AN EMBRAER 120 ON APCH AT 2500 FT HAS TRAILING EDGE FLAP FAILURE AT 15 DEGS. OPS MANUAL HAS NO PERFORMANCE NUMBERS FOR THIS SETTING.

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WE WERE APCHING TO LAND AT ZZZ ARPT ABOUT 7 MILES OUT I CALLED; AS FLYING PLT; FLAPS 15 (INITIAL FLAP POSITION FOR LNDG). THE FLAPS MOVED TO APPROX 15 DEG AND WE RECEIVED A FLAP DISAGREEMENT ON THE OUTBOARD FLAPS. WE COMPLIED WITH THE CHKLIST AND LEFT THE FLAPS AT THE FIRST SELECTED POSITION (15 DEG) FLAP DISAGREEMENTS ARE VERY COMMON WITH THE EMB 120 AND TYPICALLY HAPPEN WHEN FLAPS ARE SELECTED TO 25 DEG OR 45 DEG. WE ELECTED TO LAND FLAPS 15 DEG AND CAPT STATED WE HAD PERFORMANCE TO LAND BUT UPON LNDG AND CLOSER REVIEW THIS WAS A GREY AREA. THE RWY ANALYSIS PROHIBITS A ZERO FLAP LNDG AT OUR WT BUT WE WERE NOT AT FLAPS 0 DEG. NORMAL LNDG DATA PROVIDES DATA FOR ONLY 25 DEG AND 45 DEG FLAP LNDGS. THE CHKLIST GIVES NO GUIDANCE FOR PARTIAL FLAP LANDINGS. THE CHKLIST ONLY STATES TO 'APPLY THE ZERO FLAP WT PENALTY IF LNDG FLAPS ZERO.' THERE IS NO GUIDANCE FOR LNDG FLAPS 15 DEG DUE TO A MALFUNCTION. WE ELECTED TO LAND DUE TO THE FACT THAT WE WOULD HAVE HAD TO CLB BACK TO ALTITUDE TO REACH OUR DISPATCH AND MX AND THEN IF WE WOULD HAVE DIVERTED WE WOULD HAVE LANDED WITH WELL LESS THAN RESERVE FUEL MINIMUMS. THE LNDG WAS UNEVENTFUL AND REF SPEEDS FOR 15 DEG VERSES 25 DEG ONLY ADDED ABOUT 10 KNOTS TO OUR APCH AIRSPEED. AFTER LNDG; THE MALFUNCTION CLRED AND AFTER CONFERRING WITH OUR MX CTL THEY STATED NOT TO WRITE UP THE DISCREPANCY SINCE IT CLRED. THE ONLY QUESTION IN MY MIND IS THE FACT THAT THERE WAS NO OFFICIAL LNDG DATA FOR A PARTIAL FLAP LNDG; ONLY FLAP 0 DEG WHICH WE WERE OVERWT FOR OR 25 DEG OR 45 DEG WHICH WE COULD NOT DO BECAUSE OF MALFUNCTION. HOW DO YOU KNOW IF A PARTIAL FLAP IS LEGAL? DOES ANY ACFT HAVE DATA FOR THIS? CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT FLAP FAILURE TO EXTEND BEYOND 15 DEGS IS A COMMON FAILURE ON THE EMBRAER 120 AND IS MOST OFTEN NOT WRITTEN BY CREWS. THE RPTR SAID MAINT HAS A STANDARD POLICY OF ADVISING THE CREWS WHO EXPERIENCE TRAILING EDGE FLAP FAILURE TO CHECK THE FLAPS ON THE GND AND IF OK MAKE NO DISCREPANCY LOGBOOK WRITE-UP. THE RPTR STATED IN THE LAST THREE DAYS TWO TRAILING EDGE FLAP FAILURES OCCURRED WITH ONE EVENT WITH FLAPS FULL UP. THE RPTR SAID THE FREQ OF FLAP FAILURES ARE SUCH THAT THE CREW DON'T BOTHER TO WRITE THE RPT IN THE MAINT LOG. THE RPTR STATED THE FLT OPS MANUAL NEEDS REVISION TO ADD ACFT AND ARPT PERFORMANCE NUMBERS FOR FLAPS 15 LNDGS.