Date: 201812
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: Marginal
Light: Daylight
Aircraft: Regional Jet 200 ER/LR (CRJ200)
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
ATC / advisory: TRACON ZZZ
Aircraft component: Pitot-Static System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Second reporter function: Pilot Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: After takeoff from ZZZ on Runway XXR; we received an EFIS COMP MON message for ALT and Airspeed. After comparing between the Captain (CA) PFD; Standby Instruments; and my PFD; we determined the CAs was the bad one. We then transferred the autopilot to my side and switched the transponder to my side. CAs side Airspeed and Altitude was off by as much as 50 knots airspeed and 300-400 feet on altitude. After switching autopilot and transponder to my side; I took the controls and radios. We believed this to be an ADC1 failure; and ran the appropriate QRH procedures which led to us selecting ADC2 for the CA. We continued on to [destination] until we heard from Maintenance and Dispatch on what they wanted to do. After contacting them via ZZZ Radio; they determined they wanted us to return to ZZZ. We informed ATC that we were not an emergency aircraft; but we needed to return to ZZZ due to a maintenance issue. After starting towards ZZZ; we determined we were about 1;500-2;000 LBS over max landing weight with our current fuel. This was complicated because we couldn't view our normal indications on the MFD that tell us what our landing fuel is and times. We asked for delay vectors from approach and they gave us multiple delay vectors at 7;000 feet while we tried to burn fuel. We eventually got down to max landing weight and safely landed on Runway XXR and taxied in. During the flight; we believed it to be an ADC1 failure. Once we were on the ground and Maintenance looked it over; their preliminary hypothesis was that it was water trapped in the CAs side pitot tube and inside the avionics bay. I don't know if that was the final result; because we left to take another plane and continue on with the flight.


Second narrative: Departing ATL RWY 27R; I was the Pilot Flying. During the initial climb; at approximately 300 feet AGL; the CA side PFD 'ALT' box began flashing and EFIS COMP MON caution message posted. I cross referenced the First Officer (FO) side PFD and the Standby Altimeter and noted the CA side altimeter was indicating 200-300 feet lower than the other 2 altimeters. Then; the CA side PFD 'IAS' box began flashing. Again; I cross referenced the FO side PFD and the Standby Airspeed indicator and noted the CA side airspeed was indicating 20 knots lower than the other two airspeed indicators. At this time; I suspected that the Air Data represented on my PFD was erroneous. We continued climbing and once we began cleaning up the aircraft above 1;000 feet AGL; I transferred the controls to the FO. We transferred the autopilot and the transponder to the FO side. I completed the QRH procedure for the EFIS COMP MON message. That procedure had us select the Air Data Source to ADC 2. That selection produced MACH TRIM and STALL FAIL caution messages. I completed the QRH procedures for those messages as well. Once the QRH procedures were complete; I contacted our Dispatcher through ZZZ Radio who conferred with Maintenance Control. All 3 parties agreed on an air return to ZZZ. We diverted back to ZZZ. On the descent into ZZZ; we discovered that our current weight would exceed the Max Gross Landing Weight. We elected to fly delay vectors from ZZZ Approach to burn off fuel so that we would land under the 47;000 LB limitation. We had no other issues. I'm not sure as to the exact cause of the flight instrument issues we had. The aircraft had not flown for 3 days prior to this flight. There was very light rain on the field at departure time. However; it's not my opinion that the amount of rain experienced would be enough to cause water to enter the pitot-static system and cause these issues. After arriving in ZZZ; Line Maintenance did start working on the aircraft right away. I overheard them stating there large amounts of water in the CA side pitot-static system. In my opinion; this water had to have entered the system during much heavier rain. This would have had to occur on a different day as today's rainfall was very light. Perform more thorough pitot-static system checks and inspect system water drains more often.


Synopsis: CRJ-200 flight crew reported returning to departure airport after noting EFIS COMP MON anomalies that Maintenance later said were related to water in the Captain's pitot/static system.