B737-800
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe; Deviation - Speed All Types
Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport
Aircraft
On takeoff the First Officer stated 'airspeed' at 100kts. Decision was made to continue. During the climb Captain's airspeed accelerated to 380-420 kts. First Officer's airspeed was normal. Passing through 6;000 feet the clacker came on. The First Officer went through the unreliable airspeed check list [and] altimeter disagree checklist. We transferred controls and contacted Maintenance and Dispatch. Suggestion was made to activate the stall protection system test to get the clacker off. This did not work. After determining the First Officer's information was accurate the First Officer flew the airplane. At one point ATC was showing that the Captain's data was correct. The First Officer pointed to the standby information and we switched to number two transponder. This gave ATC accurate information. We planned a 15 mile final and configured early. The clacker went off while descending through 6;000 feet. We selected flaps 30 and the flaps indicator showed 25 based on Captain's information. We selected 40 and the indicator went to 30 as we were on short final we selected flaps 30 and flaps stayed at 30. Captain's airspeed slowed to about 120 as the plane approached to land.
During takeoff roll F/O made '100 kts' callout when CA-side airspeed indicator read 80 kts. The decision was made to continue. After takeoff; CA's airspeed indication climbed faster than FO's. Around 6;000 ft CA's airspeed indication exceeded 250kts; maxing out at around 400 kts by 15;000 ft. We [advised ATC] for unreliable instruments and were assigned 14;000 ft. At that time it was discovered that the altimeters disagreed by about 1;000 ft with the CA's reading higher. QRH procedures for unreliable airspeed and altimeter disagree were performed.
B737-800 flight crew reported returning to the departure airport after noting the Captain's side airspeed and altimeter were unreliable.