Narrative:

While cruising at FL390 in the vicinity of eld, ar, we encountered what appeared to be a dual altimeter malfunction. The aircraft in question is a C560. I was the captain occupying the right seat acting as the PNF. At the time of the incident we were level at FL390 in IMC conditions. Although technically IMC we knew we were in the tops of the clouds due to the occasional encounters with blue sky above. The IMC conditions existed for approximately the previous 50 or 60 mi. I and my copilot had just finished making plans to deviate around thunderstorms approximately 60 mi ahead on the arrival into hou when I noticed both ivsi's begin a 300-500 FPM climb. The pilot's altimeter began a corresponding descent and the flight director was pitching down. The copilot's altimeter was not moving. The flight director was indicating level pitch and the airspeed was slightly decreasing. I pointed this out to the PF and he disengaged the autoplt. I immediately requested and was granted a block of altitude from ATC from FL390 to FL410. In approximately the next 30-45 seconds the pilot's altimeter broke loose. It was descending several thousand ft then climbing rapidly occasionally showing an intermittent off flag. At the same time the TCASII and GPWS became inoperative. I instructed the PF to begin a shallow climb in an attempt to climb out of the tops of the clouds. Shortly thereafter I asked ATC what our mode C readout was and they replied 40400 ft. The pilot's altimeter indicated 38000 ft and the copilot's altimeter indicated approximately 41260 ft. At that time I knew the pilot's altimeter was grossly in error but I also could not with any certainty rely on the copilot's altimeter. It appeared there was some kind of pitot-static problem because in addition to the altimeter problems the airspds were split with the copilot's airspeed indicating approximately 5-7 KTS slower. I then declared an emergency with ATC informing them of our situation and I requested a 180 degree turn to the left in order to return to the VFR conditions approximately 50 mi behind us, and once we encountered VFR conditions we wanted to divert to the nearest suitable airport. ATC complied with our request. During this time I continued to act as the PNF monitoring the instruments and working the radios. Once we encountered VFR conditions we elected to land at monroe, la. I informed the passenger of our intentions and we landed without incident. Upon landing however, the pilot's altimeter indicated 100 ft below field elevation and the copilot's altimeter indicated 100 ft above field elevation both with the current altimeter setting dialed in. Callback conversation with reporter revealed the following information: reporter stated that maintenance found the problem in the flight data computer was caused by a piece of air duct debris getting caught in the computer and causing false readings to all the equipment for which it provides information. However, the first officer altimeter was not one of them and no cause has been found for its slow and faulty readings during at the time of incident.

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Original NASA ASRS Text

Title: CAPT OF A CESSNA CITATION V, C560, DECLARED AN EMER WHEN BOTH ALTIMETERS BECAME ERRATIC IN THEIR INDICATIONS AND THE TCASII AND GPWS BECAME INOP. THE FLC DIVERTED TO LAND.

Narrative: WHILE CRUISING AT FL390 IN THE VICINITY OF ELD, AR, WE ENCOUNTERED WHAT APPEARED TO BE A DUAL ALTIMETER MALFUNCTION. THE ACFT IN QUESTION IS A C560. I WAS THE CAPT OCCUPYING THE R SEAT ACTING AS THE PNF. AT THE TIME OF THE INCIDENT WE WERE LEVEL AT FL390 IN IMC CONDITIONS. ALTHOUGH TECHNICALLY IMC WE KNEW WE WERE IN THE TOPS OF THE CLOUDS DUE TO THE OCCASIONAL ENCOUNTERS WITH BLUE SKY ABOVE. THE IMC CONDITIONS EXISTED FOR APPROX THE PREVIOUS 50 OR 60 MI. I AND MY COPLT HAD JUST FINISHED MAKING PLANS TO DEVIATE AROUND TSTMS APPROX 60 MI AHEAD ON THE ARR INTO HOU WHEN I NOTICED BOTH IVSI'S BEGIN A 300-500 FPM CLB. THE PLT'S ALTIMETER BEGAN A CORRESPONDING DSCNT AND THE FLT DIRECTOR WAS PITCHING DOWN. THE COPLT'S ALTIMETER WAS NOT MOVING. THE FLT DIRECTOR WAS INDICATING LEVEL PITCH AND THE AIRSPD WAS SLIGHTLY DECREASING. I POINTED THIS OUT TO THE PF AND HE DISENGAGED THE AUTOPLT. I IMMEDIATELY REQUESTED AND WAS GRANTED A BLOCK OF ALT FROM ATC FROM FL390 TO FL410. IN APPROX THE NEXT 30-45 SECONDS THE PLT'S ALTIMETER BROKE LOOSE. IT WAS DSNDING SEVERAL THOUSAND FT THEN CLBING RAPIDLY OCCASIONALLY SHOWING AN INTERMITTENT OFF FLAG. AT THE SAME TIME THE TCASII AND GPWS BECAME INOP. I INSTRUCTED THE PF TO BEGIN A SHALLOW CLB IN AN ATTEMPT TO CLB OUT OF THE TOPS OF THE CLOUDS. SHORTLY THEREAFTER I ASKED ATC WHAT OUR MODE C READOUT WAS AND THEY REPLIED 40400 FT. THE PLT'S ALTIMETER INDICATED 38000 FT AND THE COPLT'S ALTIMETER INDICATED APPROX 41260 FT. AT THAT TIME I KNEW THE PLT'S ALTIMETER WAS GROSSLY IN ERROR BUT I ALSO COULD NOT WITH ANY CERTAINTY RELY ON THE COPLT'S ALTIMETER. IT APPEARED THERE WAS SOME KIND OF PITOT-STATIC PROB BECAUSE IN ADDITION TO THE ALTIMETER PROBS THE AIRSPDS WERE SPLIT WITH THE COPLT'S AIRSPD INDICATING APPROX 5-7 KTS SLOWER. I THEN DECLARED AN EMER WITH ATC INFORMING THEM OF OUR SIT AND I REQUESTED A 180 DEG TURN TO THE L IN ORDER TO RETURN TO THE VFR CONDITIONS APPROX 50 MI BEHIND US, AND ONCE WE ENCOUNTERED VFR CONDITIONS WE WANTED TO DIVERT TO THE NEAREST SUITABLE ARPT. ATC COMPLIED WITH OUR REQUEST. DURING THIS TIME I CONTINUED TO ACT AS THE PNF MONITORING THE INSTS AND WORKING THE RADIOS. ONCE WE ENCOUNTERED VFR CONDITIONS WE ELECTED TO LAND AT MONROE, LA. I INFORMED THE PAX OF OUR INTENTIONS AND WE LANDED WITHOUT INCIDENT. UPON LNDG HOWEVER, THE PLT'S ALTIMETER INDICATED 100 FT BELOW FIELD ELEVATION AND THE COPLT'S ALTIMETER INDICATED 100 FT ABOVE FIELD ELEVATION BOTH WITH THE CURRENT ALTIMETER SETTING DIALED IN. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT MAINT FOUND THE PROB IN THE FLT DATA COMPUTER WAS CAUSED BY A PIECE OF AIR DUCT DEBRIS GETTING CAUGHT IN THE COMPUTER AND CAUSING FALSE READINGS TO ALL THE EQUIP FOR WHICH IT PROVIDES INFO. HOWEVER, THE FO ALTIMETER WAS NOT ONE OF THEM AND NO CAUSE HAS BEEN FOUND FOR ITS SLOW AND FAULTY READINGS DURING AT THE TIME OF INCIDENT.

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.