Narrative:

I was assigned as captain of gulfstream X, a g-iib, en route from vny to cou. I was well rested, not having flown this, nor any other aircraft for 10 days. The departure from vny and en route to a point 180 mi west of cou was normal. The controller at that point asked us when we would like to commence our descent into cou. He was informed that we would like to begin our descent by reaching 125 NM from cou. After proceeding beyond the request point, we requested descent. We were cleared to descend and maintain FL430. The first officer entered this altitude in the altitude selector controller (VNAV). I commenced the descent. At the same time, the first officer left the radio to obtain ATIS information and to contact the FBO. We were left with less than desirable time for our descent, and relying on the autoplt to capture the assigned altitude, my attention was diverted to calculating the required rate of descent for the remainder of the descent. We detected a TCASII target less than 2000 ft below us, checked our altitude with the VNAV setting, and realized that the controller and autoplt had failed to capture the FL430 altitude. At FL418, we realized what had occurred and commenced an immediate climb to FL430. The aircraft was hand flown for the remainder of the leg. On the subsequent leg from cou to cmh, the VNAV was closely monitored, and once again failed to capture the programmed altitude on initial descent into cmh, and was leveled off manually at the assigned altitude. The VNAV was disabled and placarded in accordance with company MEL procedures. The trip was completed 3 days later and the aircraft was returned to vny for replacement of the VNAV altitude selector. As PIC, I was responsible for safe conduct of the flight. While at no time was there imminent danger due to reduced separation, I should not have totally relied on the autoplt to capture the selected altitude. Mitigating factors on this were: ATC -- ATC's failure to anticipate 'slam dunk' descent. They could have and should have initiated the descent prior to the 125 NM if descent was not possible at that point. Equipment and location of equipment -- altitude selector failed to capture preset altitude. Altitude selector is located on center console out of either pilot's instrument scan. Recent experience -- this was my first trip as captain on this aircraft. My only other recent experience was 5 legs as copilot, with no left seat time and no time at the controls. I had not previously operated a gulfstream ii or III for the previous 3 1/2 yrs. I did recently complete g-ii initial training and a successful competency, instrument and part 135 chkride. Copilot duties -- the delayed descent rushed the flight crew. The copilot's attention was diverted to gathering ATIS information when the incident occurred. Supplemental information from acn 480414: when TCASII alerted and ATC questioned, we started a climb immediately back to FL430 and hand flew the rest of the flight. Only correction would be to not rely on equipment and not allow crew to take attention away while in any descent or climb.

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Original NASA ASRS Text

Title: A G-II FLC ALLOWS THEIR ACFT TO DSND BELOW THE ASSIGNED ALT WHICH PRODUCED A POTENTIAL CONFLICT WITH ANOTHER ACFT 100 MI W OF COU, MO.

Narrative: I WAS ASSIGNED AS CAPT OF GULFSTREAM X, A G-IIB, ENRTE FROM VNY TO COU. I WAS WELL RESTED, NOT HAVING FLOWN THIS, NOR ANY OTHER ACFT FOR 10 DAYS. THE DEP FROM VNY AND ENRTE TO A POINT 180 MI W OF COU WAS NORMAL. THE CTLR AT THAT POINT ASKED US WHEN WE WOULD LIKE TO COMMENCE OUR DSCNT INTO COU. HE WAS INFORMED THAT WE WOULD LIKE TO BEGIN OUR DSCNT BY REACHING 125 NM FROM COU. AFTER PROCEEDING BEYOND THE REQUEST POINT, WE REQUESTED DSCNT. WE WERE CLRED TO DSND AND MAINTAIN FL430. THE FO ENTERED THIS ALT IN THE ALT SELECTOR CONTROLLER (VNAV). I COMMENCED THE DSCNT. AT THE SAME TIME, THE FO LEFT THE RADIO TO OBTAIN ATIS INFO AND TO CONTACT THE FBO. WE WERE LEFT WITH LESS THAN DESIRABLE TIME FOR OUR DSCNT, AND RELYING ON THE AUTOPLT TO CAPTURE THE ASSIGNED ALT, MY ATTN WAS DIVERTED TO CALCULATING THE REQUIRED RATE OF DSCNT FOR THE REMAINDER OF THE DSCNT. WE DETECTED A TCASII TARGET LESS THAN 2000 FT BELOW US, CHKED OUR ALT WITH THE VNAV SETTING, AND REALIZED THAT THE CTLR AND AUTOPLT HAD FAILED TO CAPTURE THE FL430 ALT. AT FL418, WE REALIZED WHAT HAD OCCURRED AND COMMENCED AN IMMEDIATE CLB TO FL430. THE ACFT WAS HAND FLOWN FOR THE REMAINDER OF THE LEG. ON THE SUBSEQUENT LEG FROM COU TO CMH, THE VNAV WAS CLOSELY MONITORED, AND ONCE AGAIN FAILED TO CAPTURE THE PROGRAMMED ALT ON INITIAL DSCNT INTO CMH, AND WAS LEVELED OFF MANUALLY AT THE ASSIGNED ALT. THE VNAV WAS DISABLED AND PLACARDED IN ACCORDANCE WITH COMPANY MEL PROCS. THE TRIP WAS COMPLETED 3 DAYS LATER AND THE ACFT WAS RETURNED TO VNY FOR REPLACEMENT OF THE VNAV ALT SELECTOR. AS PIC, I WAS RESPONSIBLE FOR SAFE CONDUCT OF THE FLT. WHILE AT NO TIME WAS THERE IMMINENT DANGER DUE TO REDUCED SEPARATION, I SHOULD NOT HAVE TOTALLY RELIED ON THE AUTOPLT TO CAPTURE THE SELECTED ALT. MITIGATING FACTORS ON THIS WERE: ATC -- ATC'S FAILURE TO ANTICIPATE 'SLAM DUNK' DSCNT. THEY COULD HAVE AND SHOULD HAVE INITIATED THE DSCNT PRIOR TO THE 125 NM IF DSCNT WAS NOT POSSIBLE AT THAT POINT. EQUIP AND LOCATION OF EQUIP -- ALT SELECTOR FAILED TO CAPTURE PRESET ALT. ALT SELECTOR IS LOCATED ON CTR CONSOLE OUT OF EITHER PLT'S INST SCAN. RECENT EXPERIENCE -- THIS WAS MY FIRST TRIP AS CAPT ON THIS ACFT. MY ONLY OTHER RECENT EXPERIENCE WAS 5 LEGS AS COPLT, WITH NO L SEAT TIME AND NO TIME AT THE CTLS. I HAD NOT PREVIOUSLY OPERATED A GULFSTREAM II OR III FOR THE PREVIOUS 3 1/2 YRS. I DID RECENTLY COMPLETE G-II INITIAL TRAINING AND A SUCCESSFUL COMPETENCY, INST AND PART 135 CHKRIDE. COPLT DUTIES -- THE DELAYED DSCNT RUSHED THE FLC. THE COPLT'S ATTN WAS DIVERTED TO GATHERING ATIS INFO WHEN THE INCIDENT OCCURRED. SUPPLEMENTAL INFO FROM ACN 480414: WHEN TCASII ALERTED AND ATC QUESTIONED, WE STARTED A CLB IMMEDIATELY BACK TO FL430 AND HAND FLEW THE REST OF THE FLT. ONLY CORRECTION WOULD BE TO NOT RELY ON EQUIP AND NOT ALLOW CREW TO TAKE ATTN AWAY WHILE IN ANY DSCNT OR CLB.

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.