COMMUTER ACFT LANDS ON RWY NOTAMED CLOSED WITH STIPULATIONS.

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I WAS THE PF INBOUND TO KMLI AND THE CAPT AS THE PNF PICKED UP THE ATIS FOR KMLI AND RELAYED THE INFO TO ME. THERE WAS NO MENTION OF RWY 31 BEING UNUSABLE. WE SAW THE ARPT BEACON AND AS WE CONTINUED TOWARDS KMLI; WE FOUND OURSELVES LINED UP FOR RWY 31. WE REQUESTED A STRAIGHT-IN VISUAL TO RWY 31 FROM THE QUAD CITIES APCH CTLR. WE RECEIVED CLRNC FOR THE VISUAL APCH TO RWY 31 AND WE WERE INSTRUCTED TO CONTACT THE TWR. THE CAPT CONTACTED QUAD CITIES TWR AND WE WERE GIVEN CLRNC TO LAND ON RWY 31. AFTER WE LANDED; WE WERE ASKED TO PHONE THE TWR. THE CAPT PHONED THE TWR AND THE TWR CTLR SAID WE WERE NOT APPROVED FOR LNDG ON RWY 31 AND HE REFED A NEW NOTAM FOR KMLI. IN OUR WX PACKAGE THERE WAS A NEW NOTAM THAT READ 'MLI 31 CLOSED LNDG ACR PS ATX PS ACFT WITH APCH SPD OVER 120 KTS.' HAVING READ THIS PRIOR TO LNDG; WE APPEARED LEGAL SINCE WE WERE GIVEN CLRNC TO LANDON RWY 31 AND OUR APCH SPD IS NOT OVER 120 KTS. I BELIEVE THIS POSSIBLE INFRACTION COULD HAVE BEEN PREVENTED BY THE FLC'S BETTER UNDERSTANDING OF WHAT THE POORLY WRITTEN NOTAM INTENDED TO SAY. WE INTERPRETED THE NOTAM AS RWY 31 BEING CLOSED TO ACR/ATX ACFT WITH AN APCH SPD OVER 120 KTS; NOT AS BEING CLOSED TO ALL ACR AND ATX ACFT. A MENTION OF THE RWY NOT BEING USABLE IN THE ATIS WOULD HAVE ALSO PREVENTED THIS INFRACTION. AS A LAST RESOURCE; NOTIFICATION FROM THE APCH OR FROM TWR SAYING THE RWY IS NOT USABLE TO ACR ACFT; BUT INSTEAD THEY GAVE US CLRNC FOR BOTH THE VISUAL APCH TO RWY 31 AND LNDG CLRNC TO RWY 31. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT HE CALLED HIS DISPATCH OFFICE FOR CLARIFICATION OF THE NOTAM. DISPATCH INDICATED THAT THE PS EQUALS A PLUS SIGN WHICH THE EQUIP CANNOT MAKE. IN SPEAKING WITH CTLR; HE INDICATED IT WAS HIS INTERPRETATION THAT THE NOTAM MEANT NO ACR ACFT COULD USE THE RWY. IF THIS WAS THE CASE; RPTR WONDERS WHY HE WOULD BE CLRED TO LAND ON THAT RWY??? RPTR WAS ADVISED OF THE HOT LINE NUMBER TO FURTHER FOLLOW UP ON THE PROB. SUPPLEMENTAL INFO FROM ACN 292915: WE ASKED FOR A VISUAL STRAIGHT-IN TO RWY 31. THIS WAS APPROVED BY APCH CTL. CHKING IN WITH TWR; A LNDG CLRNC WAS OBTAINED FOR RWY 31. ANY ONE OF SEVERAL THINGS WOULD HAVE PREVENTED THIS: THE FLC'S BETTER UNDERSTANDING OF A POORLY WRITTEN NEW NOTAM; A MENTION OF THE CHANGE ON THE ATIS OR; AS A LAST LINE OF DEFENSE; NOTIFICATION FROM APCH OR TWR CTL; THE APPROVING AGENCY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS ADVISED THAT 'PS' IS A LEGAL FAA ABBREVIATION FOR 'PLUS;' THEREFORE; HIS ACR ACFT WITH AN APCH SPD LESS THAN 120 KTS WAS INCLUDED IN THIS PROSCRIPTION.