Cessna 310/T310C
Air Traffic Control Fully Certified
Communication Breakdown; Human-Machine Interface; Situational Awareness
ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
General None Reported / Taken
ATC Equipment / Nav Facility / Buildings; Human Factors; Procedure
Aircraft X departed CGI airport pop up wanting an IFR clearance to ZZZ at 030. I cleared him and climbed him to his requested altitude; not thinking about the MIA northeast of CGI being 035 because of the towers. I was concerned more about his speed and losing 5 miles separation with an arrival to CGI from the northwest and getting him to report the field in sight and then he did and cancelled IFR. I was busy tending to other traffic and the MSAW alert never went off. The MSAW normally would go off instantly when the aircraft departed northeast bound because it's a large area that has to be missed. The tower had gone home almost an hour before. They would have asked about it also. When he was roughly 1 or 2 miles east of the tallest tower; a controller at ZKC called me and asked if the aircraft was really at 030. I realized immediately what had happened and I said yes. It was too late to to anything different with the aircraft at that point. He was missing the towers. But after I hung up the landline I realized the MSAW never was flashing telling me he was at an unsafe altitude. I called the other controller back and asked him if his MSAW was going off. He said right after he had called me it started to go off. I fully accept responsibility for making the mistake of not realizing 030 was an unsafe altitude on his route. However; the MSAW should have been going off for several minutes and this is a safety issue. I have been working at this facility for almost 30 years and have never even known it would be possible for me to disable the MSAW and I would never try to do such. The first thing the tech ops guy asked my supervisor was if it was turned on. Like I said; I didn't know it could be turned off on an individual sector any more than you could disable Conflict alert. You can suppress it individually for and aircraft but it is always on as far as I know.
ZME Controller reported MSAW alert failure contributed to improperly issuing an IFR clearance to aircraft below the MVA.