Date: 201304
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: IMC
Weather elements / visibility: Icing
Light: Daylight
Aircraft: MD-11
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Air Data Computer
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Distraction; Confusion; Training / Qualification
Second reporter function: Pilot Not Flying; Captain
Anomaly: Aircraft Equipment Problem Critical; Inflight Event / Encounter Weather / Turbulence
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Aircraft Equipment Problem Dissipated; Flight Crew Took Evasive Action; Flight Crew FLC complied w / Automation / Advisory; General Declared Emergency; General Maintenance Action
Contributing factors: Aircraft; Weather
Primary problem: Ambiguous

Narrative: In cruise; after about 1.5 hours; without warning; the autopilot and autothrottles disconnected while IMC. Several indicator lights and CAS messages appeared and subsequently disappeared initially. Over the next 30 seconds with mushy aircraft controls and with the 'what's happening' conversation; we indicated 300 FT high from our assigned altitude. Malfunctions did disappear and we were able to use autopilot and autothrottles again briefly; only to have similar or worse indications occur again a few minutes later. Indicated airspeed comparator was indicated sporadically and the crew noted 215 KTS vs 290 KTS difference from Captain side to First Officer side at one point during the flight. After approximately the third loss of autopilot; autothrottles; and having IAS differences between Captain and First Officer sides; we declared an emergency and received priority handling. Complied with the 'airspeed lost; suspect; or erratic' checklist and started our descent about 15 minutes earlier than planned. Airspeeds eventually became closer to normal during approach and landing.


Second narrative: After cruising for over an hour and a half in the clouds with engine anti-ice ON; the autopilot and autothrottles disconnected with several malfunctions indicated. Most indicated malfunctions disappeared shortly; and we were able to reengage the automation. Our peek-a-boo malfunctions occurred again. This time I notified ATC that we were negative RVSM due to autopilots. I also requested FL250 to make it easier to hand fly the aircraft. After analyzing and running the appropriate checklists; we were led to the Airspeed: Lost; Suspect; or Erratic checklist. Indications included IAS Comparator indications; at one point there was an airspeed difference of over 50 KTS between the Captain and First Officer. At this point; we declared an emergency. By the time we passed 15;000 FT]; all three airspeed indicators (Captain; First Officer; STBY) were within 10 KTS of each other.

Second callback: The reporter stated that the Number 1 Air Data Computer feeding data to the Captain's instruments was changed by Maintenance. The aircraft has sat at the departure airport during heavy rain prior to departure but the reporter did not know if that was a factor. The pitot-static anti-ice system appeared to be operating normally during the entire time they were IMC; conditions which they encounter shortly after takeoff and remained in over an hour.

Synopsis: A MD-11 developed a nearly 75 KT difference between airspeed indications after an hour of IMC flight at FL350 but upon descent the Captain; First Officer; and Standby airspeeds again agreed and autothrottles and autopilot came functional. Maintenance changed the number 1 CADC.