A C402B CPR ACFT RUNS OFF THE END OF RWY 12 DURING A HIGH SPD TAXI TEST AND HAS THE NOSE GEAR COLLAPSE AS THE ACFT ENTERED THE OVERRUN AREA AT ABQ; NM.

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AT APPROX XA20 ON 07/XA/02; I RECEIVED CLRNC TO TAXI ONTO RWY 12 AT ABQ ARPT TO CONDUCT A HIGH SPD TAXI TEST OF MY C402B. CONDITIONS WERE VFR; TEMP APPROX 70 DEGS F WITH WIND LIGHT AND VARIABLE. CALCULATED ACFT WT WAS 6250 LBS WITH MYSELF; AN AVIATION MECH AND APPROX 160 GALLONS OF FUEL ON BOARD. THE TEST WAS TO BE A CHK OF MAINT DONE TO CORRECT A SEVERE NOSEWHEEL SHAKE ENCOUNTERED DURING THE TKOF ROLL OF A FLT ATTEMPTED EARLIER THAT MORNING. THE MAINT PERFORMED CONSISTED OF ADJUSTING THE NOSEWHEEL STEERING CABLES AND SVCING THE STEERING DAMPENER. BECAUSE THE NOSEWHEEL SHAKE HAD APPEARED AT BTWN 50-60 KTS; I PLANNED TO ACCELERATE THE ACFT TO 70 KTS BEFORE SHUTTING DOWN IN ORDER TO VERIFY THE CONDITION WAS TRULY REPAIRED. I DECIDED TO REQUEST A RWY FOR THE TEST TO TAKE ADVANTAGE OF ITS EXTRA LENGTH AND WIDTH IN THE EVENT THAT PROBS WERE ENCOUNTERED. THE ARPT WAS VERY BUSY AT THIS TIME WITH LNDG AND DEPARTING TFC USING RWY 3 AND RWY 8I CHOSE 6000 FT X 150 FT RWY 12 FOR THE TEST AFTER REVIEWING THE ACCELERATE/STOP DATA CONTAINED IN THE ACFT POH AND BECAUSE I FELT THAT ARPT OPS WOULD PRECLUDE MY USING A LONGER RWY FOR THE TEST. THE PUBLISHED DATA SHOWED A REQUIRED DISTANCE OF 4100 FT FOR OUR CONDITIONS. TO THIS FIGURE; I ADDED 20% FOR 4920 FT REQUIRED THEN ROUNDED UP TO 5200 FT. I PLANNED TO BEGIN MY TEST FROM A POINT ON RWY 12 S OF THE ILS CRITICAL AREA FOR THE ACTIVE RWY 8. ALTHOUGH THIS WOULD SHORTEN THE LENGTH OF THE RWY AVAILABLE FOR THE TEST TO APPROX 5800 FT I FELT SATISFIED THAT I HAD BUILT SUFFICIENT CUSHION INTO MY CALCULATIONS TO COMPENSATION FOR REAL WORLD CONDITIONS; ESPECIALLY SINCE I WOULD BE ACCELERATING TO JUST OVER 70 KTS AND SHUTTING DOWN RATHER THAN TO THE 91 KT ROTATION SPD USED TO DERIVE THE POH DATA. I ADVANCED THE THROTTLES AND ACCELERATED THE ACFT TO 70 KTS. ACCELERATION APPEARED TO BE NORMAL AND WAS WITHOUT VIBRATION. SATISFIED THAT THE ORIGINAL PROB CONDITION HAD BEEN REPAIRED I REDUCED THE THROTTLES TO IDLE AT JUST OVER 70 KTS AND BEGAN HVY BRAKING. THE ACFT TRACKED STRAIGHT AHEAD; BUT DID NOT APPEAR TO SLOW ALTHOUGH THE BRAKE PEDAL PRESSURE FELT NORMAL. CONTINUED HVY BRAKING RESULTED IN ONLY SLOW DECELERATION. WITH THE AVAILABLE RWY RAPIDLY DECREASING I DECIDED AGAINST LOCKING THE BRAKES OR USING FLAPS IN AN EFFORT TO SLOW THE ACFT AS I FELT THAT EITHER OF THESE ACTIONS WOULD BE INEFFECTIVE AND WOULD RESULT IN A LOSS OF CTL. IN ADDITION; BECAUSE I HAD SLOWED TO LESS THAN 50 KTS; ADVANCING THE THROTTLES; ATTEMPTING TO ACCELERATE TO TKOF SPD AND TAKING OFF IN THE DISTANCE REMAINING WAS NOT AN OPTION. I CHOSE INSTEAD TO CONTINUE HVY BRAKING AND RIDE THE ACFT ONTO THE OVERRUN AREA IF WE HAD NOT STOPPED UPON REACHING THE END OF THE RWY. THE ACFT HAD SLOWED TO APPROX 20-25 KTS AS IT EXITED THE RWY ONTO THE OVERRUN AREA. JUST PRIOR TO END OF THE RWY; I REDUCED THE MIXTURES TO IDLE CUTOFF SO THAT THE ENGS WOULD BE SHUT DOWN AND HOPEFULLY SUSTAIN LESS DAMAGE SHOULD THE NOSE GEAR COLLAPSE. THE OVERRUN AREA CONSISTED OF A FLAT; SANDY AREA THAT DSNDED FROM RWY ELEVATION ABOUT 1000 FT TO THE ARPT PERIMETER ROAD. THE ACFT TRACKED STRAIGHT AHEAD AND CONTINUED TO SLOW AS WE PROCEEDED DOWN THE HILL. THE NOSE OF THE ACFT BEGAN TO PORPOISE DUE TO THE UNEVEN NATURE OF THE GND WITH THE NOSEWHEEL EVENTUALLY DIGGING INTO THE SAND AND THE STRUT FOLDING BACK. PRIOR TO THE ACFT STOPPING; I TURNED OFF THE FUEL VALVES AND SWITCHED OFF THE MAGNETOS AND MASTER SWITCH. THE ACFT CAME TO REST WITHOUT INJURY TO PERSONS ON BOARD OR FIRE. THE MECH AND I EXITED THE ACFT AND WAITED FOR EMER PERSONNEL TO ARRIVE.