Narrative:

We were flying the fuzzy arrival into las. ZOA gave us a clearance to cross fuzzy intersection at FL190. The captain, who was flying, typed 19000 ft into the CDU so that the plane would cross fuzzy in VNAV at 19000 ft as we were cleared for. I did see 19000 ft in the box at fuzzy and 19000 ft set into the altitude alerter. I got busy getting ATIS and calling the company for a gate assignment and when I came back we were already in our descent to cross fuzzy at FL190. Suddenly the captain made a comment and extended the spoilers to increase our rate of descent. When I looked at the CDU the 19000 ft crossing restr that had been set for fuzzy had dropped out and we were now high, but we would still have no problem crossing fuzzy at FL190 with the speed brakes fully extended and the airspeed on vmo. Just before we got to fuzzy we were handed off to las approach and the controller started questioning our altitude as we leveled at FL190. I told him that we were level at FL190 across fuzzy and he said that he showed us initially 1200 ft high and then about 400-500 ft high. I've always been under the impression that our mode C readout would lag our actual altitude, especially when rapidly descending like we were. When we finally leveled off, our altitude did agree with what the controller showed. When we started our descent out of FL190 I asked the controller for a check on our mode C readout and it still lagged by about 400 ft or so. We switched mode C to the 'B' transponder and everything worked fine. We then went back to 'a' transponder and that worked fine as well. Since it worked on our last check, we landed and didn't write it up. On departure from las about 30 mins later, the same controller seemed overly interested in our altitude and comparing it to our mode C readout. It lagged by about 400 ft in the climb and the controller handed us off to ZOA by making a comment on our mode C. When we got to our destination we did write up the transponder since the las controller had questioned it so much, even though we both believed it was probably working just fine! Even with the computer screwing up and dumping our descent altitude out we did catch the problem and we did make the restr at fuzzy. We're both still puzzled today though as to why the controller seemed so interested in questioning our mode C readout, especially when we were climbing out of las with no crossing restrs to make.

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Original NASA ASRS Text

Title: FLC OF B737-400 HAS MODE C MALFUNCTION INDICATING 800 FT HIGHER THAN THEY ACTUALLY WERE. SECOND MODE C ACTIVATED TO COMPARE AND INDICATED CORRECTLY TO CTLR.

Narrative: WE WERE FLYING THE FUZZY ARR INTO LAS. ZOA GAVE US A CLRNC TO CROSS FUZZY INTXN AT FL190. THE CAPT, WHO WAS FLYING, TYPED 19000 FT INTO THE CDU SO THAT THE PLANE WOULD CROSS FUZZY IN VNAV AT 19000 FT AS WE WERE CLRED FOR. I DID SEE 19000 FT IN THE BOX AT FUZZY AND 19000 FT SET INTO THE ALT ALERTER. I GOT BUSY GETTING ATIS AND CALLING THE COMPANY FOR A GATE ASSIGNMENT AND WHEN I CAME BACK WE WERE ALREADY IN OUR DSCNT TO CROSS FUZZY AT FL190. SUDDENLY THE CAPT MADE A COMMENT AND EXTENDED THE SPOILERS TO INCREASE OUR RATE OF DSCNT. WHEN I LOOKED AT THE CDU THE 19000 FT XING RESTR THAT HAD BEEN SET FOR FUZZY HAD DROPPED OUT AND WE WERE NOW HIGH, BUT WE WOULD STILL HAVE NO PROB XING FUZZY AT FL190 WITH THE SPD BRAKES FULLY EXTENDED AND THE AIRSPD ON VMO. JUST BEFORE WE GOT TO FUZZY WE WERE HANDED OFF TO LAS APCH AND THE CTLR STARTED QUESTIONING OUR ALT AS WE LEVELED AT FL190. I TOLD HIM THAT WE WERE LEVEL AT FL190 ACROSS FUZZY AND HE SAID THAT HE SHOWED US INITIALLY 1200 FT HIGH AND THEN ABOUT 400-500 FT HIGH. I'VE ALWAYS BEEN UNDER THE IMPRESSION THAT OUR MODE C READOUT WOULD LAG OUR ACTUAL ALT, ESPECIALLY WHEN RAPIDLY DSNDING LIKE WE WERE. WHEN WE FINALLY LEVELED OFF, OUR ALT DID AGREE WITH WHAT THE CTLR SHOWED. WHEN WE STARTED OUR DSCNT OUT OF FL190 I ASKED THE CTLR FOR A CHK ON OUR MODE C READOUT AND IT STILL LAGGED BY ABOUT 400 FT OR SO. WE SWITCHED MODE C TO THE 'B' XPONDER AND EVERYTHING WORKED FINE. WE THEN WENT BACK TO 'A' XPONDER AND THAT WORKED FINE AS WELL. SINCE IT WORKED ON OUR LAST CHK, WE LANDED AND DIDN'T WRITE IT UP. ON DEP FROM LAS ABOUT 30 MINS LATER, THE SAME CTLR SEEMED OVERLY INTERESTED IN OUR ALT AND COMPARING IT TO OUR MODE C READOUT. IT LAGGED BY ABOUT 400 FT IN THE CLB AND THE CTLR HANDED US OFF TO ZOA BY MAKING A COMMENT ON OUR MODE C. WHEN WE GOT TO OUR DEST WE DID WRITE UP THE XPONDER SINCE THE LAS CTLR HAD QUESTIONED IT SO MUCH, EVEN THOUGH WE BOTH BELIEVED IT WAS PROBABLY WORKING JUST FINE! EVEN WITH THE COMPUTER SCREWING UP AND DUMPING OUR DSCNT ALT OUT WE DID CATCH THE PROB AND WE DID MAKE THE RESTR AT FUZZY. WE'RE BOTH STILL PUZZLED TODAY THOUGH AS TO WHY THE CTLR SEEMED SO INTERESTED IN QUESTIONING OUR MODE C READOUT, ESPECIALLY WHEN WE WERE CLBING OUT OF LAS WITH NO XING RESTRS TO MAKE.

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.