Date: 201311
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Flight conditions: IMC
Light: Night
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight phase: Climb
ATC / advisory: Center ZZZZ
Aircraft component: Air Data Computer
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Second reporter function: Captain; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Critical
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Aircraft Equipment Problem Dissipated; Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: I was PF flying in IMC conditions; night during climbout on last leg of pairing for day. PM in left seat questioned IAS and relayed that he was seeing IAS rollback; altitude fluctuate on PFD display and matched in HUD. PF had associated IAS and ALT disagree annunciations matched on bottom of PFD. Both FMC/CDU screens became degraded. Flight Director and map displays were normal. Ex: winds aloft data both on IRS and on FMC were not accurate with report info from PIREPS as relayed by ATC. No Master Caution illuminated. No CBs popped. PM discovered EEC [in] alt mode on overhead; depressed recall then engine light noted. PF sought VMC conditions with ATC and maintained aircraft control and ATC comms. PM performed QRH checklist; comms with Dispatch and Maintenance via phone patch as VHF comm frequencies on release weren't satisfactory. Dispatch recommended ZZZ (close by) and later [continue to destination]. Crew chose to maintain VMC above undercast with good ambient lighting all the way to destination reported with straight-in long runway and VMC conditions. Fuel log with Dispatch; IAS and ALT cross checks with ATC were performed while troubleshooting malfunctioning systems. Landed uneventfully at destination; maintained VMC entire portion of leg. System operated normally on descent after Descent Checklist completion. Air data computer failure of this nature not common. Never experienced kind of indications without associated Master Caution prior to this event. Felt risk of descending into IMC conditions or congested airport environment with degraded flight instruments could have become compounded further with any more malfunctions of PFD instruments PF was monitoring. Both PF and PM had never been exposed to a similar malfunction in training. Exposing this scenario would be valuable. We discussed [previous] pitot static aircraft problems where PF used HUD as only info available to maintain aircraft control and situational awareness. PM PFD erroneous indications matched in HUD and EEC soft alt mode switchover.


Second narrative: At approximately 15;000 feet in IMC; icing conditions; all probe heat and engine anti ice on; PM (me) noticed a/s alt roll back. Queried PF--he had more reliable data; and maintained a/c control. I believe I heard a lone callout of 'airspeed' from the GPWS. I thought; why aren't we getting the stall warning? Tried to analyze all seemingly unrelated malfunctions: Bad data in HUD (269 knot tailwind) and CDU's; checked circuit breakers (all in). No master caution light or display source light illuminated. Noticed EEC's in soft alt; hit Master Caution and ENG remained illuminated. Pilots conferred and agreed that returning not as desirable as seeking VMC conditions to get out of a possible cause (icing conditions). Wanted VMC to analyze the situation; in case it worsened and the only way was to climb. Climbing to better fuel economy altitudes would also buy us more time; and provide more options if we had to divert; once we knew what was wrong. Verified our altitude and airspeed with ATC and requested block altitudes. After being stabilized; attempted contact with Dispatch/Maintenance. Could not raise on several frequencies; got phone patch. Was hoping for an easy fix from Maintenance. Phone patch became unreliable; made contact on VHF. PF continued to verify alt and a/s with each controller. PM made fuel checks with dispatch. At some point Dispatch offered ZZZ as a divert. We did not like this idea as we still did not fully know what was wrong; would have to go into IMC; and it was close so we would have been rushed. ZZZ1 was also not liked as an option for similar reasons and traffic. The previous leg we flew was from ZZZ2 so we knew the further east we went the more VMC we'd have. At one point Maintenance asks if we had a 'display source' light; we did not. When referencing the display source checklist; our problem and overall situational awareness became clearer: With the opposite autopilot on; a DEU failure will cause the erroneous readings (failures) to the other pilots displays. Also it says the EEC's will be in soft alt and to run the EEC failure checklist. We were unable to restore anything; but at least we had a pretty good idea of the nature of the problem. At this point; our original destination; long runways and VMC conditions; seemed like the best option and we were almost there; so we continued to original destination and considered preparations for a safe arrival. At 13;000 feet; for no apparent reason; all of our instrumentation returned.


Synopsis: B737-700 flight crew experiences airspeed and altitude anomalies on the Captain's displays during climb. Crew elects to maintain an intermediate altitude in VMC and continue to destination also VMC. During descent the Captain's instruments return to normal.