Major Tom:
I guess this is why its called a 'research assignment' - yet i simply cant find anything in regards to 'cons' of RNP.
Here are a few cons:
1. Equippage with at least one grossly expensive IRU for RNP of less than RNP 0.30 or missed approach of less than RNP 1.0 It's a freebie for the airlines because 3 IRUs are cost effective in an $80 million airframe and needed for oceanic or remote land mass operations.
2. Expensive independent database validation of all RNP AR IAPs on a continuing basis. The only practical solution for the non-airline operation is to pay a Honeywell twice for the same database; i.e., it makes the RNP AR database "golden."
3. Extra training costs and extensive line qualification requirements. And, the invarible struggle with the occasional obnoxious FAA inspector who doesn't like you so he makes it very difficult to obtain your approval.