CPR X LANDED ON OCCUPIED RWY. NON ADHERENCE TO ATC INSTRUCTION GAR HAD LTSS FROM TWR COMMANDER. SYS ERROR. TWIN COMMANDER NON ADHERENCE TO ATC INSTRUCTION.

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G4 WAS FLOWN UNDER IFR FLT PLAN; VNY TO SLC. 2 CREW; NO PAX. NORMAL FLT TILL SLC TWR CLRED ACFT FOR A HIGH VISUAL APCH FROM A CLOSE-IN BASE LEG TO RWY 17 AT SLC; FROM AN ALT OF 9000 FT. OUR ACFT WAS FOLLOWING A TWIN COMMANDER ON SHORT FINAL TO RWY 17. FINAL APCH SQUARED AT HIGH RATE OF DSCNT PER SLC TWR INSTRUCTIONS. ON 2 MI FINAL OUR ACFT CLRED TO LAND; WE WERE FLYING 145-150 KTS. TWIN COMMANDER AHEAD OF US LANDED ON RWY 17 AND MISSED SEVERAL TURNOFFS TO THE L. TWR INSTRUCTED HIM TO EXIT NEXT TXWY JUST AS WE WERE INITIATING FLARE NEAR RWY THRESHOLD. THE TWIN COMMANDER WAS TOLD TO EXPEDITE OFF RWY WHEN WE WERE QUITE CLOSE TO TOUCHDOWN AT FAIRLY HIGH RATE OF DSCNT. TWIN COMMANDER WAS NEAR TXWY TURNOFF WHEN SLC TWR CALLED 'GAR.' WE WERE ALREADY COMMITTED TO LAND ON THE G4 AND FO ADVISED ATC WE WERE COMMITTED TO LAND. WE TOUCHED DOWN SECONDS LATER AND INITIATED FULL THRUST REVERSE AND BRAKING SLOWING TO LESS THAN 20 MPH BY THE TIME WITHIN 600-800 FT OF TWIN COMMANDER WHICH SLOWLY; BUT FINALLY; EXITED TO TXWY L OF RWY 17 FROM A HIGH RATE OF DSCNT FINAL APCH AS A RESULT OF VECTORING BY ATC TO A HIGH CLOSE-IN BASE LEG. THE PIC JUDGED SUFFICIENT RWY REMAINING TO EXECUTE A SAFE LNDG IN THE REMAINING APPROX 3500-4000 FT ON RWY 17 AT SLC. PIC'S DECISION TO LAND SAFELY WAS PARTLY AS A RESULT OF JUDGING THAT A SAFE GAR FROM SUCH LOW ALT NEAR VREF AT A RELATIVELY HIGH FINAL DSCNT RATE WITH FULL FLAPS WAS NOT A SAFE MANEUVER AND COULD HAVE RESULTED IN STRUCTURAL DAMAGE TO THE ACFT IN A HIGH ROTATION ANGLE TO ACHIEVE CLB IN A LOW TAIL POS OF THE ACFT RELATIVE TO THE GND. ATC NEVER REVOKED OUR CLRNC TO LAND AND THEIR LATE GAR ADVISORY SEEMED PRECAUTIONARY; WHILE TWR ATC WAS WORKING TO COMMUNICATE WITH THE TWIN COMMANDER AHEAD OF US TO EXIT ON THE TXWY. THE PIC NEVER COMPROMISED THE SAFETY OF OUR ACFT OR ANY OTHER ACFT IN THIS LNDG PHASE AT SLC. THE TWIN COMMANDER FAILED TO FOLLOW PROPER RWY EXIT PROCS AFTER BEING REQUESTED TO EXPEDITE TAXIING OFF SLC RWY 17. SUPPLEMENTAL INFO FROM ACN 297325: WE WERE ON 2 MI FINAL AND TWR CLRED US TO LAND. THE TWIN MISSED SEVERAL TURNOFFS AND WAS ADVISED BY TWR TO EXPEDITE TO THE NEXT LEFT. OUR POS WAS SHORT FINAL; PIC/PF WAS SLOWING FROM VREF PLUS 10 TO VREF AND TRANSITIONING TO LNDG FLARE. I ROGERED THE XMISSION. LNDG CLRNC WAS NOT CANCELED. I ADVISED PIC AND EXPECTED THE GAR PROC TO COMMENCE BUT HE ADVISED ME HE WAS COMMITTED WHICH I RPTED TO THE TWR. AFTER OBTAINING TAXI CLRNC TO FBO I SWITCHED TO FBO FREQ TO CHK ON PAX; QUICK TURN; ETC. PIC HAD ATC COMS DURING THIS TIME. UPON TAXI OUT FOR RETURN TO VNY; GND REQUESTED WE CONTACT THEM VIA LANDLINE. I TRIED CELLULAR AND SATCOM BUT APPARENTLY THEY HAD GIVEN ME A BAD NUMBER. I CALLED THEM AFTER RETURNING TO VNY AND WAS QUERIED BY THE TWR SUPVR ABOUT THE CHAIN OF EVENTS AND WAS ADVISED THEY WOULD BE REVIEWING THE TAPES.