Date: 201901
Local time of day: 1201-1800
Location: ZZZ.TRACON, US
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class C ZZZ
ATC / advisory: TRACON ZZZ
Reporter organization: Government
Reporter function: Approach
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Situational Awareness
Second reporter function: Captain; Pilot Flying
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: Aircraft X was being vectored for RWY XX ILS approach in low IFR conditions. The altimeter at the time was approximately 29.20. The aircraft was assigned 2;000 feet; and I observed them descend through approximately 1;800 [feet] which is the MVA in the area they were in. At that point; I queried them about their altitude and restated their assigned altitude. The pilot(s) disputed what I indicated their altitude to be. I issued a low altitude alert without stating the MVA/MSA and a climb back to 2;000 [feet]. In my judgementm the pilots did not believe they were below their assigned altitude and I felt stating the relevant MVA/MSA would not hasten a climb back to a safe altitude. The aircraft descended to approximately 1;200 [feet] before leveling and climbing ultimately to 3;000 feet as I subsequently assigned. Once established in a climb; the pilot asked for the altimeter and stated they had programmed an altimeter setting of 30.20; rather than 29.20; into the aircraft. The aircraft was given the altimeter on initial contact; shortly after when I read them the latest METAR (the ATIS was out of date on initial contact); and the altimeter was broadcast a third time when I announced the new hourly ATIS code while the aircraft was on my frequency. To my recollection; the aircraft did not read back the altimeter setting at any point. I did not issue a Brasher warning. My perception was the pilots were rattled (they asked me to repeat their assigned heading three times) and I felt adding the perceived threat of a pilot deviation would not help them safely navigate an instrument approach in low IFR weather. Local Management was made aware of the incident while the aircraft was still on final approach. There was adequate time to issue a Brasher warning on local or ground control frequencies and they chose not to or failed to do so. I can think of no air traffic control mechanism that will ensure aircrews properly program their equipment. I did not use exactly correct phraseology in my low altitude alert.


Second narrative: We entered (and read back) different altimeter setting from Approach 1 that differed from Approach 2 actual altimeter. Approach 2 asked for altitude report which differed from our actual altitude. They told us to climb to higher altitude at the same time I queried as to the local altimeter; suspecting we had a different setting. This turned out to be true resulting in approximately being 400-500 feet below assigned altitude. We reset our altimeters and climbed back to correct altitude and completed the approach and landing.


Synopsis: Approach Controller and air carrier flight crew reported incorrect altimeter setting in aircraft which led to an airspace violation due to the proximity to the ground.