In the list of approaches on the Arrivals page in the FMC, I often see something that says TACAN (then RWY X). What is this, what does it stand for, & how does it differ from the other approaches? I know what ILS approaches are, and VOR, VORDME, but I am not sure what this one is. Also, what does RNAV stand for on the approach page. & is this yet ANOTHER type of approach?Thanks for the help.Chris Catalano

TACtical Air NavigationRNAV"Area Navigation (RNAV) can be defined as a method of navigation that permits aircraft operation on any desired course within the coverage of station-referenced navigation signals or within the limits of a self contained system capability, or a combination of these.RNAV was developed to provide more lateral freedom and thus more complete use of available airspace. This method of navigation does not require a track directly to or from any specific radio navigation aid, and has three principal applications: A route structure can be organized between any given departure and arrival point to reduce flight distance and traffic separation; Aircraft can be flown into terminal areas on varied pre-programmed arrival and departure paths to expedite traffic flow; and Instrument approaches can be developed and certified at certain airports, without local instrument landing aids at that airport. Navigation systems which provide RNAV capability include VOR/DME, DME/DME, LORAN C, GPS, OMEGA and self contained Inertial Navigation Systems (INS) or Inertial Reference Systems (IRS)."Andy B

Chris,"TACAN" is basically a military approach and is not used in the civilian world (at least in the US -- requires additional equipment).Here is a link to help further, and more technically, answer your question:http://www.radarpages.co.uk/mob/navaids/tacan/tacan1.htmRegards,Sean

TACAN is a UHF military variant of a civilian VOR/DME. A VORTAC is a combination of both a VOR/DME and a TACAN. Functionally, they are essentially the same to the pilot except instead of frequencies, TACAN uses channels. The only significant functional difference is that holding directly over a TACAN is prohibited due to the wider cone of confusion.RegardsBob ScottATP IMEL Gulfstream II-III-IV-V L-300Washington, D.C.