Narrative:

After takeoff from dca at approximately 3000' the autoplt was engaged by the PIC utilizing heading mode with altitude preselect. When a new heading was given for vectors to an arwy intercept by ATC, the PIC noticed that the aircraft was slow to respond to new heading inputs, stopping 15-20 degrees short of selected heading with wings level. At this point the altitude preselect warning sounded indicating 1000' to go to assigned altitude. PIC noticed the select armed light (amber light) on the autoplt annunciator began blinking. This is not a normal indication. PIC disconnected the autoplt and began to manually level the aircraft at the assigned altitude. At approximately the assigned initial altitude, the pilot's altimeter and vertical speed indicator began showing a rapid increase in altitude and vertical speed, with a corresponding increase (rapid) in airspeed. PIC responded by pushing the nose of the aircraft down while reducing power in an effort to maintain altitude and airspeed. Within seconds (approximately 5) the pilot's vertical speed indicated a 6000 FPM climb rate with an altimeter approximately 200' above assigned altitude. Airspeed indicator pegged to maximum. A quick reference of the copilot's instruments at this time showed assigned altitude minus 2000' with a normal rate of descent and normal airspeed. PIC turned controls over to the copilot. Copilot corrected to assigned altitude and heading. During this time, transponder systems (both #1 and #2) were reported intermittent to inoperative by ATC. Prior to his taking the controls, the copilot had been involved with checking circuit breaker's and switches for the transponder(south). ATC was notified of a flight director and autoplt problem and a request was made for a lower than planned altitude and airspeed. PIC made the determination to proceed to destination (bed) where the WX was VMC. Pilot's instruments remained unusable for the balance of flight, as indications were erratic. Approximately 30 mins into the flight PIC noticed the air data computer fail (red light) on autoplt annunciator panel flickering. Approximately 40 mins into the flight the air data computer light illuminated steady red. The maximum mmo bell and light came on and off intermittently, so airspeed indicator fluctuated erratically. The flight continued to bed and a visibility approach with a normal landing ensued. Upon inspection maintenance revealed that water had entered the fuselage from the base of the HF antenna on top of the fuselage directly above the air data computer. The previous evening's rain at dca had apparently entered and soaked the computer causing the malfunction.

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Original NASA ASRS Text

Title: REPORTER ACFT HAD A FAILURE OF THE AIR DATA COMPUTER.

Narrative: AFTER TKOF FROM DCA AT APPROX 3000' THE AUTOPLT WAS ENGAGED BY THE PIC UTILIZING HDG MODE WITH ALT PRESELECT. WHEN A NEW HDG WAS GIVEN FOR VECTORS TO AN ARWY INTERCEPT BY ATC, THE PIC NOTICED THAT THE ACFT WAS SLOW TO RESPOND TO NEW HDG INPUTS, STOPPING 15-20 DEGS SHORT OF SELECTED HDG WITH WINGS LEVEL. AT THIS POINT THE ALT PRESELECT WARNING SOUNDED INDICATING 1000' TO GO TO ASSIGNED ALT. PIC NOTICED THE SELECT ARMED LIGHT (AMBER LIGHT) ON THE AUTOPLT ANNUNCIATOR BEGAN BLINKING. THIS IS NOT A NORMAL INDICATION. PIC DISCONNECTED THE AUTOPLT AND BEGAN TO MANUALLY LEVEL THE ACFT AT THE ASSIGNED ALT. AT APPROX THE ASSIGNED INITIAL ALT, THE PLT'S ALTIMETER AND VERT SPD INDICATOR BEGAN SHOWING A RAPID INCREASE IN ALT AND VERT SPD, WITH A CORRESPONDING INCREASE (RAPID) IN AIRSPD. PIC RESPONDED BY PUSHING THE NOSE OF THE ACFT DOWN WHILE REDUCING PWR IN AN EFFORT TO MAINTAIN ALT AND AIRSPD. WITHIN SECS (APPROX 5) THE PLT'S VERT SPD INDICATED A 6000 FPM CLB RATE WITH AN ALTIMETER APPROX 200' ABOVE ASSIGNED ALT. AIRSPD INDICATOR PEGGED TO MAX. A QUICK REF OF THE COPLT'S INSTRUMENTS AT THIS TIME SHOWED ASSIGNED ALT MINUS 2000' WITH A NORMAL RATE OF DSCNT AND NORMAL AIRSPD. PIC TURNED CONTROLS OVER TO THE COPLT. COPLT CORRECTED TO ASSIGNED ALT AND HDG. DURING THIS TIME, XPONDER SYSTEMS (BOTH #1 AND #2) WERE RPTED INTERMITTENT TO INOP BY ATC. PRIOR TO HIS TAKING THE CONTROLS, THE COPLT HAD BEEN INVOLVED WITH CHKING CB'S AND SWITCHES FOR THE XPONDER(S). ATC WAS NOTIFIED OF A FLT DIRECTOR AND AUTOPLT PROB AND A REQUEST WAS MADE FOR A LOWER THAN PLANNED ALT AND AIRSPD. PIC MADE THE DETERMINATION TO PROCEED TO DEST (BED) WHERE THE WX WAS VMC. PLT'S INSTRUMENTS REMAINED UNUSABLE FOR THE BAL OF FLT, AS INDICATIONS WERE ERRATIC. APPROX 30 MINS INTO THE FLT PIC NOTICED THE ADC FAIL (RED LIGHT) ON AUTOPLT ANNUNCIATOR PANEL FLICKERING. APPROX 40 MINS INTO THE FLT THE ADC LIGHT ILLUMINATED STEADY RED. THE MAX MMO BELL AND LIGHT CAME ON AND OFF INTERMITTENTLY, SO AIRSPD INDICATOR FLUCTUATED ERRATICALLY. THE FLT CONTINUED TO BED AND A VIS APCH WITH A NORMAL LNDG ENSUED. UPON INSPECTION MAINT REVEALED THAT WATER HAD ENTERED THE FUSELAGE FROM THE BASE OF THE HF ANTENNA ON TOP OF THE FUSELAGE DIRECTLY ABOVE THE AIR DATA COMPUTER. THE PREVIOUS EVENING'S RAIN AT DCA HAD APPARENTLY ENTERED AND SOAKED THE COMPUTER CAUSING THE MALFUNCTION.

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.