Date: 201707
Local time of day: 0001-0600
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Airspeed Indicator
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: On descent from FL380-340 (First Officer (FO) flying) IAS DISAGREE illuminated on Captain side; then FO side. Noticed it airspeed split with Captains side being true compared to standby airspeed indicator. Within 3 secs noticed ALT DISAGREE on Captain side-no DSPLY SOURCE indication. Simultaneously autopilot disconnected and dual EEC ALTN and ENG 'six pack' light illuminated. Requested continued descent out of RVSM airspace. Noticed FO airspeed then spool down to 45 kts with altitude split of 500-1;000 ft. ATC then cleared aircraft to descend via KIPPR arrival. After reviewing QRH EEC ALTN procedures with no corrections and discussing IAS/ALT DISAGREE QRC coupled with unreliable airspeed/altitude procedures; Captain elected to disconnect autopilot (after trying reengage) and auto throttle (after trying reengage IAW QRH) and proceed visually to airfield. Crew briefed; no cabin prep initiated and emergency declared. Passing 12;000 ft to 11;000 ft all indications went away. At 11;000 ft all indications returned (IAS/ALT DISAGREE; EEC ALTN; engine indicators of N1/TAT/CLBCRZDES thrust mode display lights out). No A/T LIM light was noted at all. Continued visually to [the Runway]-during LOC intercept Captain/FO ILS frequency changed to amber with line through freq. Aural frequency verification was normal as well as all inputs to GS and LOC capture. Normal approach to an uneventful landing. All above noted indexes remained until engine shutdown at gate.

Callback: The reporter stated that the first indication is between FL380 and FL370 they had a 1;000 FT altimeter split between the Captain's and FO's altimeters. At first the Captain's ALT was the same as the SAI then suddenly it wasn't. The Reporter also stated both EEC's went into ALTN mode. The reporter stated all indications went away; (normal);  passing through 12;000 FT; then below 11;000 FT all indications returned. The reporter stated that after landing all indications remained until the engines were shut down.

Second narrative: [Report narrative contained no additional information.]


Synopsis: B737 flight crew reported that the First Officer's airspeed indicator started decreasing while the Captain's remained steady. Both engines went into 'ALTN' mode on the EEC's.