Loss of air data computer causes altdevs of plus minus 500 ft.

Attributes
ACN	243300
Time
Date	199306
Day	Tue
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : cid
State Reference	IA
Altitude	msl bound lower : 34500 msl bound upper : 35500
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zmp
Operator	general aviation : corporate
Make Model Name	Light Transport
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 100 flight time total : 8000 flight time type : 600
ASRS Report	243300
Person 2
Affiliation	Other
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude inflight encounter : weather
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	none taken : unable
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

2 hours after takeoff cruising at FL350, pilot's airspeed indicator slowly moved to 0. Shortly thereafter air data computer failure for left side instruments. Autoplt disconnected and then we hit severe wave action causing plus or minus 500 ft deviations of altitude to keep aircraft from red lining. With air data computer failure, mode C terminated and center called about it. #2 transponder was selected and showed 400 ft high. Correction was made to fly aircraft from #2 altimeter which required about a 300 ft error correction to bring mode C to proper altitude. Air data computer then came back on line and intermittently tripped off line the remainder of flight to salt lake city. The problem disappeared through 14000 ft as the pilot's airspeed indicator worked again. Probable cause: static ice.

Original NASA ASRS Text

Title: LOSS OF AIR DATA COMPUTER CAUSES ALTDEVS OF PLUS MINUS 500 FT.

Narrative: 2 HRS AFTER TKOF CRUISING AT FL350, PLT'S AIRSPD INDICATOR SLOWLY MOVED TO 0. SHORTLY THEREAFTER AIR DATA COMPUTER FAILURE FOR L SIDE INSTS. AUTOPLT DISCONNECTED AND THEN WE HIT SEVERE WAVE ACTION CAUSING PLUS OR MINUS 500 FT DEVS OF ALT TO KEEP ACFT FROM RED LINING. WITH AIR DATA COMPUTER FAILURE, MODE C TERMINATED AND CTR CALLED ABOUT IT. #2 XPONDER WAS SELECTED AND SHOWED 400 FT HIGH. CORRECTION WAS MADE TO FLY ACFT FROM #2 ALTIMETER WHICH REQUIRED ABOUT A 300 FT ERROR CORRECTION TO BRING MODE C TO PROPER ALT. AIR DATA COMPUTER THEN CAME BACK ON LINE AND INTERMITTENTLY TRIPPED OFF LINE THE REMAINDER OF FLT TO SALT LAKE CITY. THE PROB DISAPPEARED THROUGH 14000 FT AS THE PLT'S AIRSPD INDICATOR WORKED AGAIN. PROBABLE CAUSE: STATIC ICE.

Data retrieved from NASA's ASRS site as of July 2007 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.