Narrative:

In cruise flight the captain's airspeed started to slowly decrease. First officer and stby agreed; trim wheel was stable; so we agreed we had a problem on the captain side. The captain's altimeter started slow climb. Master caution came on. I looked on overhead and both eec in altitude mode. ATC called and said you are showing 300 ft high. I told him we had a computer failure and would like a block FL380-FL390 and he granted it. I then switched from #1 transponder to #2 so it would agree with the working instruments. Center said we were showing right on FL380. While first officer flew the aircraft I checked all circuit breakers and none were out. I then grabbed the QRH and complied with the non-normal checklist for the eec's. Then the non-normal for unreliable airspeed and unreliable altitude. I asked the first officer if he would be comfortable if I recycled the pitot heat. He said yes; and I did. Captain's instruments came back slowly (2 mins approximately). All instruments agreed and CDU airspeed disagreement note cleared. The last thing we had to decide was stay in or out of rvsm. Everything was working now except we had left the eec's in the altitude position as per QRH. I felt the engines were stable and thought it was now safe to give up 10000 ft and go to FL280 the last hour of the flight (plenty of fuel). We notified dispatch and maintenance en route and wrote it up. We had been in a steady cirrus layer for about 45 mins prior to the event. I think it was about -62; but I really think it was a static port or a pitot problem. I am a big fan of heated static ports. Although I am also not an expert it could have been a computer. We never had a pitot annunciator.callback conversation with reporter revealed the following information: the reporter was not aware of the maintenance procedures required to return the aircraft to service. The eec's do have a pitot static input through the air data computer's so the two anomalies were almost certainly related. The fact that the pitot static problems disappeared after cycling the pitot heat switch is a strong indication that pitot heat was the culprit in this incident. No pitot heat warnings were ever received; except when the switch was cycled off.

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Original NASA ASRS Text

Title: B737-700 CAPT RPTS AIRSPEED AND ALTIMETER ANOMALIES AT FL380; EEC'S GO TO ALTERNATE MODE. CYCLING PITOT HEAT SWITCH SEEMS TO SOLVE THE PROB.

Narrative: IN CRUISE FLT THE CAPT'S AIRSPEED STARTED TO SLOWLY DECREASE. FO AND STBY AGREED; TRIM WHEEL WAS STABLE; SO WE AGREED WE HAD A PROBLEM ON THE CAPT SIDE. THE CAPT'S ALTIMETER STARTED SLOW CLB. MASTER CAUTION CAME ON. I LOOKED ON OVERHEAD AND BOTH EEC IN ALT MODE. ATC CALLED AND SAID YOU ARE SHOWING 300 FT HIGH. I TOLD HIM WE HAD A COMPUTER FAILURE AND WOULD LIKE A BLOCK FL380-FL390 AND HE GRANTED IT. I THEN SWITCHED FROM #1 TRANSPONDER TO #2 SO IT WOULD AGREE WITH THE WORKING INSTRUMENTS. CTR SAID WE WERE SHOWING RIGHT ON FL380. WHILE FO FLEW THE ACFT I CHECKED ALL CIRCUIT BREAKERS AND NONE WERE OUT. I THEN GRABBED THE QRH AND COMPLIED WITH THE NON-NORMAL CHECKLIST FOR THE EEC'S. THEN THE NON-NORMAL FOR UNRELIABLE AIRSPEED AND UNRELIABLE ALTITUDE. I ASKED THE FO IF HE WOULD BE COMFORTABLE IF I RECYCLED THE PITOT HEAT. HE SAID YES; AND I DID. CAPT'S INSTRUMENTS CAME BACK SLOWLY (2 MINS APPROX). ALL INSTRUMENTS AGREED AND CDU AIRSPEED DISAGREEMENT NOTE CLEARED. THE LAST THING WE HAD TO DECIDE WAS STAY IN OR OUT OF RVSM. EVERYTHING WAS WORKING NOW EXCEPT WE HAD LEFT THE EEC'S IN THE ALT POSITION AS PER QRH. I FELT THE ENGINES WERE STABLE AND THOUGHT IT WAS NOW SAFE TO GIVE UP 10000 FT AND GO TO FL280 THE LAST HOUR OF THE FLT (PLENTY OF FUEL). WE NOTIFIED DISPATCH AND MAINT ENRTE AND WROTE IT UP. WE HAD BEEN IN A STEADY CIRRUS LAYER FOR ABOUT 45 MINS PRIOR TO THE EVENT. I THINK IT WAS ABOUT -62; BUT I REALLY THINK IT WAS A STATIC PORT OR A PITOT PROB. I AM A BIG FAN OF HEATED STATIC PORTS. ALTHOUGH I AM ALSO NOT AN EXPERT IT COULD HAVE BEEN A COMPUTER. WE NEVER HAD A PITOT ANNUNCIATOR.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS NOT AWARE OF THE MAINTENANCE PROCS REQUIRED TO RETURN THE ACFT TO SERVICE. THE EEC'S DO HAVE A PITOT STATIC INPUT THROUGH THE ADC'S SO THE TWO ANOMALIES WERE ALMOST CERTAINLY RELATED. THE FACT THAT THE PITOT STATIC PROBLEMS DISAPPEARED AFTER CYCLING THE PITOT HEAT SWITCH IS A STRONG INDICATION THAT PITOT HEAT WAS THE CULPRIT IN THIS INCIDENT. NO PITOT HEAT WARNINGS WERE EVER RECEIVED; EXCEPT WHEN THE SWITCH WAS CYCLED OFF.

Data retrieved from NASA's ASRS site as of May 2009 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.