Manual Input Of RNP and UNABLE RNP Message
 

I've noticed whilst sitting at the gate, than I was unable to generate an ">UNABLE RNP" EICAS message (747-400) by manually entering an RNP smaller than the displayed ANP. I waited about a minute for the message to appear to see if there was a time delay for the message, but I imagine that you would want to see a message relating to this ASAP.

Would there be an air/ground inhibit on this message or something more specific? e.g. LNAV and/or VNAV arm/engage

Thanks.
Cheers
NSEU

AFAIK the logic is inhibited on the ground after a certain software version - noticed that the ANP was exceeding 10 NM while the IRS were aligning the other day and the aircraft didnt say a thing. On another ship (with older software) it immediately complained (UNABLE RNP) when the IRSs were turned on for alignment...

stbyrud:
Perhaps the newer software was RNP AR compliant, which has far more demanding performance requirements than "plain vanilla" RNP.

That might well be the case - the aircraft in question (with the newer software) also continously displays ANP and RNP in the ND and has the ANP deviation bars on the PFD...

Thanks, gentlemen. Lots of food for thought.

Actually, I wasn't aware of the new "AR" software.

Cheers
NSEU

NSEU:
That is speculation on my part. A load with RNP AR capabilities replacing a load without those capabilities would tend to show sublte changes of the nature described.

Improved software without RNP AR capability would likely have improvements of some type.