FLC OF AN SMT CPR ACFT OVERSHOT ASSIGNED ALT ON DSCNT DUE TO A FAULTY XPONDER ALT CODE READOUT.

Narrative

DUE TO STRONG WINDS AND LIGHT TO MODERATE TURB; A LOWER ALT OF FL220 WAS REQUESTED; AND SHORTLY A DSCNT CLRNC TO FL220 WAS RECEIVED FROM ATC. THE ACFT WAS BEING FLOWN ON AUTOPLT WITH THE SOFT RIDE FUNCTION OF THE PITCH AXIS ENGAGED DUE TO THE TURB FLT CONDITIONS. AT THE TIME OF THE REQUESTED LOWER ALT OUR FLT LEVEL WAS FL240. UPON RECEIPT OF A CLRNC TO FL220; A DSCNT WAS INITIATED ON AUTOPLT BY SELECTING FL220 ON THE ALT SELECTOR/ALERTER CTL AND DEPRESSING OF THE DSCNT MODE BUTTON ON THE AUTOPLT CTL PANEL; WHICH INITIATED A STANDARD PROFILE DSCNT RATE FROM THE AUTOPLT DSCNT PROFILE PROGRAM. WHILE MONITORING THE DSCNT AND UPON LEAVING FL230; THE ALT ENCODER READOUT ON THE XPONDER CTL HEAD WAS OBSERVED TO BE IN ERROR BY A 300 FT DIFFERENCE BTWN THE READOUT AND THE INDICATED ALT OF THE ALTIMETER. THE #2 ENCODER WAS SELECTED IN ORDER TO TROUBLESHOOT AND VERIFY AND FURTHER DISCREPANCY ON THE ALT/XPONDER/ENCODER SYS. THE AUTOPLT OVERSHOT THE ALT LEVEL OUT BY 200 FT; AND THEN RETURNED THE ACFT TO FL220 AFTER A GENTLE ROUNDOUT; WHICH IS A NORMAL OCCURRENCE IN TURBULENT FLT CONDITIONS WITH THE SOFT RIDE FUNCTION ENGAGED. AT THE TIME OF LEVELOFF; ATC ADVISED THAT WE CHK OUR ALT; WHICH AT THE TIME WAS FL220; WITH ALT HOLD ENGAGED AND DISPLAYED ON THE AUTOPLT/FLT DIRECTOR ANNUNCIATOR PANELS. THE REASON FOR THE SUBMISSION OF THIS RPT IS IN ORDER TO NOT BE CHARGED WITH THE ALTDEV EVENT THAT DID NOT OCCUR. THE PROBABLE CAUSE FOR ATC NOTIFYING OF THE ALT DISCREPANCY WAS DUE TO HAVING SWITCHED FROM ENCODER 1 TO ENCODER 2 DURING THE DSCNT AS DESCRIBED HEREIN; AND THE SLIGHT OVERSHOOT ON LEVELOFF BY THE AUTOPLT. IN CONCLUSION; THE DESCRIBED ABOVE EVENTS IS POSITIVE PROOF THAT IN TODAY'S TIMES OF ELECTRONICS AND COMPUTERS; NO SYS IS FOOLPROOF AND THAT ERRONEOUS DISCREPANCIES DO OCCUR FOR WHICH THE PLT IS NEITHER RESPONSIBLE OR ACCOUNTABLE FOR DUE TO ELECTRONIC 'GLITCHES' IN THE SYS AND EQUIP.