Narrative:

Aircraft X was being vectored for runway 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The aircraft was given the altimeter on initial contact; shortly after when I read them the latest metar (the ATIS was out of date on initial contact); and the altimeter was broadcast a third time when I announced the new hourly ATIS code while the aircraft was on my frequency.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.

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Original NASA ASRS Text

Title: 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.

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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.