positioning eqation until the aircraft is powered down. So they work just fine during the RNP approach and can augment the positioning. The problem is that they shouldn't. The RNP approach cannot be executed with other than GNSS positioning sensor unless specifically authorised by state (which is rare). I haven't seen single pilot who switch off DME/DME, INS and VOR/DME during the approach in order to have pure GNSS-generated ANP. So it might happen that despite the lack of RAIM the ANP will not rise or rise very slowly. I can't say I have it all right since I can't find any proof but certainly some FIRs/airlines forbid the approach in case of RAIM loss and some don't (some even encourage their crews to try!). I can't figure out why.