Date: 202111
Local time of day: 1201-1800
Location: ZZZ.TRACON, US
Flight conditions: IMC
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class C ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Altimeter
Component problem: Improperly Operated
Reporter organization: Government
Reporter function: Approach
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: ATC Issue All Types; Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Automation Air Traffic Control; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Overcame Equipment Problem; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Contributing factors: Human Factors; Aircraft
Primary problem: Human Factors

Narrative: While working arrival/approach; Aircraft X was given a vector and descent for final approach course Runway XXR at ZZZ. At that time; the altimeter was 28.86. Aircraft X was on a base turn and assigned 2;000 ft. Aircraft X was observed descending below 2;000 ft. and low altitude alert/alarm triggered. Lowest observed altitude was 1;300 ft. A low altitude alert was issued to the aircraft. They asked for confirmation that the altimeter was 28.86 and said that they were level at 2;000 ft. I confirmed that the altimeter was 'low' at 28.86 and instructed them to stop altitude squawk. I am unsure if this was a faulty Mode C; or if the pilot had the incorrect altimeter setting.


Second narrative: Duty day started with a XA:00 show to ZZZ1 for a 3.5hr flight to ZZZ. Departure corridor had several pockets of moderate turbulence and the first two hours of flight was spent searching for an altitude with less than constant light and occasional moderate chop. The METAR and D-ATIS for ZZZ showed 075SCT 090OVC conditions with a TAF forecasting deteriorating vis and -SN. The altimeter setting was 28.86 but no low pressure warning caveats existed in D-ATIS. The CA (Captain) was PF (Pilot Flying) and briefed an ILS to [Runway] XXR with expectation to most likely break out of weather on base. The aircraft was cleared to descend via ZZZZZ4 and alerter was set to 5;000 ft. Passing thru transition level; the pilot's descent flows were interrupted by Center who wanted a relay call to a lost comm aircraft on Guard. The attempt was unsuccessful and the aircraft was pushed to approach. Approach mentioned the ILS XXR was out and offered the ILS XXL or the RNAV XXR. The pilots chose the latter and built and briefed the new approach. In lieu of the request and approach change; the descent checklist was not completed until the low teens (18K is standard). ATC instructed aircraft to delete the arrival speeds (e.g. 210 [kts.] at ZZZZZ1) and descend to 2;000 ft. The PF took the hint and expedited the descent without slowing. Next came the close in base turn and the approach clearance (2;000 ft. until established). The FAF altitude is 1;600 ft. Reaching 2;000 ft. and established inbound ATC issued a command to check altitude immediately. No GPWS flags or warning were present. PF disconnected autopilot and asked PM (Pilot Monitoring) to confirm correct altimeter setting. Correct altimeter setting was 28.86. Pilots determined aircraft altimeters were incorrectly set to 29.86. The error was corrected and visual contact with terrain and the airport was also made. PF stated 'Runway Landing' and maneuvered aircraft to correct GP (glidepath). ATC asked pilots to confirm established on approach. Once confirmation was made ATC instructed pilots to contact ZZZ Tower. No instructions were given to contact ATC by phone. Tower cleared aircraft to land. Final approach and landing were uneventful.After an early show and a long battle with turbulence and headwinds the pilots were experiencing some early signs of fatigue. The initial arrival brief was not rushed but ample acknowledgement of the unusual altimeter setting was not given. The final descent was a bit chaotic given the ATC request; the approach change and the slam dunk out of arrival. Unfortunately; their were not any intermediate level offs during the descent where an opportunity to discover the discrepancy at a higher altitude could have occurred. The ceiling and vis were lower than expected and any early warning (by visual cues) that the aircraft might be lower than it should be was not afforded. Bottom line: The correct altimeter was not set and confirmed effectively during the descent checklist. The pilots attribute this error to confirmation bias due to the abnormally low setting and to cognitive capture due to a simultaneous approach change (ILS to RNAV) and a request from ATC to contact a lost comm aircraft while passing through transition altitude.The pilots operated two subsequent flights on this day and noticed that a low baro caveat was added to D-ATIS and ATC was stating 'Low' in front of all altimeter read backs. This was very helpful. After significant debrief the pilots have reaffirmed the notion that 28.xx settings are a very real threat in places like State; that transition altitude is a sacred moment to ensure safety and success in the subsequent approach; and that external requests (even outside of a critical phase of flight) are sometimes best politely declined.


Synopsis: Controller and First Officer reported incorrect altimeter setting resulted in an alert for descending below MVA.