A C172 CFI EXPERIENCED AN NMAC WITH A TOW PLANE AND GLIDER NEAR DSV ARPT.

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I WAS GIVING DUAL INSTRUCTION TO A STUDENT PLT. WE WERE DSNDING TOWARD DSV; WHICH IS LOCATED IN A VALLEY WITH 1000 FT HIGH HILLS (ABOVE ARPT). WE CALLED OUR APCH/DSCNT ON 123.0 MHZ AND WERE MONITORING THE FREQ FOR TFC. THE SUN WAS IN OUR 11 O'CLOCK POS. WE WERE ACTIVELY LOOKING FOR AIRPLANES AND GLIDERS AS WE APCHED THE ARPT. THE TOW PLANE AND GLIDER CLBED BY US; HEAD ON; ON THE L SIDE AT ABOUT 300 FT HORIZ DISTANCE. THE TOW PLANE WAS NOT USING THE UNICOM AND DID NOT SEE US UNTIL HE PASSED US BY; AS HE THEN MADE A COMMENT ON UNICOM THAT HE SAW US. SIT LEADING TO PROB: TOW PLANE DIDN'T CALL ON UNICOM THAT HE WAS DEPARTING OR HIS POS. HE WAS BELOW THE TOP OF THE S RIDGE; AND HAD HIS BACK TO THE SUN. GND CLUTTER OBSCURED HIS PRESENCE. HE DID NOT SEE US. WE WERE ABOVE HIM AND SHOULD HAVE BEEN EASILY VISIBLE AS WE WERE NOT OBSCURED BY GND CLUTTER ON THE N RIDGE. RECOMMENDATIONS: TOW PLANES SHOULD USE THE CTAF WHEN DEPARTING; CLBING; DSNDING; AND LNDG. TOW PLANES SHOULD USE AN OPERATING MODE 'C' XPONDER AND BE EQUIPPED WITH TCASII. WE ARE GOING TO EQUIP OUR CESSNA WITH TCASII. WE CURRENTLY HAVE A CERTIFIED MODE C XPONDER; AS WE OPERATE OUT OF CLASS C AIRSPACE AND WE USE ACFT FOR IFR WORK. WE WILL USE MORE DSNDING TURNS FOR 'SEE AND BE SEEN.' WE WILL DSND FURTHER AWAY FROM THE FIELD; USE THE LNDG LIGHT; AND CONTINUE USING STROBES; BEACON; AND MODE C XPONDER.