Date: 202010
Local time of day: 1801-2400
Location: ZZZ.Tower, US
Light: Daylight
Aircraft: Small Aircraft
Operator: Personal
Operating under: Part 91
Flight plan: IFR
Mission: Personal
Flight phase: Descent
Airspace: Class D ZZZ
ATC / advisory: Tower ZZZ
Second aircraft: Small Transport; Low Wing; 2 Turbojet Eng
Reporter organization: Government
Reporter function: Local
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Confusion; Workload; Training / Qualification; Situational Awareness
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Airspace Structure; Human Factors; Procedure
Primary problem: Procedure

Narrative: I was on the Local Control position and traffic was light. Initially coordinated with Approach Control; Aircraft X was approved for a late frequency change to tower as it would be number two behind Aircraft Y. With this coordination I was expecting Aircraft Y to be first followed by Aircraft X; but Aircraft X came to tower first approximately two miles north of the VOR. At this time Aircraft Y was still outside of 10 miles from the field and I cleared Aircraft X to land. Approximately on a two mile final; Aircraft X requested a left 360 due to him still being too high. I asked Aircraft X to verify they had the field in sight; but there was no response. The CIC (Controller in Charge) responded to me that he requested a left 360. I observed outside the window that Aircraft X was in his left turn executing a 360 prior to my approval. At this time; Aircraft Y was approximately 8 to 9 nm north of the field. Immediately we coordinated this with Approach letting them know Aircraft X was too high and needed to execute a 360 and was instructed a missed approach; but at the time Aircraft X was just completing his left 360 with Aircraft Y approaching approximately a five mile 5 mile final. With coordination for the missed approach and Aircraft X nearly completing a 360; I advised Aircraft X of the inbound traffic and instructed him to turn northbound and to contact Approach Control. During this time; separation between both aircraft was lost.I believed what led to this event was initially the miscommunication between tower and approach with the sequence of the inbound. I was preparing for Aircraft X to be shipped late behind Aircraft Y; but there wasn't any communication that the sequence has changed.  We are a VFR tower and I haven't had the experience with dealing or the knowledge on how to deal with such a situation; but this incident was definitely a learning experience. I believe Aircraft X also went below the MVA; which is 3000 ft. and they were at 2000 ft. I have not seen or experienced this situation.In my opinion; I believed this occurred because of the miscommunication and both the lack of knowledge from both my position and the pilot in command of Aircraft X. I am aware of the required separation and that IFR procedure. With the last minute decision of Aircraft X to execute the 360 on his own led me to approve his request when I know he must break of the approach and cancel IFR; otherwise they should execute the published missed approach. I would recommend for this VFR tower to include the rare; but possible situations that could occur during training. This situation was unexpected and rare; but it did happen. I would recommend to go over the separation needed for IFR aircraft and the MVAs surrounding the airport. Prior to this situation; I should've noted the altitude; this would have prevented the situation.




Synopsis: Local Controller reported an aircraft broke off its approach into a left 360 without approval below MVA to conflict with inbound traffic.