Date: 202005
Local time of day: 1201-1800
Location: ZBW.ARTCC, NH
Flight conditions: Marginal
Aircraft: Skyhawk 172/Cutlass 172
Operator: Personal
Operating under: Part 91
Flight plan: IFR
Mission: Personal
Flight phase: Final Approach
Airspace: Class E ZBW
ATC / advisory: Center ZBW
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Human-Machine Interface; Situational Awareness
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance
Contributing factors: Airspace Structure; Human Factors; Weather
Primary problem: Human Factors

Narrative: I was working on one scope with light traffic. C172 was an IFR arrival into a non towered airport which does not have instrument approaches. Weather station at the field reported clear but PIREPs and weather stations nearby reported scattered clouds in the 6000 ft. to 7000 ft. range. I observed a VFR aircraft at a Mode C indicated altitude of 6;500 ft. about 10 miles south of the airport so I believed the pilot would be able to pick up the field visually. C172 requested a descent from a cruise altitude of 7000 ft. I cleared the aircraft to 6000 ft. and advised this was the minimum IFR altitude. The pilot rogered me and indicated he would cancel IFR when he saw the field. I observed the aircraft descend to 6000 ft.During subsequent scans; I observed C172s altitude vary between 5;800 ft. and 6;200 ft. These variations are within mode C error tolerance so I did not query the pilot. I believed the pilot may have been experiencing turbulence; but radio coverage is spotty in that area at 6;000 ft. and I did not wish to increase the pilot's workload during a critical phase of flight so I did not solicit a PIREP. I did not have any turbulence PIREPSs in the area; but I also did not have any aircraft at low altitude. Gusty and variable winds at the various weather stations coupled with mountainous terrain in the area usually indicate low-level turbulence.On a subsequent scan; I observed C172's mode C altitude indicate 5;500 ft. I immediately keyed up and issued a low altitude alert to C172; but transposed the 'Low Altitude Alert' and callsign when issuing the alert. C172 rogered my warning; and I observed the aircraft at 5;700 ft. on the next radar hit; and within mode C conformance for 6;000 ft. on the next hit. I did not query the pilot as to his actual altitude at this point; as it was more important to get the alert out. I was therefore unable to determine if the pilot had deviated from his clearance; or if he had experienced a downdraft; or if there had been a bad radar hit on the Mode C. I only observed one Mode C readout at 5;500 ft..A few minutes later; the pilot reported the field in sight and requested to cancel IFR. Since I did not have concrete evidence of a pilot deviation and because I wished to avoid more distraction than necessary for a pilot about to conduct a VFR approach to a short runway in gusty wind conditions; I did not issue the pilot deviation phraseology to the aircraft. I instructed the pilot to squawk VFR and change frequencies; and wished him a nice day. The pilot acknowledged the instruction and I observed the aircraft squawk VFR and descend toward the airport until radar contact was lost at an expected altitude due to the mountains in the area.I was unable to determine why the aircraft indicated 5;500 ft. for one radar hit. I issued a low altitude alert immediately; but my phraseology was incorrect as I said the callsign before 'Low Altitude Alert'. This happened because safety alerts are a rare instance where the callsign is not the first thing in the phraseology. They are also thankfully rare so it is not something said often. Recurrent training has emphasized this in the past; but I question if the benefits outweigh the difficulty of getting the phraseology correct in an already potentially stressful moment. It may be better to make the phraseology like most other instructions; with the callsign first.




Synopsis: A Center Controller reported they used incorrect phraseology to an aircraft which deviated from its assigned altitude and descended below the Minimum IFR Altitude.