LR-45 FLT CREW RETURNED TO LAND AFTER L ENG SHUTDOWN AND SUBSEQUENTLY LEARNED THAT THE FAA THOUGHT THE ACFT HAD BEEN OPERATING IN A NON-AIRWORTHY STATUS SINCE RETURN FROM MAINT.

Narrative

WE WERE CLBING THROUGH FL230 ON THE WAY TO FL270 ENRTE TO ZZZ1. WE HAD JUST SWITCHED TO CENTER AND WERE CLRED TO CLB TO FL270. I WAS THE FO; PF. AFTER THEY CLRED US TO FL270; AS THE AUTOPLT WAS ALREADY ON; I DIALED IT IN AND WENT BACK TO LOOK AT THE ENG INSTS AND THE L ENG ITT (INTERSTAGE TURBINE TEMP) WAS RUNNING AWAY ON US AND HAD GONE RED AND ABOVE 1000 FT. I QUICKLY REACHED OVER AND BROUGHT THE PWR WAY BACK ON THAT ENG AND ALERTED THE CAPT ABOUT IT. AS I BROUGHT THE PWR BACK THE ITT CAME BACK DOWN; BUT NOT AS FAR AS IT SHOULD; SO WE DECIDED TO BRING THE PWR BACK UP A LITTLE JUST TO SEE WHAT HAPPENS. AS WE DID; THE ITT CLBED RAPIDLY BACK UP; SO WE MADE THE DECISION TO SHUT THE L ENG DOWN. WE TOLD CENTER WE JUST HAD TO SHUT THE L ENG DOWN AND NEEDED TO RETURN TO ZZZ AND WOULD LIKE 'TO DECLARE.' THEY ASKED US THE SOULS ON BOARD; FUEL ON BOARD; AND IF WE NEEDED ASSISTANCE. WE SAID WE WOULD LIKE THE FIRE TRUCKS STANDING BY. THE CAPT RAN THE APPROPRIATE CHKLISTS AND GOT US SET UP FOR A SINGLE ENG LNDG WHILE I FLEW THE AIRPLANE AND HANDLED THE RADIOS. WE RETURNED TO ZZZ WITHOUT INCIDENT AND THE FIRE TRUCKS FOLLOWED US IN AND ASKED IF EVERYTHING WAS OK; WHICH IT WAS; AND THEN LEFT. NOW IS WHERE THE PROB BEGINS. BACK IN AUGUST; WE HAD OUR LEAR 45 DO SOME FLT TESTING FOR AN ONGOING PROB WE HAVE BEEN HAVING WITH THE ACFT. THEY PUT THE AIRPLANE ON AN EXPERIMENTAL CERTIFICATE SO THEY COULD DO WHAT THEY NEEDED TO. WHEN THEY WERE DONE WITH THE FLT TESTING; WE WENT BACK TO PICK UP THE ACFT. THE FAA CALLED TODAY INVESTIGATING THE INFLT ENG SHUTDOWN OCCURRENCE AND SAID THE AIRPLANE WAS STILL ON AN 'EXPERIMENTAL CERTIFICATE.' WE SAID 'WHAT!?' THEY SAID IF IT WAS THAT; WE WOULD BE VIOLATED FOR EVERY TKOF AND LNDG THAT WAS DONE SINCE THEN; WHICH WOULD BE AROUND 100. WE CLRLY HAVE THE LOGBOOK ENTRY AND RELEASE FROM THEM RETURNING THE ACFT BACK TO REGULAR AIRWORTHINESS AND THEY ALSO GAVE US OUR AIRWORTHINESS CERTIFICATE BACK. I THINK IT IS JUST A MATTER OF PAPERWORK NOT FINDING THE CORRECT PATH. WE KNOW WHAT OUR AIRFRAME LOGBOOK SAYS AND ARE NOT CONCERNED. I LEARNED FROM THIS TO REALLY DOUBLECHK EVERYTHING WHEN YOU ACCEPT AN ACFT BACK AFTER MAINT. EVERYBODY IS HUMAN AND MAKES MISTAKES; THE PROB IS THAT THEIR MISTAKES CAN COST YOU YOUR LICENSE OR A VIOLATION.