Old Akro you really don't know what you're talking about in this instance.
A typical FMC in a current model wide, or narrow body, jet CAN get input from VOR or DME but they also get input from dual GPS and VOR/DME input is typically inhibited. With RNP AR they must be inhibited to ensure the required FMS accuracy to fly the approach.
RNP AR does not require WAAS.
Not all FMCs can fly RF (Radius to Fix) legs which is a requirement for RNP AR approaches. The AR btw stands for Authorisation Required...the plates are not published for public consumption.
I very much doubt Thai is RNP AR approved...they MAY be GNSS conversant which is the GPS approach available to GA aircraft. GNSS is a VERY different animal to RNP AR.
An FMC gives you along track distance not slant distance direct to the station like a DME.
That a particular FMC can't do RF legs doesn't mean they can't fly curved tracks - they can.
ANY 777 FMC is perfectly capable of providing vertical guidance, that's what VNAV is, and the aircraft will follow it very well if the auto thrust is working. The LNAV (lateral guidance) and the VNAV is fed to the Computers and presented to the crew and auto pilots via the Flight Director. IT LOOKS JUST LIKE YOU'RE FLYING AN ILS.
When you programme the FMC to fly an approach a bunch of height/speed way points are coded into the legs starting at the RWXX waypoint (50' over the threshold) and back through the FF waypoint, CF waypoint and on back around the DME arc (in this case) and through the IAL transition all the way to TOPD.
In the cae of the 777 if you leave the damn thing alone in LNAV VNAV it will accurately fly the correct profile and decelerate to the required speeds at each constraining waypoint. LITERALLY all you have to do is configure the aircraft and set the MCP ALT (alt window) to each successive altitude constraint until the FAF then to the minima (once you're at least 300' below the missed approach altitude you set that and the aircraft will continue descending to the minima)....extend the flaps when the command bug tells you to and put the wheels down before it gets expensive.
It's EXACTLY like flying an ILS from TOPD...for all intents and purposes. Yes to a higher minima but from an flight instrument interpretation and manipulative stand point an NPA in a modern jet is functionally identical to an ILS.
Now if you insist on not using VNAV and instead use VS (Vertical Speed) or FLCH (Flight Level Change) all bets are off...You better know how to do that pilot sh!t...and you damn well better understand the differences and what happens when the Auto Flight Guidance System goes to 'On Approach' mode.
As an aside RNP AR is a thing of beauty...although not perfect. I'd suggest within 5 years, 10 tops, ILSs will be as rare as NDBs.
Last edited by Chimbu chuckles; 27th Feb 2013 at 09:41.