What happened
The FAA’s narrative, in sentence case. The FAA writes it in capitals and removes private details.
As stated by the pilot:
The pilot contacted Hillsboro ground, letting them know of my intentions to do a brake burn-in, and that this would be a ground operation only, and we would exit the runway taxi back to parking on completion. I requested access to the non-active runway but was denied, and informed I would have to use the active runway.
After waiting at intersection bravo on 31L, the tower cleared me for a fast taxi on the active, but asked me to expedite as there was traffic on a long final (an MU-2). The pilot confirmed with tower his intention to taxi the remaining length of the runway and exit at the FAR end, but that the piplot would do his best to expedite.
After a few thousand feet of the brake burn-in, but well short of the end of the runway, the tower asked the pilot to exit the runway. At that point the brakes were too soft from intentional overheating to stop the airplane, so I missed the first taxiway. A second, urgent call from the tower to exit immediately was received, so the pilot made a best effort to turn off at the next taxiway, kilo. The brakes were still overheated as the pilot had not yet begun any sort of cool down phase, so it was difficult to negotiate this turn. As the pilot came around, he could see that I was going to overshoot, so he simply aimed between two taxi lights, assuming that it would result in an overrun onto the grass. The aircraft is equipped with big tires for off-field operation.
The pilot did not realize was that all of the turf in this area was new, and very soft. As the pilot rolled off of the taxiway at a fairly low speed, the nose gear sank into the soft ground, as did the right main to a lesser degree, with the aircraft coming to a complete stop in less than 10 seconds.
As the FAA filed it
AS STATED BY THE PILOT:
THE PILOT CONTACTED HILLSBORO GROUND, LETTING THEM KNOW OF MY INTENTIONS TO DO A BRAKE BURN-IN, AND THAT THIS WOULD BE A GROUND OPERATION ONLY, AND WE WOULD EXIT THE RUNWAY TAXI BACK TO PARKING ON COMPLETION. I REQUESTED ACCESS TO THE NON-ACTIVE RUNWAY BUT WAS DENIED, AND INFORMED I WOULD HAVE TO USE THE ACTIVE RUNWAY.
AFTER WAITING AT INTERSECTION BRAVO ON 31L, THE TOWER CLEARED ME FOR A FAST TAXI ON THE ACTIVE, BUT ASKED ME TO EXPEDITE AS THERE WAS TRAFFIC ON A LONG FINAL (AN MU-2). THE PILOT CONFIRMED WITH TOWER HIS INTENTION TO TAXI THE REMAINING LENGTH OF THE RUNWAY AND EXIT AT THE FAR END, BUT THAT THE PIPLOT WOULD DO HIS BEST TO EXPEDITE.
AFTER A FEW THOUSAND FEET OF THE BRAKE BURN-IN, BUT WELL SHORT OF THE END OF THE RUNWAY, THE TOWER ASKED THE PILOT TO EXIT THE RUNWAY. AT THAT POINT THE BRAKES WERE TOO SOFT FROM INTENTIONAL OVERHEATING TO STOP THE AIRPLANE, SO I MISSED THE FIRST TAXIWAY. A SECOND, URGENT CALL FROM THE TOWER TO EXIT IMMEDIATELY WAS RECEIVED, SO THE PILOT MADE A BEST EFFORT TO TURN OFF AT THE NEXT TAXIWAY, KILO. THE BRAKES WERE STILL OVERHEATED AS THE PILOT HAD NOT YET BEGUN ANY SORT OF COOL DOWN PHASE, SO IT WAS DIFFICULT TO NEGOTIATE THIS TURN. AS THE PILOT CAME AROUND, HE COULD SEE THAT I WAS GOING TO OVERSHOOT, SO HE SIMPLY AIMED BETWEEN TWO TAXI LIGHTS, ASSUMING THAT IT WOULD RESULT IN AN OVERRUN ONTO THE GRASS. THE AIRCRAFT IS EQUIPPED WITH BIG TIRES FOR OFF-FIELD OPERATION.
THE PILOT DID NOT REALIZE WAS THAT ALL OF THE TURF IN THIS AREA WAS NEW, AND VERY SOFT. AS THE PILOT ROLLED OFF OF THE TAXIWAY AT A FAIRLY LOW SPEED, THE NOSE GEAR SANK INTO THE SOFT GROUND, AS DID THE RIGHT MAIN TO A LESSER DEGREE, WITH THE AIRCRAFT COMING TO A COMPLETE STOP IN LESS THAN 10 SECONDS.