SW2-32 GLIDER PLT ON APCH TO LAND HAD A TWIN CESSNA BACK TAXI ON THE RWY IN USE FOR HIS LNDG. GLIDER PLT MUST LAND OVER THE CESSNA WITH APPROX 45 FT VERT SEPARATION.

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I WAS FLYING A SW2-32 GLIDER ON AN FBO SCENIC FLT OUT OF BAR HARBOR; BHB; WITH 2 PAX. IT WAS MY FIRST FLT OF THE MORNING AND MY ACFT WAS EQUIPPED WITH A PANEL-MOUNT RADIO AND HEADSET. WX WAS VFR (CLR AND 30 MI VISIBILITY); WITH WINDS OUT OF THE SW AT 10-15 KTS. THE ACTIVE RWY WAS RWY 22 AND TFC WAS MODERATE. I WAS MONITORING UNICOM ON 123.0. AS I APCHED BHB FROM THE W I NOTICED SEVERAL ACFT HAD TAXIED OUT FROM THE FBO ON THE SE SIDE OF THE FIELD. THESE ACFT TAXIED UP RWY 35; HELD SHORT OF RWY 22; AND AS THE ACTIVE RWY BECAME AVAILABLE; THEY BACK-TAXIED APPROX 2000 FT DOWN RWY 22 FOR DEP. AT A POINT 1 TO 1 1/2 MI W OF THE FIELD I MADE THE FOLLOWING CALL TO ADVISE ACFT OF THE GLIDER'S IMPENDING ENTRY INTO THE PATTERN: 'BAR HARBOR TFC; GLIDER XXX 1 MI W OF THE FIELD AT 1500 FT...WILL BE ENTERING THE PATTERN IN 1 1/2 MINS.' I OBSERVED AN ACFT (TYPE UNKNOWN) BACK TAXI DOWN RWY 22 AND DEPART; WHILE A TWIN CESSNA HELD SHORT OF RWY 22 ON RWY 35. I ENCOUNTERED SOME 'SINK' AFTER MY RADIO CALL AND MY ALT QUICKLY DECREASED TO 900 FT. (GLIDER HAS A 34:1 GLIDE RATIO.) WHEN THE DEPARTING ACFT WAS ON THE TKOF ROLL I CROSSED OVER MID-FIELD ON A SOUTHEASTERLY HEADING; MAKING A MID-FIELD XWIND ENTRY TO THE PATTERN. I SIMULTANEOUSLY BROADCAST ON UNICOM: 'BAR HARBOR TFC; GLIDER XXX; MID- FIELD XWIND; L TFC; RWY 22; BAR HARBOR.' PLTS FREQUENTLY REPEAT THE LOCATION 'BAR HARBOR' AS 123.0 IS SHARED BY SEVERAL OTHER ACTIVE ARPTS AND CONFUSION OCCASIONALLY RESULTS. AS I PASSED OVER MID-FIELD; THE TWIN CESSNA WAS STILL HOLDING ITS POS SHORT OF THE ACTIVE RWY AND FACING WESTERLY. MY GLIDER SHOULD HAVE BEEN CLRLY VISIBLE TO THAT ACFT. MY ALT WAS APPROX 700 FT ON THE XWIND AS I HAD DELAYED ENTERING THE PATTERN FOR THE FIRST DEP. AS I TURNED L DOWNWIND; I SAW THE TWIN CESSNA TAXI ONTO RWY 22 AND SLOWLY START TO BACK TAXI FOR A FULL LENGTH DEP. I MADE THE FOLLOWING CALL: 'BAR HARBOR TFC; GLIDER XXX L DOWNWIND RWY 22; TIGHT PATTERN.' I MADE ADDITIONAL CALLS ANNOUNCING BASE AND FINAL AND EMPHASIZED THAT MY ACFT WAS A GLIDER. THE TWIN CESSNA CONTINUED BACK TAXIING TO THE END OF RWY 22; EVEN AS I WAS ON SHORT FINAL (1/8-1/4 MI). AT THIS POINT MY ALT WAS 150-200 FT AGL AND DSNDING; AND MY AIRSPD WAS APPROX 80 KTS. I CONTINUED ON FINAL AND PASSED DIRECTLY OVER THE TOP OF THE TWIN CESSNA AT APPROX 40-50 FT AGL. THE TWIN CESSNA WAS STILL FACING ME AS I PASSED OVERHEAD. I DID NOT MOVE TO THE E OR W SIDES OF THE RWY AS THE ILS ANTENNAS; VASI LIGHTS AND OTHER OBSTACLES WERE BLOCKING THOSE LOCATIONS. I ALSO WAS CONCERNED THE ACFT MIGHT ATTEMPT TO TURN QUICKLY AND START A TKOF ROLL TO GET OUT OF THE WAY (THIS WOULD HAVE CREATED A DANGEROUS SIT BECAUSE I HAD LIMITED OPTIONS FOR LNDG). I WANTED THE TWIN CESSNA TO RECOGNIZE MY ACFT WAS A GLIDER AND HOPED HE WOULD JUST HOLD HIS POS WHEN HE REALIZED MY ACFT'S LIMITATIONS. OTHER RADIO TFC PREVENTED ME FROM MAKING ADDITIONAL CALLS ON UNICOM. THE TWIN CESSNA DID NOT MOVE OFF THE ACTIVE RWY AS THERE ARE NO PARALLEL TXWYS AND THAT ACFT'S ONLY OTHER OPTION WOULD HAVE BEEN TO PULL INTO THE GRASS. I HAD AMPLE AIRSPD AND ALT TO PASS OVERHEAD OF THE TWIN CESSNA WITHOUT DIFFICULTY AND COMPLETE MY LNDG. I RULED OUT MAKING A 360 DEG HOLDING TURN TO DELAY MY LNDG BECAUSE OF THE 'SINK' IN THE AREA. I FELT A 360 DEG TURN WOULD CREATE MORE RISKS THAN NECESSARY. I DO NOT RECALL HEARING THE TWIN CESSNA ANNOUNCE ITS BACK-TAXI ON UNICOM. I REMEMBER HEARING IT ANNOUNCE ITS TKOF ROLL ONCE I CLRED THE RWY. I LATER LEARNED MY RADIO WAS PERIODICALLY UNREADABLE WHEN XMITTING; ALTHOUGH IT 'RECEIVED' XMISSIONS FINE. I SUSPECT THE TWIN CESSNA DID NOT HEAR MY RADIO CALLS. POSSIBLE CONTRIBUTING FACTORS: UNFAMILIARITY OF TWIN CESSNA PLT WITH GLIDER OPS AND 'TIGHT' PATTERN NATURE OF GLIDER-TYPE ACFT; FAILURE OF GLIDER'S RADIO FOR XMISSION PURPOSES (ALTHOUGH RADIO NOT REQUIRED BY BHB) AND RECENT ADDITION OF RWY MARKERS AND OBSTACLES ON W AND E SIDES OF RWY AS PART OF RECENT FAA-BACKED 'SAFETY IMPROVEMENTS' AT FIELD REDUCED AVAILABLE ALTERNATE LNDG SPOTS FOR THEGLIDERS. BEFORE THESE 'IMPROVEMENTS' WERE INSTALLED; AND BEFORE THE RWY WAS 'NARROWED' FROM 150 FT WIDE TO 100 FT WIDE AS PART OF THE SAME 'IMPROVEMENT' PROJECT; THE GLIDERS COULD LAND ON VARIOUS SEGMENTS OF GRASS IN AN EMER. THE RECENT NARROWING OF THE PAVEMENT AND INSTALLATION OF RWY SIGNS AND MARKERS WHICH ARE HIGHER THAN THE HT OF THE GLIDER WINGS HAS DECREASED ARPT SAFETY BY INCREASING THE NUMBER OF OBSTRUCTIONS. GLIDERS MUST NOW LAND ON THE CTRLINE OF THE PAVEMENT TO AVOID HITTING RWY LIGHTS ON MUCH OF THE ARPT. (THE ARPT 'LOWERED' THE RWY LIGHTS IN THE VICINITY OF THE GLIDER RAMP TO ACCOMMODATE GLIDER TFC PULLING OFF THE RWY.) I DID NOT PERCEIVE THE SIT AS CRITICAL AND I KNEW THAT I HAD SUFFICIENT SPD AND ALT TO LAND ON THE ACTIVE RWY BEYOND THE TAXIING ACFT. THE TWIN CESSNA WAS ALSO STILL ON THE 'BACK TAXI' AND THE PLT COULD CLRLY SEE ME FLY OVERHEAD HIS ACFT. I KNEW HE WOULD NOT BEGIN A TKOF ROLL UNTIL I CLRED.