Small Aircraft; Low Wing; 1 Eng; Fixed Gear
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 5
Communication Breakdown; Confusion; Situational Awareness
ATC Issue All Types; Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Aircraft; Chart Or Publication; Human Factors
Aircraft X checked on approximately 30 [miles] north [of] CUL airport requesting to descend. Marginal VFR to IFR conditions in airspace. Attempted to request type approach and verification from pilot. Pilot advised they could not hear me and switched radios. The pilot switched but was broken and unreadable and when the pilot called back; I attempted to switch to standby transmitter but the pilot could only get me '2x5'. I told the aircraft to proceed direct the IAF for the RNAV 18 approach at CUL. The pilot responded 'proceeding direct HGOOD; cleared RNAV 18 approach.' I chose not to respond to the pilot as the pilot seemed to be having difficulty possibly with radios or navigation with the weather; so I chose not to reiterate 'cleared RNAV 18 approach'. Moments later I observed the aircraft at 2;000 feet level direct to the IAF (not on a published portion of the approach). The MVA in the area is 2;500. I then pulled the approach plate to verify the crossing altitude at the IAF (2;100 feet) and pulled up the VFR chart located above the position to verify obstructions in the area. There were no obstructions on the route of flight above 800 feet which the aircraft was more than 1;000 feet above. I chose not to climb the aircraft and accepted the altitude as variance of what I thought was an appropriate altitude considering the following: 7110.65 4-8-5 Specifying Altitude:'Specify in the approach clearance the altitude shown in the approach procedures when adherence to that altitude is required for separation. When vertical separation will be provided from other aircraft by pilot adherence to the prescribed maximum; minimum; or mandatory altitudes; the controller may omit specifying the altitude in the approach clearance.'  Additionally; I was thinking of the example given in 4-8-1 for RNAV approaches of the 7110.65:'Aircraft 1 can be cleared direct to CENTR. The intercept angle at that IAF is 90 degrees or less. The minimum altitude for IFR operations (14 CFR; section 91.177) along the flight path to the IAF is 3;000 feet.'After the event a controller who had seen the altitude asked what was going on with the Aircraft X from earlier at 2000. I told him the situation and what I thought was an appropriate response. He told me that he does not believe that applied to this circumstance. I followed up with two additional more senior radar controllers who agreed that an altitude should be given but didn't know what 4-8-5 was in reference to. The next morning I asked a supervisor about the scenario and asked what 4-8-5 refers to; he showed me an approach plate for ORD with multiple crossing altitudes on the approach. I asked him how he knew that this is what that paragraph referred to and he just noted experience.I would recommend clarification of 4-8-5 in the 7110.65 to be more easily understood to which scenario this applies. Additionally; in the example given in 4-8-1 for RNAV application; the use of minimum IFR altitude is confusing as this altitude could be lower than an MVA for the area. I would change this to MVA; or DVA if applicable; to ensure better understanding that a lower MIA than MVA is not acceptable on an unpublished route.
EVV TRACON controller reported misunderstanding JO 7110.65 approach clearance procedure which resulted in aircraft being below the MVA.