OBSERVER RPT REGARDING A MIDAIR COLLISION BTWN 2 C172 ACFT IN ATA.

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THE OCCURRENCE THAT I WOULD LIKE TO DESCRIBE IS A CONFLICT BTWN 2 CESSNA 172'S ON THE DOWNWIND LEG OF THE TFC PATTERN FOR NAPA COUNTY ARPT'S RWY 18R. ALTHOUGH I WAS SAFELY ON THE GND; I FEEL MY DESCRIPTIONS MAY HAVE SOMETHING TO ADD THAT MAY BE BENEFICIAL IN FUTURE RESEARCH. ON THE DAY OF THE CONFLICT; I WAS ON GND CTL TAXIING OUT AS THE 2 ACFT WERE COMING BACK IN AND I COULD HEAR THEIR EXCITEMENT OVER THE RADIO. BOTH ACFT TAXIED OVER TO THE MAINT HANGAR. I COULD HEAR THE CTLR INSTRUCTING BOTH PLTS TO PHONE THE TWR AT THEIR NEXT POSSIBLE OPPORTUNITY. LATER THAT AFTERNOON I TALKED WITH A STUDENT OF MINE WHO WAS IN THE PATTERN AT THE TIME. HE SAID HE WAS ABEAM THE NUMBERS OF RWY 18R BEGINNING HIS DSCNT FOR LNDG PROCS; WHEN HE HEARD THE COMMOTION ON DOWNWIND OVER THE RADIO. HE COULD NOT SEE THEM AHEAD OF HIM SO HE CONTINUED WITH HIS APCH. ON SHORT FINAL HE WAS TOLD TO 'GO MISSED APCH;' WHICH HE CORRECTLY INTERPRETED AS THE SAME AS 'GAR.' HE WAS MORE CONCERNED WITH CTLING HIS OWN ACFT SO HE COULD NOT REMEMBER THE EXACT WORDS BTWN THE 2 172'S; OR BTWN THE AIRPLANES AND THE CTL TWR; IF THERE WERE ANY. I ALSO WORK AS AN A&P MECH SO I HAVE LOOKED AT THE DAMAGE TO BOTH ACFT. THE YELLOW 172 WAS HIT ON THE L WINGTIP AND DOWN BY THE WING STRUT SUPPORT (VERY MINIMAL DAMAGE). THE BLUE 172 HAS DAMAGE TO THE R HORIZ STABILIZER AND ELEVATOR; PROBABLY REQUIRING REPLACEMENT OF BOTH. THE ELEVATOR STILL MOVES FREELY AND A SAFE LNDG WAS COMPLETED BY BOTH PLTS. FROM AN INSTRUCTOR'S POINT OF VIEW; I HAVE HAD CONTACT WITH THE PLT OF THE BLUE 172. HE IS A LCL PLT KNOWN BY MANY AT THE ARPT. I FLEW WITH HIM A WHILE BACK (I BELIEVE IT WAS LAST SUMMER) FOR HIS BIENNIAL FLT REVIEW. I REMEMBER HIM DOING A FINE JOB OF CTLING THE ACFT; BUT I DID ASK HIM TO COME BACK FOR A SECOND APPOINTMENT AS WE WERE NOT ABLE TO COVER ALL OF THE AREAS I FELT WE SHOULD IN 1 MEETING. I CAN RECALL HE HAD A WEAK AREA IN FLT LOGBOOK DOCUMENTATION AND RADIO PROCS. IT HAS BEEN MY PAST EXPERIENCE THAT LOGBOOKS CAN BE A TOUCHY AREA WITH SOME PLTS. THEY VIEW IT AS ALMOST AN INVASION OF PRIVACY (SIMILAR TO LOOKING THROUGH SOMEONE'S CHECKBOOK WAS ONE ANALOGY I WAS TOLD). TO HELP THE PLT WITH HIS RADIO PROCS I EVEN PROVIDED HIM WITH SOME CRIB SHEETS TO HELP ORGANIZE HIS RADIO WORK. HOPEFULLY MY INSTRUCTION WAS OF BENEFIT TO HIM AS I DO STRESS EMER PROCS AND TFC PATTERN OPS DURING MY BIENNIALS. SUCH AN OCCURRENCE AS THESE 2 172'S HAD; COULD OBVIOUSLY HAVE BEEN PREVENTED BY PROPER COLLISION AVOIDANCE TECHNIQUES AS WELL AS BETTER COORD WITH THE CTL TWR AS TO THE LOCATION OF OTHER ACFT.