Date: 201602
Local time of day: 1201-1800
Location: K90.TRACON, MA
Light: Daylight
Aircraft: Small Aircraft; High Wing; 1 Eng; Fixed Gear
Operator: Air Taxi
Operating under: Part 91
Flight plan: VFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class E K90
ATC / advisory: TRACON K90
Reporter organization: Government
Reporter function: Approach
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Human-Machine Interface; Situational Awareness; Training / Qualification
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Separated Traffic
Contributing factors: ATC Equipment / Nav Facility / Buildings
Primary problem: ATC Equipment / Nav Facility / Buildings

Narrative: Shortly after taking position I was monitoring Aircraft X level at 3500 feet and noticed the Mode C drop to 3100 feet. I believed he was in a VFR descent so I didn't question it; until 2-3 minutes later when he stated he was starting his descent from 3500 feet; and he was still indicating 3100 feet. Then [a separate aircraft] called up for VFR advisories and reported level at 2000 feet; his Mode C indicated 1600 feet. At the same time [another aircraft] called for advisories. After radar identifying both aircraft and confirming local altimeter of 30.29 both aircraft's Mode C indicated 400 feet lower than actual altitude. I then took a handoff [who] checked in at 3500 feet verified Mode C.  They showed 3500 feet. Than their Mode C indicated 3100 feet. I verified with the pilot he was still level at 3500 feet and he confirmed. At this fourth aircraft confirmation I informed the Controller In Charge (CIC) that the radar was not useable due to the Mode C jumping 400 feet.  We were operating in CENRAP due to our radar out of service. My radar display was indicating a normal function of local altimeter setting of 30.39. After a couple of hours the CIC received a call stating that ARTCC was using a altimeter setting of 29.92 and this was been sent to the CENRAP display and this pressure change was causing the 400 foot discrepancy and the problem had been fixed.There is a general lack of understanding on the use of CENRAP; its functions and limitations at the controller level both from the TRACON and probably at ARTCC. How the radar presentation can use an altimeter setting other than the one displayed on the scope is a serious factor that none of the local controllers knew about. A vast majority of the controllers don't know which radar sensor inputs are being displayed when in CENRAP or how different radar sensor outages can effect CENRAP. There should also be some protections to prevent ARTCC from changing altimeter settings without notifying underlying facility when a TRACON is using CENRAP.




Synopsis: Due to a radar outage; K90 TRACON was receiving their radar information from ZBW ARTCC. The ARTCC has to make a data entry of the current altimeter for the TRACON airspace yet the ARTCC did not realize this requirement. Therefore the Mode C readouts of aircraft in the TRACON airspace were 400 feet off from the actual altitude of the aircraft.