A md-11 developed a nearly 75 KT difference between airspeed indications after an hour of IMC flight...

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.

Original NASA ASRS Text

Title: 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.

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.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1079922
Time
Date	201304
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Make Model Name	MD-11
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Air Data Computer
Person 1
Function	First Officer Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Pilot Not Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Critical Inflight Event / Encounter Weather / Turbulence

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.