Narrative:

Gulfstream aircraft was proceeding wbound direct to flyby intersection just west of lvs VOR at FL430 when ZAB cleared them to descend to FL350. The first officer reset the altitude alert to 35000 ft and the captain started the descent on autoplt using vertical speed mode and approximately 1500-1800 FPM rate of descent set on the spz-800 flight director/autoplt control. As the aircraft passed through FL360 the first officer called out 1000 ft to go and the captain slowed the rate of descent slightly. The descent proceeded normally until approaching FL350. At 35100 ft the first officer called out that the autoplt was not capturing the selected altitude. The captain immediately selected altitude select on the flight guidance control panel but the aircraft continued through the altitude. The autoplt was then disconnected by the captain and a pull-up maneuver executed. The aircraft started to level and reverse direction as the altimeter indicated about 34700 ft and was level at 35000 ft approximately 10 seconds later. I believe the contributing factors to this incident to be the possible malfunction of the flight director/autoplt altitude hold function and the crew's failure to realize that the autoplt was not doing its job until too late. Another factor was the captain had just completed 21 day initial training on an aircraft with a very different flight guidance controller (fmz-8400). Subsequent corrective actions: 1) a visit to ZAB was made the next day by the pilot. 2) all aircraft system including the air data computer's, altimeters, flight director computers and autoplt were checked by the avionics shop in albuquerque the next day. Two discrepancies were found: a) the #1 transponder could not be supplied data from the #2 air data computer, and B) on one attempt to capture altitude, the #2 flight director failed. 3) crew procedures are being changed so that in addition to the normal callout at 1000 ft above or below the assigned altitude or restr the copilot will also state 'rate of descent 1000 FPM or less and flight director armed.' 4) the new procedure will be sent to flight safety to be incorporated into our flight crew semi-annual training program. 5) a crew meeting has been scheduled to inform all pilots of the new procedure and to explain further the limitations of the autoplt system. Supplemental information from acn 396174: immediately following this an MD80 airliner at FL330 reported to ATC they were receiving a TA alert on TCASII. At that time we were questioned by ATC and told to call center on the ground. From that point on a normal descent and landing were made. We have two types of aircraft with two different flight guidance computers. One does not require activation of altitude select to capture the preselected altitude. After flying this one type for a number of trips and then switching aircraft, it does take a continuous effort to make sure altitude select is selected. Recommendations: the PNF should call '1000 ft to go, altitude select.'

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Original NASA ASRS Text

Title: A G3 OVERSHOT THEIR ASSIGNED ALT WHEN THE AIR DATA COMPUTER FAILS TO RECOGNIZE THE ALT CAPTURE ALT AND DOES NOT LEVEL THE ACFT AT THE ALT ALERTER SETTING OF FL350. AN OPPOSITE DIRECTION MD80 GETS A TCASII TA.

Narrative: GULFSTREAM ACFT WAS PROCEEDING WBOUND DIRECT TO FLYBY INTXN JUST W OF LVS VOR AT FL430 WHEN ZAB CLRED THEM TO DSND TO FL350. THE FO RESET THE ALT ALERT TO 35000 FT AND THE CAPT STARTED THE DSCNT ON AUTOPLT USING VERT SPD MODE AND APPROX 1500-1800 FPM RATE OF DSCNT SET ON THE SPZ-800 FLT DIRECTOR/AUTOPLT CTL. AS THE ACFT PASSED THROUGH FL360 THE FO CALLED OUT 1000 FT TO GO AND THE CAPT SLOWED THE RATE OF DSCNT SLIGHTLY. THE DSCNT PROCEEDED NORMALLY UNTIL APCHING FL350. AT 35100 FT THE FO CALLED OUT THAT THE AUTOPLT WAS NOT CAPTURING THE SELECTED ALT. THE CAPT IMMEDIATELY SELECTED ALT SELECT ON THE FLT GUIDANCE CTL PANEL BUT THE ACFT CONTINUED THROUGH THE ALT. THE AUTOPLT WAS THEN DISCONNECTED BY THE CAPT AND A PULL-UP MANEUVER EXECUTED. THE ACFT STARTED TO LEVEL AND REVERSE DIRECTION AS THE ALTIMETER INDICATED ABOUT 34700 FT AND WAS LEVEL AT 35000 FT APPROX 10 SECONDS LATER. I BELIEVE THE CONTRIBUTING FACTORS TO THIS INCIDENT TO BE THE POSSIBLE MALFUNCTION OF THE FLT DIRECTOR/AUTOPLT ALT HOLD FUNCTION AND THE CREW'S FAILURE TO REALIZE THAT THE AUTOPLT WAS NOT DOING ITS JOB UNTIL TOO LATE. ANOTHER FACTOR WAS THE CAPT HAD JUST COMPLETED 21 DAY INITIAL TRAINING ON AN ACFT WITH A VERY DIFFERENT FLT GUIDANCE CTLR (FMZ-8400). SUBSEQUENT CORRECTIVE ACTIONS: 1) A VISIT TO ZAB WAS MADE THE NEXT DAY BY THE PLT. 2) ALL ACFT SYS INCLUDING THE ADC'S, ALTIMETERS, FLT DIRECTOR COMPUTERS AND AUTOPLT WERE CHKED BY THE AVIONICS SHOP IN ALBUQUERQUE THE NEXT DAY. TWO DISCREPANCIES WERE FOUND: A) THE #1 XPONDER COULD NOT BE SUPPLIED DATA FROM THE #2 ADC, AND B) ON ONE ATTEMPT TO CAPTURE ALT, THE #2 FLT DIRECTOR FAILED. 3) CREW PROCS ARE BEING CHANGED SO THAT IN ADDITION TO THE NORMAL CALLOUT AT 1000 FT ABOVE OR BELOW THE ASSIGNED ALT OR RESTR THE COPLT WILL ALSO STATE 'RATE OF DSCNT 1000 FPM OR LESS AND FLT DIRECTOR ARMED.' 4) THE NEW PROC WILL BE SENT TO FLT SAFETY TO BE INCORPORATED INTO OUR FLC SEMI-ANNUAL TRAINING PROGRAM. 5) A CREW MEETING HAS BEEN SCHEDULED TO INFORM ALL PLTS OF THE NEW PROC AND TO EXPLAIN FURTHER THE LIMITATIONS OF THE AUTOPLT SYS. SUPPLEMENTAL INFO FROM ACN 396174: IMMEDIATELY FOLLOWING THIS AN MD80 AIRLINER AT FL330 RPTED TO ATC THEY WERE RECEIVING A TA ALERT ON TCASII. AT THAT TIME WE WERE QUESTIONED BY ATC AND TOLD TO CALL CTR ON THE GND. FROM THAT POINT ON A NORMAL DSCNT AND LNDG WERE MADE. WE HAVE TWO TYPES OF ACFT WITH TWO DIFFERENT FLT GUIDANCE COMPUTERS. ONE DOES NOT REQUIRE ACTIVATION OF ALT SELECT TO CAPTURE THE PRESELECTED ALT. AFTER FLYING THIS ONE TYPE FOR A NUMBER OF TRIPS AND THEN SWITCHING ACFT, IT DOES TAKE A CONTINUOUS EFFORT TO MAKE SURE ALT SELECT IS SELECTED. RECOMMENDATIONS: THE PNF SHOULD CALL '1000 FT TO GO, ALT SELECT.'

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.