Narrative:

We were issued a discretionary descent clearance to cross 30 mi south of morgantown VOR at FL240. At the time of receipt of this clearance, we were approximately 20 mi from this point. We were trying to insert this crossing restriction in the FMS' vertical navigation path descent. This computer takes a while to compute the descent requirements and we were a little negligent in manually computing the descent this clearance required. If I may be candid about the shortcomings of this FMS VNAV function, it isn't rapid enough or simple enough to be of use in today's ATC environment. When on a profile descent such as lax you can count on the speed and altitude restrictions and plan ahead accordingly with the FMS. But in most route and terminal areas the traffic/WX/active runway vary enough to make planned vertical navigation a futile game of guess work. All too often I've programmed a descent only to be vectored or given speed restrictions or different altitude. Until the system can be made more predictable or our VNAV computers able to produce instantaneous information in response to a rapidly changing air traffic environment, we will need to emphasize the manual method of attaining proper altitude at crossing restrictions. Supplemental information from acn 146738: green restriction to cross 30 mi south of mgw at FL240, VNAV function not monitored closely enough and took 5 mi too far to reach FL240. Should have taken over manually to ensure exact altitude at exact DME.

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Original NASA ASRS Text

Title: CROSSING RESTRICTION NOT MET AS FLT CREW FAILS TO PROPERLY HANDLE DESCENT THROUGH FMC.

Narrative: WE WERE ISSUED A DISCRETIONARY DSNT CLRNC TO CROSS 30 MI S OF MORGANTOWN VOR AT FL240. AT THE TIME OF RECEIPT OF THIS CLRNC, WE WERE APPROX 20 MI FROM THIS POINT. WE WERE TRYING TO INSERT THIS XING RESTRICTION IN THE FMS' VERT NAV PATH DSNT. THIS COMPUTER TAKES A WHILE TO COMPUTE THE DSNT REQUIREMENTS AND WE WERE A LITTLE NEGLIGENT IN MANUALLY COMPUTING THE DSNT THIS CLRNC REQUIRED. IF I MAY BE CANDID ABOUT THE SHORTCOMINGS OF THIS FMS VNAV FUNCTION, IT ISN'T RAPID ENOUGH OR SIMPLE ENOUGH TO BE OF USE IN TODAY'S ATC ENVIRONMENT. WHEN ON A PROFILE DSNT SUCH AS LAX YOU CAN COUNT ON THE SPD AND ALT RESTRICTIONS AND PLAN AHEAD ACCORDINGLY WITH THE FMS. BUT IN MOST RTE AND TERMINAL AREAS THE TFC/WX/ACTIVE RWY VARY ENOUGH TO MAKE PLANNED VERT NAV A FUTILE GAME OF GUESS WORK. ALL TOO OFTEN I'VE PROGRAMMED A DSNT ONLY TO BE VECTORED OR GIVEN SPD RESTRICTIONS OR DIFFERENT ALT. UNTIL THE SYS CAN BE MADE MORE PREDICTABLE OR OUR VNAV COMPUTERS ABLE TO PRODUCE INSTANTANEOUS INFO IN RESPONSE TO A RAPIDLY CHANGING AIR TFC ENVIRONMENT, WE WILL NEED TO EMPHASIZE THE MANUAL METHOD OF ATTAINING PROPER ALT AT XING RESTRICTIONS. SUPPLEMENTAL INFO FROM ACN 146738: GREEN RESTRICTION TO CROSS 30 MI S OF MGW AT FL240, VNAV FUNCTION NOT MONITORED CLOSELY ENOUGH AND TOOK 5 MI TOO FAR TO REACH FL240. SHOULD HAVE TAKEN OVER MANUALLY TO ENSURE EXACT ALT AT EXACT DME.

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.