Widebody transport 2 man aircraft at cruise had malfunction of air data system giving flight crew erratic readings on altitude, vertical speed, airspeed.

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Attributes
ACN	194786
Time
Date	199111
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : grd airport : atl
State Reference	SC
Altitude	msl bound lower : 28000 msl bound upper : 29000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : ztl
Operator	common carrier : air carrier
Make Model Name	Widebody, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	climbout : intermediate altitude cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : commercial pilot : instrument
Experience	flight time last 90 days : 150 flight time total : 19000 flight time type : 3500
ASRS Report	194786
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 180 flight time total : 13800
ASRS Report	194884
Events
Anomaly	aircraft equipment problem : critical altitude deviation : overshoot altitude deviation : excursion from assigned altitude other anomaly other anomaly
Independent Detector	other flight crewa
Resolutory Action	flight crew : overcame equipment problem flight crew : declared emergency flight crew : regained aircraft control other
Consequence	Other Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

Autoplt failed to level off aircraft at FL280, disengaged autoplt and leveled aircraft at about FL290. Descended back towards FL280. Both altimeters continued to indicate descent while instruments indicated climb. Level off difficult to accomplish at FL280. Altimeters indicated climb and instruments descent. Requested return to atl and block altitudes to clear traffic. Altimeters were in excess of 1000 ft between captain and first officer. ATC advised us we were at altitudes other than indicated on either altimeter. ATC also advised us that, at times, we were either climbing or descending when we thought we were in level flight. Callback conversation with reporter revealed the following information: maintenance performed on aircraft after parking. Test flight revealed same symptoms. Second go at maintenance where most of the air data system was replaced corrected the problem. With altimetry involved in the malfunction of the air data system we suspected only a plugged static line but there was apparently some internal, electrical, computer malfunction as well.

Original NASA ASRS Text

Title: WDB 2 MAN ACFT AT CRUISE HAD MALFUNCTION OF AIR DATA SYS GIVING FLC ERRATIC READINGS ON ALT, VERT SPD, AIRSPD.

Narrative: AUTOPLT FAILED TO LEVEL OFF ACFT AT FL280, DISENGAGED AUTOPLT AND LEVELED ACFT AT ABOUT FL290. DSNDED BACK TOWARDS FL280. BOTH ALTIMETERS CONTINUED TO INDICATE DSCNT WHILE INSTS INDICATED CLB. LEVEL OFF DIFFICULT TO ACCOMPLISH AT FL280. ALTIMETERS INDICATED CLB AND INSTS DSCNT. REQUESTED RETURN TO ATL AND BLOCK ALTS TO CLR TFC. ALTIMETERS WERE IN EXCESS OF 1000 FT BTWN CAPT AND FO. ATC ADVISED US WE WERE AT ALTS OTHER THAN INDICATED ON EITHER ALTIMETER. ATC ALSO ADVISED US THAT, AT TIMES, WE WERE EITHER CLBING OR DSNDING WHEN WE THOUGHT WE WERE IN LEVEL FLT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: MAINT PERFORMED ON ACFT AFTER PARKING. TEST FLT REVEALED SAME SYMPTOMS. SECOND GO AT MAINT WHERE MOST OF THE AIR DATA SYS WAS REPLACED CORRECTED THE PROBLEM. WITH ALTIMETRY INVOLVED IN THE MALFUNCTION OF THE AIR DATA SYS WE SUSPECTED ONLY A PLUGGED STATIC LINE BUT THERE WAS APPARENTLY SOME INTERNAL, ELECTRICAL, COMPUTER MALFUNCTION AS WELL.

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.