MD-83
Flight Crew Air Transport Pilot (ATP)
Workload; Troubleshooting; Time Pressure; Human-Machine Interface; Distraction; Confusion
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Aircraft Equipment Problem Dissipated; Flight Crew FLC complied w / Automation / Advisory; Flight Crew FLC Overrode Automation; Flight Crew Overcame Equipment Problem; Flight Crew Took Evasive Action; General Maintenance Action
Aircraft; Procedure; Weather
First Officer is flying. FMS airplane at FL330; IMC; deviating around storms. Very little showing up on radar but moderate turbulence and precipitation on front windows. Engine and airfoil anti-ice has been on for quite awhile. A lot of static on radios making it extremely difficult to hear/talk to ATC and distracting to cockpit communications also. On FMS I see CADC1 (central air data computer) data fail and 2 data fail at same time as autopilot and autothrottle disconnects. This is not CADC failure but CADC data fail. No master warning or caution and no off flags anywhere. While I'm flying the Captain says we also have Mach trim compensator inoperative and engine anti-ice valve light on. Captain starts looking through QRH etc.. To figure out what is going on while I continued deviating; flying and dealing with turbulence.  Soon I notice the airplane doesn't feel right followed by inability to hold altitude. I tell Captain to tell Center we're coming down to FL310 while I increase power to climb power and fly the plane down. At the same time I look at Captain's airspeed and it's at 220 kts and increasing. First Officer instruments are still showing 260 kts and .74 Mach. I level off at FL310 and 275 kts and accomplish memory items for unreliable airspeed. There was power showing on all selections of meter selector. Engine and airfoil anti-ice still on also. Very shortly after that we exited precipitation. All indications returned to normal. First Officer instruments all started working normally. Autopilot and autothrottle engaged normally. We were on arrival at that point and continued uneventfully to our filed destination. The altitude deviation was about 400 feet before the Captain was able to get ahold of ATC and tell them we were descending. ATC was fine with that and cleared us to FL310. They didn't question the deviation at all. No emergency was declared but we would have with our next transmission if ATC didn't want us to descend.I don't know what Maintenance was able to find wrong with the plane; if anything. We suspect an icing event of some sort.  Things I could have done better. When transitioning to hand flying cross check power settings more often. Throttle position felt normal and performance indications looked normal on my side. I suspect that when the autothrottle disconnected the power was slightly low due to turbulence deviations and since the performance looked ok on my side I didn't notice it was low. When in icing it would be a good idea to cross check standby and Captain's instruments as well. Pilot not flying can't stop monitoring aircraft performance when troubleshooting either. As usual in an incident/accident chain there are a lot of things that lead up to it. This included deviations due to storms; moderate turbulence; IMC; very abnormal aircraft malfunctions affecting the pilot flying instruments only and static on radios hindering communication and adding another distraction.
The reporter stated that Maintenance found no system anomalies to explain this event. Other long time MD-80 pilots had never experience this type of event either and this reporter is guessing that the pitot system was unable to prevent ice buildup and therefore affect the central air data computer systems.
There were never any fail flags in the flight instruments. We were not sure if the airspeed readings were caused by the data fail messages. At FL310 the turbulence lessened and all fail indications went away.
A MD-83 CADC DATA 1; CADC DATA 2 and MACH TRIM COMPENSATOR Fail lights alerted at the same time the autothrottles disconnected and the ENGINE ANTI-ICE VALVE alerted. The radar was not painting weather but ice was accumulating on the window. After entering VMC conditions all systems returned to normal indicating a potentially overwhelmed pitot anti-ice system.