AN AIR AMBULANCE HELI TAKES OFF FROM A PART TIME ATCT ARPT WITHOUT CALLING FOR TKOF CLRNC AT ZZZ; US.

Narrative

WAS ON THE GND AT ZZZ ARPT GETTING FUEL. AFTER REFUEL; RECEIVED ANOTHER CALL (THIS IS A MEDICAL ACFT). I WAS TO RESPOND BACK TO THE HOSPITAL 1/2 MI W OF ZZZ AND PICK UP MY CREW AND RESPOND TO A PERSON HIT BY A CAR. DURING RUN-UP; I FELT SOME PRESSURE TO GET GOING QUICKLY. I CALLED MY COMS CTR AND TOLD THEM I WAS DEPARTING ZZZ BACK TO THE HOSPITAL TO PICK UP MY CREW. I WAS HAVING MIKE PROBS AND IT TOOK ME SEVERAL ATTEMPTS TO CONTACT MY COMS. I THOUGHT 'JUST GET TO THE HOSPITAL' AND 'I'LL FIX THE MIKE WHILE THE CREW IS LOADING.' I DEPARTED WITHOUT A CLRNC; THINKING THAT THE TWR WAS CLOSED. IN FACT; THE TWR WAS STILL OPEN. TIME WAS PA40 AND THE TWR CLOSES AT PN00. I WAS UNABLE TO CONTACT TWR DUE TO THE MIKE PROB. THERE WAS NO TFC. I DID A CLRING TURN AND DEPARTED. ON FINAL AT THE HOSPITAL; MY MIKE BEGAN TO WORK AGAIN AND I DID SPEAK WITH THE TWR. I SHOULD HAVE SLOWED DOWN AND FIXED THE MIKE BEFORE TKOF FROM ZZZ. I SHOULD HAVE BEEN BETTER AWARE OF THE TIME AND SHOULD HAVE REALIZED THE TWR WAS STILL OPEN. I NEED TO SLOW DOWN AND DO IT ALL BY THE BOOK. DON'T LET THE EXCITEMENT OF THE SIT TAKE OVER! I WILL NOT LET THIS HAPPEN AGAIN.