Air data computer malfunction account moisture or icing in pitot static system.

Attributes
ACN	96782
Time
Date	198810
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : hvq
State Reference	WV
Altitude	msl bound lower : 34700 msl bound upper : 35000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zid artcc : zdc
Operator	general aviation : corporate
Make Model Name	Light Transport, Low Wing, 2 Turbojet Eng
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : first officer oversight : pic
Qualification	pilot : atp pilot : cfi
Experience	flight time last 90 days : 150 flight time total : 15000 flight time type : 2000
ASRS Report	96782
Person 2
Affiliation	Other
Function	flight crew : captain
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : undershoot
Independent Detector	aircraft equipment other aircraft equipment : unspecified
Resolutory Action	none taken : anomaly accepted
Consequence	faa : reviewed incident with flight crew
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

After flying 3 legs of a 5 leg trip we experienced a partial loss of our #1 air data computer. Prior to the problem on leg #4 we had been flying in heavy rain and also had quick turned our last stop. I believe the problem was caused by ice accumulation in the static system. As we were climbing through FL330 for FL350 the pilot in the left seat noticed a 200-300' difference in the altimeters. After turning off the autoplt and manually leveling off the aircraft at FL350 we switched the autoplt to the right side. After leveling off we noticed the vsi and altimeter on the left side were moving up and down 50-100', and still showing us to be about 300' low. We then descended to a lower altitude and hand flew the aircraft the last 200 mi to our destination. After descending to a lower altitude the problem seemed to correct itself. After sitting on the ground for about 40 mins (OAT and 65 degrees F) and seeing that all again looked normal, I decided to ferry the aircraft home at a lower altitude. The trip home was uneventful and everything worked normally. I don't know what could be done to correct the situation, but I do know that a good instrument scan by both pilots will help reduce the possibility of busting an altitude in this type situation.

Original NASA ASRS Text

Title: AIR DATA COMPUTER MALFUNCTION ACCOUNT MOISTURE OR ICING IN PITOT STATIC SYSTEM.

Narrative: AFTER FLYING 3 LEGS OF A 5 LEG TRIP WE EXPERIENCED A PARTIAL LOSS OF OUR #1 AIR DATA COMPUTER. PRIOR TO THE PROB ON LEG #4 WE HAD BEEN FLYING IN HEAVY RAIN AND ALSO HAD QUICK TURNED OUR LAST STOP. I BELIEVE THE PROB WAS CAUSED BY ICE ACCUMULATION IN THE STATIC SYS. AS WE WERE CLBING THROUGH FL330 FOR FL350 THE PLT IN THE LEFT SEAT NOTICED A 200-300' DIFFERENCE IN THE ALTIMETERS. AFTER TURNING OFF THE AUTOPLT AND MANUALLY LEVELING OFF THE ACFT AT FL350 WE SWITCHED THE AUTOPLT TO THE RIGHT SIDE. AFTER LEVELING OFF WE NOTICED THE VSI AND ALTIMETER ON THE LEFT SIDE WERE MOVING UP AND DOWN 50-100', AND STILL SHOWING US TO BE ABOUT 300' LOW. WE THEN DSNDED TO A LOWER ALT AND HAND FLEW THE ACFT THE LAST 200 MI TO OUR DEST. AFTER DSNDING TO A LOWER ALT THE PROB SEEMED TO CORRECT ITSELF. AFTER SITTING ON THE GND FOR ABOUT 40 MINS (OAT AND 65 DEGS F) AND SEEING THAT ALL AGAIN LOOKED NORMAL, I DECIDED TO FERRY THE ACFT HOME AT A LOWER ALT. THE TRIP HOME WAS UNEVENTFUL AND EVERYTHING WORKED NORMALLY. I DON'T KNOW WHAT COULD BE DONE TO CORRECT THE SITUATION, BUT I DO KNOW THAT A GOOD INSTRUMENT SCAN BY BOTH PLTS WILL HELP REDUCE THE POSSIBILITY OF BUSTING AN ALT IN THIS TYPE SITUATION.

Data retrieved from NASA's ASRS site as of August 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.