B737-700
Workload; Time Pressure; Confusion; Distraction
Aircraft Equipment Problem Critical; Inflight Event / Encounter Weather / Turbulence
Flight Crew Became Reoriented; Flight Crew FLC Overrode Automation; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Took Evasive Action; General Maintenance Action
Aircraft; Weather
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.
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.
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.