Narrative:

On initial descent from FL330 to FL310, a distraction in the form of a minor system problem focused attention away from the front panel. During the discussion, the 1ST altitude alert went unnoticed by the pilots. Autoplt continued descent through FL310 until at approximately 250' low, the altitude alert sounded and the recovery was initiated immediately with maximum error of approximately 500'. Moments later, clearance was received to continue descent with no unusual comments. Factors involved in this error were a failure of the pilot flying to remain exclusively the master of the aircraft and remove himself from non flying duties, a failure of pilot non flying to cross check pilot flying, an overreliance in the form of 'knee jerk' response on automated system and reminders. Reflection on this mistake has motivated me towards improved awareness and attention to detail. Also, thoughts on how to reduce the maximum error when this occurrence inevitably happens are possible improvements of the altitude alert system. 1) narrow the alert envelope from plus/minus approximately 250' to plus/minus approximately 175'. This should alert the pilots sooner to a deviation. 2) integrate vertical speed logic into the system such that when passing the programmed altitude with vertical speed in excess of 500'/min, the alert will sound off. I hope these suggestions might be considered because the human factor will always be present, therefore, the sooner such errors are detected, the sooner they can be corrected.

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Original NASA ASRS Text

Title: LGT OVERSHOOTS ASSIGNED ALT DURING DESCENT.

Narrative: ON INITIAL DSCNT FROM FL330 TO FL310, A DISTR IN THE FORM OF A MINOR SYS PROB FOCUSED ATTN AWAY FROM THE FRONT PANEL. DURING THE DISCUSSION, THE 1ST ALT ALERT WENT UNNOTICED BY THE PLTS. AUTOPLT CONTINUED DSCNT THROUGH FL310 UNTIL AT APPROX 250' LOW, THE ALT ALERT SOUNDED AND THE RECOVERY WAS INITIATED IMMEDIATELY WITH MAX ERROR OF APPROX 500'. MOMENTS LATER, CLRNC WAS RECEIVED TO CONTINUE DSCNT WITH NO UNUSUAL COMMENTS. FACTORS INVOLVED IN THIS ERROR WERE A FAILURE OF THE PLT FLYING TO REMAIN EXCLUSIVELY THE MASTER OF THE ACFT AND REMOVE HIMSELF FROM NON FLYING DUTIES, A FAILURE OF PLT NON FLYING TO CROSS CHK PLT FLYING, AN OVERRELIANCE IN THE FORM OF 'KNEE JERK' RESPONSE ON AUTOMATED SYS AND REMINDERS. REFLECTION ON THIS MISTAKE HAS MOTIVATED ME TOWARDS IMPROVED AWARENESS AND ATTN TO DETAIL. ALSO, THOUGHTS ON HOW TO REDUCE THE MAX ERROR WHEN THIS OCCURRENCE INEVITABLY HAPPENS ARE POSSIBLE IMPROVEMENTS OF THE ALT ALERT SYS. 1) NARROW THE ALERT ENVELOPE FROM PLUS/MINUS APPROX 250' TO PLUS/MINUS APPROX 175'. THIS SHOULD ALERT THE PLTS SOONER TO A DEV. 2) INTEGRATE VERT SPD LOGIC INTO THE SYS SUCH THAT WHEN PASSING THE PROGRAMMED ALT WITH VERT SPD IN EXCESS OF 500'/MIN, THE ALERT WILL SOUND OFF. I HOPE THESE SUGGESTIONS MIGHT BE CONSIDERED BECAUSE THE HUMAN FACTOR WILL ALWAYS BE PRESENT, THEREFORE, THE SOONER SUCH ERRORS ARE DETECTED, THE SOONER THEY CAN BE CORRECTED.

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.