ZOB Controller described a potential conflict event during late night combined operations when an IFR departure from FNT failed to comply with assigned call for release instructions and departed with traffic on final.

Narrative

I was working the late night shift; at Sector 12; which had LAN; FNT; ECK; and GAM combined with FNT Approach's airspace. An A310 called for clearance from FNT. I issued the clearance; acknowledged the read back and asked the pilot when he would be ready for departure. He stated that he would be ready for push back in 10 to 20 minutes. I advised him to hold for release; advise when ready for departure. Shortly after that; I took a hand off on a B717 inbound to FNT. About 10 minutes after I issued the clearance to the A310; they called requesting push back. I advise them that I did not have the authority to issue a push back clearance; but that they should advise on the taxiway at the end of the runway when they were ready for departure. I did not specify again the hold for release. A couple of minutes later; one of the A310 pilots came on the frequency and said; 'Attention all aircraft; A310...' and then stopped transmitting. I thought they were attempting to transmit on the CTAF that they were taxiing on the field and realized that they were on the wrong frequency. But I also wondered if maybe they were getting ready to depart. I thought about calling them to make sure; but decided not to; figuring that they knew the correct procedures to follow; and I had told them to hold for release and call me at the end. The B717 said that they had the field in sight about 12 miles out; I cleared them for a visual and put them on the CTAF. As the B717 was about 6 miles from the airport and descending through about 3;500; the A310 called airborne! I quickly advised them of the traffic. The pilot stated that he had 'visual contact' with the B717 and had talked with him on the CTAF. I reminded them that they were supposed to advise me on the taxiway; short of the runway when they were ready. The pilot apologized and said that he was not familiar with the procedures and had forgotten to advise me. I had them IDENT and RADAR identified them. I do not believe that they were ever within 5 miles of each other; but it was close. Recommendation; first of all; I have always believed that we should not be taking any Approach Control airspace on late night shifts because our RADAR coverage is very poor. The Approach Controls can provide a much better service than we could ever hope to. Secondly; the 2 pilots on board the A310 did not seem familiar with the procedures used when operating at an airport with a center providing the service. Lastly; I should trust my instincts more often. When I wondered if they were getting ready to depart; I should have called them on the frequency to make sure.