Narrative:

During cruise flight on autoplt INS command and automatic throttle pms (performance command system) command, we noticed discrepancy in altitude indication of 300' at 37000'. It was necessary to select 37200' on barometric altimeter window which controls pms to indicate 36900' on electric altimeter (ATC altitude reporting altimeter). Descending into phl using automatic throttle and pms we were cleared to 12000'. Pms appeared to perform normally during descent through numerous step altitude clrncs. However, with 12000' set in altitude selector center called requesting present altitude. Electric altimeter indicated 11800' at time of call, but suspect reason for ATC altitude request was deviation from assigned altitude of more than 300', although we were not showing this. At time of center request I was reprogramming INS for routing change and first officer was monitoring and working ATC radios. Second officer was talking to company and preparing for approach check. It was a busy time in a high traffic area. Center turned us over to approach control west/O commend and we flew manually remainder off flight. Maintenance was notified and log book entry made. Supplemental information from acn 108623: as the captain leveled the aircraft at 12000', I turned to look in my flight bag for the correct approach plate for phl. When I looked up I saw the aircraft had commenced a descent from our assigned 12000' and was descending fairly rapidly through about 11500'. Realizing that if I spoke to the captain a discussion would ensue with further loss of altitude. Therefore, I grabbed the yoke, disengaged the autoplt, pulled the nose up and added the power. Gave the aircraft back to the startled captain at 11800' climbing. We had had up to a 300' discrepancy between the 3 altimeters in the cockpit. Mode C was reporting from the captain's altimeter which was consistently 100-200' lower than mine. Personally, I don't use the pms which in controled airspace under positive radar control. The system is advertised to climb or descend 150' in order to maintain speed. I've seen if gain or lose 300' on some aircraft. With altimeter discrepancies of 200-300' plus the pms' sloppy flying, a potential for busting altitude certainly exists. Any system which gives fuel conservation a higher priority than altitude hold in our crowded ATC environment should be trashed.

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Original NASA ASRS Text

Title: WDB ACFT ON DESCENT ON PERFORMANCE COMMAND AUTO SYSTEM OVERSHOT ALT IN DESCENT. FO MANUALLY RECOVERED ACFT BACK TO ASSIGNED ALT.

Narrative: DURING CRUISE FLT ON AUTOPLT INS COMMAND AND AUTO THROTTLE PMS (PERFORMANCE COMMAND SYS) COMMAND, WE NOTICED DISCREPANCY IN ALT INDICATION OF 300' AT 37000'. IT WAS NECESSARY TO SELECT 37200' ON BAROMETRIC ALTIMETER WINDOW WHICH CONTROLS PMS TO INDICATE 36900' ON ELECTRIC ALTIMETER (ATC ALT RPTING ALTIMETER). DSNDING INTO PHL USING AUTO THROTTLE AND PMS WE WERE CLRED TO 12000'. PMS APPEARED TO PERFORM NORMALLY DURING DSCNT THROUGH NUMEROUS STEP ALT CLRNCS. HOWEVER, WITH 12000' SET IN ALT SELECTOR CENTER CALLED REQUESTING PRESENT ALT. ELECTRIC ALTIMETER INDICATED 11800' AT TIME OF CALL, BUT SUSPECT REASON FOR ATC ALT REQUEST WAS DEVIATION FROM ASSIGNED ALT OF MORE THAN 300', ALTHOUGH WE WERE NOT SHOWING THIS. AT TIME OF CENTER REQUEST I WAS REPROGRAMMING INS FOR ROUTING CHANGE AND F/O WAS MONITORING AND WORKING ATC RADIOS. S/O WAS TALKING TO COMPANY AND PREPARING FOR APCH CHK. IT WAS A BUSY TIME IN A HIGH TFC AREA. CENTER TURNED US OVER TO APCH CTL W/O COMMEND AND WE FLEW MANUALLY REMAINDER OFF FLT. MAINT WAS NOTIFIED AND LOG BOOK ENTRY MADE. SUPPLEMENTAL INFO FROM ACN 108623: AS THE CAPT LEVELED THE ACFT AT 12000', I TURNED TO LOOK IN MY FLT BAG FOR THE CORRECT APCH PLATE FOR PHL. WHEN I LOOKED UP I SAW THE ACFT HAD COMMENCED A DSCNT FROM OUR ASSIGNED 12000' AND WAS DSNDING FAIRLY RAPIDLY THROUGH ABOUT 11500'. REALIZING THAT IF I SPOKE TO THE CAPT A DISCUSSION WOULD ENSUE WITH FURTHER LOSS OF ALT. THEREFORE, I GRABBED THE YOKE, DISENGAGED THE AUTOPLT, PULLED THE NOSE UP AND ADDED THE PWR. GAVE THE ACFT BACK TO THE STARTLED CAPT AT 11800' CLBING. WE HAD HAD UP TO A 300' DISCREPANCY BTWN THE 3 ALTIMETERS IN THE COCKPIT. MODE C WAS RPTING FROM THE CAPT'S ALTIMETER WHICH WAS CONSISTENTLY 100-200' LOWER THAN MINE. PERSONALLY, I DON'T USE THE PMS WHICH IN CTLED AIRSPACE UNDER POSITIVE RADAR CTL. THE SYS IS ADVERTISED TO CLB OR DSND 150' IN ORDER TO MAINTAIN SPD. I'VE SEEN IF GAIN OR LOSE 300' ON SOME ACFT. WITH ALTIMETER DISCREPANCIES OF 200-300' PLUS THE PMS' SLOPPY FLYING, A POTENTIAL FOR BUSTING ALT CERTAINLY EXISTS. ANY SYS WHICH GIVES FUEL CONSERVATION A HIGHER PRIORITY THAN ALT HOLD IN OUR CROWDED ATC ENVIRONMENT SHOULD BE TRASHED.

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.