B737 flight crew reported that the first officer's airspeed indicator started decreasing while the c...

Narrative:

On descent from FL380-340 (first officer (first officer) flying) IAS disagree illuminated on captain side; then first officer side. Noticed it airspeed split with captains side being true compared to standby airspeed indicator. Within 3 secs noticed altitude disagree on captain side-no dsply source indication. Simultaneously autopilot disconnected and dual eec altn and engine 'six pack' light illuminated. Requested continued descent out of rvsm airspace. Noticed first officer 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/altitude disagree qrc coupled with unreliable airspeed/altitude procedures; captain elected to disconnect autopilot (after trying reengage) and auto throttle (after trying reengage in accordance with 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/altitude 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 localizer intercept captain/first officer ILS frequency changed to amber with line through freq. Aural frequency verification was normal as well as all inputs to GS and localizer capture. Normal approach to an uneventful landing. All above noted indexes remained until engine shutdown at gate.

Original NASA ASRS Text

Title: 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.

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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1461225
Time
Date	201707
Local Time Of Day	0001-0600
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Component
Aircraft Component	Airspeed Indicator
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Pilot Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 260 Flight Crew Total 2473.45 Flight Crew Type 2473.45
Events
Anomaly	Aircraft Equipment Problem Less Severe

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.