757/767 & RNAV (GPS) Approaches

[GalaxySam]

Are there any 757/767 Operators out there flying GPS approaches with the Pegasus FMS? If so, how do you determine a loss of signal/RAIM while on approach? Is the "Unable RNP" EICAS message deemed sufficient warning? Any details would be appreciated. Thanks.

[BN2A]

RNAV approaches and departures are used at LPFR, but can GPS be used as a primary navigation system yet?
Still the 3 IRS's, I think..

[Blacksheep]

The three IRS's give you up to RNP5 on B757/B767 aircraft (i.e. Basic RNAV, RNP10 and RNP5) The certification status of each individual aircraft should be indicated on a Certificate of Minimum Navigation Performance usually carried somewhere on the flight deck.

To get below RNP5, to RNP1/3 for example, and move to Precision Area Navigation (PRNAV) requires external equipment such as Augmented GPS; Rho-Rho navigation using DME updates isn't good enough. There are places (e.g. Frankfurt) offering PRNAV approaches to aircraft equipped and certified for them, but I don't think many aircraft are so equipped AND certified yet.

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[GalaxySam]

Yes, what I'm referrring to are 757/767 operators with GPS receivers included into their navigation suite. (i.e. 3 IRUs + a GPS receiver(s) all refining the FMC nav solution) The question is this: With continuous GPS updating to the FMCs, the naviagtion accuracy is extremely good. However, this isn't exactly a standalone "GPS Receiver" commonly used for GPS approaches. Common sense dictates that with such good nav accuracy (including GPS aided nav solutions) one should be able to shoot a GPS stand-alone approach. However, as I understand the Pegasus FMC, there isn't a "RAIM Alert" function like a normal, early generation GPS Approach receiver. The question then becomes, how do you know that you've lost the GPS signal if you are shooting a GPS approach? (and do you care if you've got an ANP/RNP comparison able to warn you of a problem)

In short, is anyone using a Pegasus FMS (with GPS receivers installed for continuous updating) able to fly stand alone GPS approaches?

[Blacksheep]

The rate at which the positional accuracy degrades when there is an equipment failure is taken into account in establishing the RNAV rating and defining what RNAV procedures must be included into the company's operations procedures. If you have GPS integrated into the navigation suite and are also certified for PRNAV - i.e. it is included in your Certificate of Minimum Navigation Performance (CMNP) - then, if the GPS is lost while performing a PRNAV approach, since there isn't an instantaneous loss of accuracy, the approach can be completed before the system's positional accuracy degrades enough to cause a problem. This performance is taken into account in issuing the CMNP for PRNAV. Its the condition of the system at the start of the precision approach that is the significant factor. As long as you are accurately placed at the beginning, the equipment is able to complete the job, despite any subsequent loss of GPS signal.

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Through difficulties to the cinema

[GalaxySam]

Thanks. That makes sense. I figured that RAIM is only needed for GPS-Only type navigators. With the integrated solution, the accuracy is very good. I appreciate your reply.