Flight Crew Air Transport Pilot (ATP)
Confusion
Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Weather / Turbulence
Aircraft Equipment Problem Dissipated; Flight Crew Took Evasive Action; Flight Crew Became Reoriented
Aircraft; Weather
During cruise at FL380; approximately 45 minutes after takeoff and in IMC; we experienced multiple airspeed and altitude reported excursions. Captain's airspeed indicated about 140 knots while First Officer's showed about 260 knots. Captain's altimeter appeared correct at FL380; but First Officer's showed FL371. The autothrottles responded to the apparent low airspeed by advancing the throttles to the engine limit. I disengaged the autothrottles and reduced the throttles manually to a normal cruise setting. However; both EEC's tripped to ALTN. After assessing which instruments reflected accurate information; we descended to FL340. During the descent and shortly after level off at FL340; all instruments began returning to normal. We continued the flight at FL340 to uneventful landing. This was caused by some unknown aircraft malfunction and; as such; there is not much we could have done to avoid it. We were presented with conflicting information and we had to methodically determine which instruments we believed to be reliable and which were not and then proceed accordingly. The symptoms we had were not unlike the simulator scenarios for unreliable airspeed and altimetry.
B737-800 Captain experiences unreliable airspeed and altimeter indications at FL380. Both EEC's were noted to have tripped to alternate. The Captain's airspeed and the First Officer's altimeter are determined to be in error and the flight is descended to FL340 where all systems return to normal.