L MAIN GEAR COLLAPSED DURING LNDG ROLLOUT ON A DA20 AT RFD.

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ATTEMPTED TO LOWER GEAR PRIOR TO LNDG USING THE ALTERNATE OR EMER PROC. GEAR FAILED TO COME DOWN. ATTEMPTED A SECOND TIME USING SAME METHOD; NOTHING. RESET ALL SWITCHES AND CTLS AND ATTEMPTED A NORMAL EXTENSION. NOTHING! SECOND ATTEMPT. NOTHING. PULLED OUT EMER EXTENSION CHKLIST. RAN CHKLIST ENTIRELY. DOORS MANAGED TO OPEN AND GEAR CAME DOWN BUT NO 'DOWN AND LOCKED' INDICATIONS. WE ACCELERATED AND YAWED THE ACFT AS PER THE CHKLIST BUT NEVER GOT A DOWN AND LOCKED INDICATION ON ALL THREE. HYD PRESSURE INDICATED NORMAL. UPON LNDG L MAIN COLLAPSED WITH MINIMAL DAMAGE TO ACFT. MECHANICAL FAILURE WAS DETERMINED AS CAUSE. SUPPLEMENTAL INFO FROM ACN 341904: A COMMON PRACTICE IN THE FALCON COMMUNITY IS TO OCCASIONALLY LOWER THE LNDG GEAR USING PROC 'A;' WHICH USES THE #2 HYD SYS AND THE EMER LNDG GEAR HANDLE TO LOWER THE GEAR. THE RESULT IS TO TRANSFER FLUID BACK FROM THE #2 SYS TO THE #1 SYS. THE PIC DECIDED TO USE THIS PROC TO ADDRESS A FLUID IMBALANCE. ON FINAL THE PIC ATTEMPTED A PROC 'A' WITH NO RESULT AT ALL. THE PIC ATTEMPTED ANOTHER PROC 'A' WITHOUT RESULT. AT THIS POINT BOTH PLTS REVIEWED THE CHKLIST AND RETURNED ALL SWITCHES TO NORMAL AND TRIED TO LOWER THE GEAR VIA THE PRIMARY #1 HYD SYS WITH NO RESULT AT ALL. WE THEN PROCEEDED TO PROC 'A' AGAIN WITHOUT RESULT. WE THEN PROCEEDED TO PROC 'B' WHICH ENTAILS MANUALLY UNLOCKING THE GEAR DOORS AND USING THE #2 HYD SYS TO EXTEND THE GEAR. THE DOORS DID UNLOCK BUT THE GEAR DID NOT EXTEND TO THE LOCKED POS. THE TWR RPTED THE GEAR HALFWAY DOWN. WE THEN WENT TO THE FINAL; LAST DITCH PROC 'C' WHICH IS A GRAVITY/AIRSTREAM EXTENSION PROC. WE ACCELERATED AND YAWED THE ACFT IN AN ATTEMPT TO GET ''DOWN AND LOCKED' INDICATIONS WITHOUT SUCCESS. THE TWR RPTED THE GEAR APPEARED 'DOWN AND LOCKED.' HAVING EXHAUSTED ALL PROCS AND FUEL RUNNING LOW WE ASKED FOR CRASH TRUCKS AND LANDED. THE PIC DID AN EXCELLENT MINIMUM DSCNT RATE LNDG AND THE ACFT ROLLED ON BOTH MAINS NORMALLY FOR ABOUT 4 SECONDS BEFORE THE L WING BEGAN TO SETTLE TOWARD THE RWY. THE ACFT WENT STRAIGHT FOR ABOUT 1000 FT BEFORE IT VEERED OFF L AND INTO THE GRASS BEFORE COMING TO A STOP. NO AMOUNT OF R BRAKING OR NOSEWHEEL STEERING COULD KEEP IT ON THE RWY. THE CAUSE WAS THE CABLE TO THE HYD VALVE THAT SWITCHED BTWN THE #1 AND #2 HYD SYS FAILED WITH VALVE BTWN THE 2 SYS. FAILURE OF THE GRAVITY FALL PROC IS STILL UNDER EXAMINATION.