Narrative:

We were being vectored for an approach to lga. We were given a turn to 330 degrees and a descent from 9000 to 7000'. The copilot was using the block V autoplt and had selected 'arm' with 7000' in the altitude window and 30.43 set for the altimeter. At approximately 7500', approach advised us we had traffic at 10 O'clock, low. The TCAS showed the traffic 1300' below us. Immediately after the traffic advisory, approach advised us to expect the ILS to 13, our third runway change. We then heard 'traffic, traffic' from the TCAS. I looked at the radar scope and noted the traffic was now 600' below us and almost 2 mi away at our 4 O'clock. The captain then noticed we had missed our level off altitude. The copilot immediately corrected back to 7000'. We had descended to approximately 6600'. We did not receive an RA from the TCAS and there was no conflict, since the traffic had already passed us and was behind. No comment was made by approach control. The autoplt had failed to capture, even though it showed it was armed. There was an information write-up that this autoplt had failed to capture 1 time, and then worked the next 4 times. No maintenance action had been taken. Supplemental information from acn 167677: factors effecting the quality of performance: lack of on-line familiarization of TCAS system--first time captain actually used TCAS with qualified first officer in actual flying situation. I am not referring to an actual equipment familiarity, but the process of incorporating a new, though positive, distracting system into a normal routine--new bells, new messages wrapped around conventional, basic habit patterns. Supplemental information from acn 168394: we realized the autoplt had not captured and continued to 6600'. An immediate climb was initiated and aircraft returned to 7000'. Flight was then cleared to 4000' and vectored to downwind for runway 13 lga. No comment was received from ATC during or after altitude excursion. Maintenance review after flight showed 1 write-up in 11/90 and numerous write-ups beginning 1/fri/91. Autoplt was placed on mco.

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Original NASA ASRS Text

Title: ALT DEVIATION ALT OVERSHOT IN DESCENT.

Narrative: WE WERE BEING VECTORED FOR AN APCH TO LGA. WE WERE GIVEN A TURN TO 330 DEGS AND A DSNT FROM 9000 TO 7000'. THE COPLT WAS USING THE BLOCK V AUTOPLT AND HAD SELECTED 'ARM' WITH 7000' IN THE ALT WINDOW AND 30.43 SET FOR THE ALTIMETER. AT APPROX 7500', APCH ADVISED US WE HAD TFC AT 10 O'CLOCK, LOW. THE TCAS SHOWED THE TFC 1300' BELOW US. IMMEDIATELY AFTER THE TFC ADVISORY, APCH ADVISED US TO EXPECT THE ILS TO 13, OUR THIRD RWY CHANGE. WE THEN HEARD 'TFC, TFC' FROM THE TCAS. I LOOKED AT THE RADAR SCOPE AND NOTED THE TFC WAS NOW 600' BELOW US AND ALMOST 2 MI AWAY AT OUR 4 O'CLOCK. THE CAPT THEN NOTICED WE HAD MISSED OUR LEVEL OFF ALT. THE COPLT IMMEDIATELY CORRECTED BACK TO 7000'. WE HAD DSNDED TO APPROX 6600'. WE DID NOT RECEIVE AN RA FROM THE TCAS AND THERE WAS NO CONFLICT, SINCE THE TFC HAD ALREADY PASSED US AND WAS BEHIND. NO COMMENT WAS MADE BY APCH CTL. THE AUTOPLT HAD FAILED TO CAPTURE, EVEN THOUGH IT SHOWED IT WAS ARMED. THERE WAS AN INFO WRITE-UP THAT THIS AUTOPLT HAD FAILED TO CAPTURE 1 TIME, AND THEN WORKED THE NEXT 4 TIMES. NO MAINT ACTION HAD BEEN TAKEN. SUPPLEMENTAL INFO FROM ACN 167677: FACTORS EFFECTING THE QUALITY OF PERFORMANCE: LACK OF ON-LINE FAMILIARIZATION OF TCAS SYS--FIRST TIME CAPT ACTUALLY USED TCAS WITH QUALIFIED F/O IN ACTUAL FLYING SITUATION. I AM NOT REFERRING TO AN ACTUAL EQUIP FAMILIARITY, BUT THE PROCESS OF INCORPORATING A NEW, THOUGH POSITIVE, DISTRACTING SYS INTO A NORMAL ROUTINE--NEW BELLS, NEW MESSAGES WRAPPED AROUND CONVENTIONAL, BASIC HABIT PATTERNS. SUPPLEMENTAL INFO FROM ACN 168394: WE REALIZED THE AUTOPLT HAD NOT CAPTURED AND CONTINUED TO 6600'. AN IMMEDIATE CLB WAS INITIATED AND ACFT RETURNED TO 7000'. FLT WAS THEN CLRED TO 4000' AND VECTORED TO DOWNWIND FOR RWY 13 LGA. NO COMMENT WAS RECEIVED FROM ATC DURING OR AFTER ALT EXCURSION. MAINT REVIEW AFTER FLT SHOWED 1 WRITE-UP IN 11/90 AND NUMEROUS WRITE-UPS BEGINNING 1/FRI/91. AUTOPLT WAS PLACED ON MCO.

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.