A HAWKER HUNTER PLT CLBED ABOVE HIS ASSIGNED ALT WHILE RETURNING TO HIS HOME ARPT AFTER AN AIRSHOW. HE IS PUZZLED AS TO WHY HE WAS NOT ADVISED OF HIS ERROR BY THE ARTCC RADAR CTLR.

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THE HAWKER HUNTER JET ACFT WAS ON DISPLAY AT AN AIRSHOW AND WAS SCHEDULED TO PERFORM A DISPLAY FLY-BY AROUND XA00 LCL. THE PLT CONTACTED ZOA BY TELEPHONE 4-5 HRS PRIOR TO HIS SCHEDULED FLT TO DISCUSS ARRANGEMENTS WHICH WOULD ALLOW HIM TO PERFORM HIS DISPLAY FLY-BY AND THEN CONTINUE ENRTE BACK TO VNY. THE AREA SUPVR FILED A FLT PLAN ON THE PLT'S BEHALF WITH A REQUESTED CRUISE ALT OF FL250. THIS ALT WAS REQUESTED SO THAT THE PLT COULD OBTAIN MORE FAVORABLE FUEL CONSUMPTION IN THE FORMER MIL JET HE WAS FLYING; EVEN THOUGH THE DURATION OF THE FLT WAS PLANNED AT ONLY 30 MINS. THE PLT FLEW HIS DISPLAY AND THEN BEGAN AN ENRTE CLB WHILE; AT THE SAME TIME; TALKING TO DEP. FLT CONDITIONS ALONG THE ENTIRE RTE AND AT ALL ALTS WERE CLR WITH OVER 25 MI OF VISIBILITY. ALTHOUGH A POP-UP CLRNC WAS PREARRANGED; ACTIVATING THE CLRNC WAS DELAYED SLIGHTLY WHICH RESULTED IN THE ACFT BEING FARTHER ALONG ITS FLT THAN ORIGINALLY ANTICIPATED. ONCE A CLRNC WAS OBTAINED; THE ACFT WAS PROGRESSIVELY STEPPED TO HIGHER ALTS; THE FINAL ASSIGNED BEING FL230. THE PLT ACKNOWLEDGED THE LAST XMISSION AND BEGAN HIS FINAL CLB. RATHER THAN STOPPING AT FL230; THE PLT INADVERTENTLY CLBED TO HIS ORIGINALLY REQUESTED FL250. A MECHANICAL ASSIGNED ALT REMINDER DEVICE; NORMALLY USED BY THE PLT; WAS NOT IN THIS ACFT WHICH MAY HAVE BEEN A CONTRIBUTING FACTOR IN THIS EVENT. DURING THE LAST STAGE OF HIS CLB; IN PARTICULAR AFTER EXCEEDING FL230; THE PLT WAS NEVER CONTACTED BY ZOA TO ALERT HIM TO THE FACT THAT HE HAD EXCEEDED HIS ASSIGNED ALT. A COURTESY ADVISORY OF THIS TYPE IS NORMALLY MADE WHEN A PLANE DEVIATES EVEN 300 FT ABOVE OR BELOW AN ASSIGNED ALT; LET ALONE WHEN IT CLBS TO 2000 FT ABOVE IT. IN FACT; IT IS THE PLT'S RECOLLECTION THAT NO MENTION OF HIS HAVING EXCEEDED THE ASSIGNED ALT WAS EVER MADE DURING THE ENTIRE FLT. THE ONLY QUESTION RAISED LATER IN THE FLT BY A CTLR WAS WHETHER OR NOT THE ACFT'S XPONDER WAS OPERATING PROPERLY. THE PLT RESPONDED THAT HE BELIEVED IT WAS. THE ACFT LEVELED OFF AND REMAINED AT FL250 FOR ONLY A FEW MINS BEFORE THE PLT REQUESTED AND OBTAINED PERMISSION TO BEGIN A DSCNT. AS PREARRANGED DURING THE ORIGINAL TELEPHONE CALL WITH ZOA; THE PLT INTENDED TO CANCEL IFR UPON DSNDING BELOW FL180 AND CONTINUE VFR TO VNY. THIS IS WHAT HE DID AND HOW THE FLT TERMINATED WITHOUT ANY FURTHER INCIDENT. ALTHOUGH THE ACFT ASCENDED TO THE WRONG ALT; THERE WERE NEVER ANY TFC CONFLICTS DURING THIS FLT. THIS IS ACCORDING TO A TELEPHONE CONVERSATION WITH THE AREA SUPVR IN ZOA HELD SHORTLY AFTER LNDG AT VNY. IT WAS DURING THIS SAME TELEPHONE CONVERSATION THAT THE FIRST EXPLICIT MENTION OF THE PLT HAVING EXCEEDED THE ASSIGNED ALT WAS EVEN MADE. IN A SUBSEQUENT TELEPHONE CALL MADE SEVERAL DAYS LATER TO ZOA BY THE PLT; ADDITIONAL INFO WAS REVEALED. APPARENTLY THE ACFT CODE ENTERED ON THE FLT PLAN WAS HWKR; HOWEVER; THIS IS NOT THE PROPER CODE FOR A HAWKER HUNTER NOR IS IT THE CODE GIVEN TO CTR OVER THE TELEPHONE BY THE PLT. IT IS NOT EVEN CLR WHETHER HWKR IS A VALID CODE AT ALL. IT APPEARS THAT THE CTR'S COMPUTER OBSERVED RATE OF CLB FOR THE ACT WAS NOT CONSISTENT WITH ITS DATABASE 'AVERAGE' VALUE FOR HWKR AND; THEREFORE; THE COMPUTER DETERMINED THAT THE ACFT'S MODE C ALT MIGHT ALSO BE IN ERROR. AS A RESULT OF THE COMPUTER'S CONCLUSION; IT DISPLAYED MULTIPLE 'X' ON THE CTLR'S SCREEN IN LIEU OF THE MODE C RPTED ALT OF FL250. THIS MAY ACCOUNT FOR WHY THE PLT WAS ASKED ABOUT THE CONDITION OF THE ACFT'S XPONDER. A DIRECT CONSEQUENCE OF THE NON SPECIFIC ALT ON THE SCREEN WAS THAT THE ZOA CTLR DID NOT REALIZE THAT THE HUNTER HAD PASSED THROUGH ITS ASSIGNED ALT AND; HENCE; DID NOT ATTEMPT TO CONTACT THE PLT AND ALERT HIM OF HIS TRANSGRESSION. DURING THIS SAME TELEPHONE CALL; IT WAS ALSO POINTED OUT THAT ALTHOUGH ZLA HAD NOTED THE ACFT AT A HIGHER ALT THAN ASSIGNED; IT APPARENTLY DID NOT NOTIFY ZOA OF ITS OBSERVATION. AND BY THE TIME THE ACFT WAS HANDED OVER TO ZLA; THE PLT WAS ALREADY REQUESTING TO DSND TO A LOWER ALT FOR THE PURPOSE OF CANCELING IFR. HE WAS CLRED TO DO SO AND CONTINUED ON TO VNY AS A VFR FLT.