Date: 201309
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: IMC
Weather elements / visibility: 1
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: INS / IRS / IRU
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Human factors: Workload; Time Pressure; Confusion; Distraction
Anomaly: Aircraft Equipment Problem Critical; Inflight Event / Encounter Weather / Turbulence
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew FLC Overrode Automation; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Took Evasive Action; General Maintenance Action
Contributing factors: Aircraft; Weather
Primary problem: Aircraft

Narrative: We were cleared direct to an arrival waypoint and to descend via the RNAV Arrival. We were in the weather and had the engine anti-ice switches on. Passing 18;000 FT; I called for the Descent Checklist. As we accomplished the checklist; my First Officer said; 'Whoa look at your airspeed.' I looked down as we got a buffet alert on the FMC and an aural airspeed low warning and saw about 120. I disengaged all automation; but continued to comply with the RNAV ground track and altitude restrictions. My First Officer then made a great call; 'My airspeed's fine indicating 250 KTS.' About that time; we had a Master Caution light and noted that both EECs had gone into the soft alternate mode. We followed the QRH and; when we put the EECs into the hard alternate mode; my correct airspeed indications came back. The rest of the flight was uneventful. When we landed; Maintenance found an ADIRU fault in both engines and grounded the aircraft. I think crews need to know that this could possibly happen with incorrect data inputs to the engines. The airspeed low warning definitely got my attention; but good pilot not flying skills by my First Officer allowed us to properly handle the problem quickly. We initially thought something might have iced up in the weather; but the anti-ice was on and Maintenance told us the faulty airspeed indication was due to the EEC problem. The issue was loss of accurate airspeed indication on Captain's side.

Callback: The reporter stated that they were descending through about 18;000 FT when he called for the Descent Checklist and at about the same time as the aircraft entered the cloud tops. The OAT was between 0C and +10C so they had turned the engine anti-ice on when the First Officer made his comment about the Captain's airspeed. The clouds were only about 4;000 FT to 5;000 FT thick and when they popped out of the clouds his airspeed recovered immediately. The increase was not gradual; it was instantaneous. The comment in his report about the weather being an issue was what he thought about first when the event began. The fact that Maintenance found a fault code on both ADIRU's with no other indications is rather strange. The reporter stated; the fault codes which Maintenance reported indicated that the aircraft was not dispatchable until further maintenance was performed.



Synopsis: A B737-800 had a dual ADIRU fault in icing conditions during descent which resulted in both engine EEC's reverting to the Soft Mode and the Captain's airspeed falling to about 120 KTS with a buffet alert and aural low speed warning.