Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe
Flight Crew Overcame Equipment Problem
Aircraft
I was the First Officer and the pilot monitoring. We were cleared to descend via the ZZZZZX. Approaching FL200 on the descent; I noticed my airspeed start to decrease rapidly without any change in airplane thrust or pitch. I called out 'airspeed' while inspecting the standby and Captain's airspeed indicator. We called unreliable airspeed and started the QRC (Quick Reference Checklist). We followed the QRC followed by the QRH (Quick Reference Handbook). We were able to confirm that the Captain's airspeed indicator was accurate based on the pitch and power settings and correlating with the standby airspeed indicator. The QRH instructed us to reengage the autopilot and change the ALT SOURCE selector to the Captain's side. This allowed my airspeed indicator to correspond with the Captain's and standby airspeed indicator. After we dictated which airspeed was accurate; ATC had given a heading and altitude to continue into ZZZ. I completed the QRH checklist for Unreliable Airspeed but kept the unreliable airspeed tables open to make sure the pitch and power settings correlated with our speed. We continued to fly the ILS approach into ZZZ without any further incident. The Captain notified Maintenance and Dispatch once we were blocked in and the door open. We were unable to conclude the cause that created the unreliable airspeed situation. But; I will become more familiar with the airspeed unreliable checklist and table so that I may refer to it and utilize it more efficiently in the event of another similar occurrence.
B737-800 First Officer reported unreliable airspeed indications on Captain's side during arrival descent.