What is the available range for DME, and why do we need EFC?
How can DME signal go from the station?



Your aircraft's DME sends a signal (when tuned to an appropriate frequency) to the ground station (suitably equipped with DME, e.g. Vortac, ILS/DME, etc.) and the station sends back an electronic reply. The aircraft's DME will measure how much time it took for the signal to travel to/from the station and translate that into a distance (in slant range).  Here is a good article explaining DME:


If you are asking what the maximum distance between the station (e.g. Vortac/VOR/DME, etc.) and the DME equipped airplane can be for a reliable DME signal, it's 199 NM (slant range).  Of course this depends on aircraft altitude and a few other variables. 

See this excerpt from the Aeronautical Information Manual for more information:



DME Article from AVWeb




  why do we need EFC?



This answer pertains to EFC, meaning "Expect Further Clearance" time, route, altitude: (U.S)


If you are flying IFR and are given holding instructions (for example) you will be given an EFC time by ATC.  This is given to you in case of lost communications.  If you loose communications with ATC you are expected to continue with your originally planned, revised, or route/approach you were told to expect by ATC at your EFC  time.

There are other occasions that you may receive an EFC from ATC.  For example, you may be taken off of a previously assigned route, and ATC might say (for example) "N12345 turn right heading 230, vector for traffic, expect direct to Omaha in five miles."  If you lost communications with ATC, then five miles from the point of the vector you should go direct to Omaha.

Or, another example might be if you are told by ATC to "maintain Flight Level 270, expect Flight Level 350 in four minutes."  If you lost communications you would be expected to have maintained Flight Level 270 and, 4 minutes after receiving this instruction, begin your climb to Flight Level 350. 

Since your question was not entirely clear regarding EFC, I included 3 different occasions that your might receive an "Expect Further Clearance" from ATC.

As a note, the actual ATC phraseology (generally speaking) where the 3 words "Expect Further Clearance" would be spoken would apply to a routing, or holding clearance (or a short term clearance to a fix/clearance limit other than an airport, with or without holding instructions). But the word "Expect," when used with regard to a route or altitude (such as "Expect Flight Level 350 in four minutes" or "Expect" direct to Omaha in five miles" is implied to be an "Expect Further Clearance." (EFC)

Finally, as noted above, EFC is directly associated with lost communication with ATC.  If you read the Federal Aviation Regulations, (FAR 91.185) you will have a better understanding of how the "Expect Further Clearance" procedures apply.