Narrative:

As we climbed through FL180 we observed a significant error between the captain's and copilot's altimeter. As we continued to the assigned altitude of FL250 the difference increased until we leveled at FL250 on the captain's altimeter, at which time the copilot's altimeter was indicating FL275. We attempted to inform ATC but frequency congestion caused a delay in this communication of our problem. When ATC informed us of opposite direction traffic at FL260 we then observed that traffic (an air carrier B widebody transport) to be below us. As we passed above the FL260 traffic the widebody transport flight observed and reported us to be above them rather than below them. We then informed ATC that our encoding altimeter was apparently in error and that we would be using the copilot's altimeter for proper altitude reference. We then proceeded to milwaukee with the assistance of ATC (additional traffic separation). Upon landing at milwaukee both altimeters were indicating field elevation but maintenance discovered and repaired a static line leak. Callback conversation with reporter revealed the following: reporter said the alternate static sources were checked on both sides. This resulted in about 100 ft change on both altimeters. This indicates that the static leak was downstream of the alternate static source valve. Reporter was surprised to learn that the standby altimeter senses the same static source as the #1 air data computer and does not resolve a suspected static leak. He said that, after the fact, someone suggested that if he had run the cabin altitude up and/or down, the faulty static source could have been determined.

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Original NASA ASRS Text

Title: LGT CARGO ACFT CLBS 2500 FT ABOVE ASSIGNED ALT BECAUSE OF A STATIC LEAK.

Narrative: AS WE CLBED THROUGH FL180 WE OBSERVED A SIGNIFICANT ERROR BTWN THE CAPT'S AND COPLT'S ALTIMETER. AS WE CONTINUED TO THE ASSIGNED ALT OF FL250 THE DIFFERENCE INCREASED UNTIL WE LEVELED AT FL250 ON THE CAPT'S ALTIMETER, AT WHICH TIME THE COPLT'S ALTIMETER WAS INDICATING FL275. WE ATTEMPTED TO INFORM ATC BUT FREQ CONGESTION CAUSED A DELAY IN THIS COM OF OUR PROB. WHEN ATC INFORMED US OF OPPOSITE DIRECTION TFC AT FL260 WE THEN OBSERVED THAT TFC (AN ACR B WDB) TO BE BELOW US. AS WE PASSED ABOVE THE FL260 TFC THE WDB FLT OBSERVED AND RPTED US TO BE ABOVE THEM RATHER THAN BELOW THEM. WE THEN INFORMED ATC THAT OUR ENCODING ALTIMETER WAS APPARENTLY IN ERROR AND THAT WE WOULD BE USING THE COPLT'S ALTIMETER FOR PROPER ALT REF. WE THEN PROCEEDED TO MILWAUKEE WITH THE ASSISTANCE OF ATC (ADDITIONAL TFC SEPARATION). UPON LNDG AT MILWAUKEE BOTH ALTIMETERS WERE INDICATING FIELD ELEVATION BUT MAINT DISCOVERED AND REPAIRED A STATIC LINE LEAK. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: RPTR SAID THE ALTERNATE STATIC SOURCES WERE CHKED ON BOTH SIDES. THIS RESULTED IN ABOUT 100 FT CHANGE ON BOTH ALTIMETERS. THIS INDICATES THAT THE STATIC LEAK WAS DOWNSTREAM OF THE ALTERNATE STATIC SOURCE VALVE. RPTR WAS SURPRISED TO LEARN THAT THE STANDBY ALTIMETER SENSES THE SAME STATIC SOURCE AS THE #1 ADC AND DOES NOT RESOLVE A SUSPECTED STATIC LEAK. HE SAID THAT, AFTER THE FACT, SOMEONE SUGGESTED THAT IF HE HAD RUN THE CABIN ALT UP AND/OR DOWN, THE FAULTY STATIC SOURCE COULD HAVE BEEN DETERMINED.

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.