Narrative:

We were at FL250 when ZNY cleared us to cross 30 mi west of lancaster at 17000 ft. The first officer as PF on autoplt dialed in 17000 ft in the altitude alerter then started programming the FMC for the crossing restr. I dialed in lancaster on my VOR. Realizing that we were fairly close to the power off descent profile, I mentioned that we were fairly close and selected level change. There was no intersection for the crossing point so the first officer had to build it, which takes time. When the FMC finished thinking, it indicated that we were well below profile, so the first officer hit VNAV which brought the descent back to 1000 FPM. That didn't make sense so I looked at the descent profile which verified what the first officer had indicated. My VOR readout and the FMC did not agree, but I did not realize what was wrong at the time. I advised that we were pretty close to the profile and once again selected level change. The first officer was confused as I was but accepted the power off descent profile. I should have mentioned what my source of information was as I realized in hindsight that he had no idea what I was basing my concern on. Passing FL200, I concluded that we would make the restr based on the VOR information but that it would be close. I advised 10 mi which probably caused more confusion since the FMC indicated that we were significantly further away. In deference to me, the first officer increased the descent speed up to our previously assigned limit speed to hasten the descent. It wasn't until we were passing FL180 that I realized that my math computations had not taken the tailwind into account and that we would not make it quite in time, but that it was too late to do anything about it. As it turned out, we crossed the restr point at 17400 ft and were level at 29.25 NM from lancaster. We were very close but not perfect. I did look at the TCASII and verified that there was no traffic anywhere around us so separation was not an issue. It took a while but I finally realized that the first officer had constructed the crossing waypoint correctly but had inserted it after the next intersection instead of before it. The FMC assumed that we were going to fly to the pre-existing intersection then back to the crossing point, which added a number of flying mi to the crossing point and led to the descent profile being in error. Unfortunately the error was caused by a reliance on modern technology which is wonderful but relies on correct inputs. The error was not immediately apparent to me and I began to question my own information which led to my lack of forcefulness in insisting on a power off descent profile until the restr was assured. We had an extended conversation after the fact where I showed the first officer how to verify that constructed intxns could be verified to be placed correctly. We also discussed how when there is any confusion as to who is correct, the appropriate solution is to follow the most conservative course of action. If it turns out that it was unnecessary and we are way early on the crossing profile, nothing is lost but a small amount of fuel. The other possibility is unacceptable. Controllers could help minimize this potential problem when dealing with modern FMC equipped aircraft by giving crossing restrs based on predefined intxns that would likely already be in our database. Any time you have to construct a crossing point, it takes a lot more time and opens up a significant increased possibility for error. It appeared in hindsight that this restr was not given for any specific traffic situation but to put us at a specific altitude for crossing a sector boundary. This is a necessary part of the ATC environment but in these cases it would be nice to have the expected crossing restr published on the STAR, give us a little more warning or to just give us the descent clearance with the words to expedite. I am once again reminded that the possibility for confusion and subsequent error is always there. If there is any continued doubt, take the most conservative of all the options and do not hesitate to ask ATC for help or, in this case, to clarify the distance to the crossing point.

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Original NASA ASRS Text

Title: B737-300 FLC MISCALCULATE FMS ENTRIES AND DISCOVER TOO LATE TO CORRECT FOR UNDERSHOOT.

Narrative: WE WERE AT FL250 WHEN ZNY CLRED US TO CROSS 30 MI W OF LANCASTER AT 17000 FT. THE FO AS PF ON AUTOPLT DIALED IN 17000 FT IN THE ALT ALERTER THEN STARTED PROGRAMMING THE FMC FOR THE XING RESTR. I DIALED IN LANCASTER ON MY VOR. REALIZING THAT WE WERE FAIRLY CLOSE TO THE PWR OFF DSCNT PROFILE, I MENTIONED THAT WE WERE FAIRLY CLOSE AND SELECTED LEVEL CHANGE. THERE WAS NO INTXN FOR THE XING POINT SO THE FO HAD TO BUILD IT, WHICH TAKES TIME. WHEN THE FMC FINISHED THINKING, IT INDICATED THAT WE WERE WELL BELOW PROFILE, SO THE FO HIT VNAV WHICH BROUGHT THE DSCNT BACK TO 1000 FPM. THAT DIDN'T MAKE SENSE SO I LOOKED AT THE DSCNT PROFILE WHICH VERIFIED WHAT THE FO HAD INDICATED. MY VOR READOUT AND THE FMC DID NOT AGREE, BUT I DID NOT REALIZE WHAT WAS WRONG AT THE TIME. I ADVISED THAT WE WERE PRETTY CLOSE TO THE PROFILE AND ONCE AGAIN SELECTED LEVEL CHANGE. THE FO WAS CONFUSED AS I WAS BUT ACCEPTED THE PWR OFF DSCNT PROFILE. I SHOULD HAVE MENTIONED WHAT MY SOURCE OF INFO WAS AS I REALIZED IN HINDSIGHT THAT HE HAD NO IDEA WHAT I WAS BASING MY CONCERN ON. PASSING FL200, I CONCLUDED THAT WE WOULD MAKE THE RESTR BASED ON THE VOR INFO BUT THAT IT WOULD BE CLOSE. I ADVISED 10 MI WHICH PROBABLY CAUSED MORE CONFUSION SINCE THE FMC INDICATED THAT WE WERE SIGNIFICANTLY FURTHER AWAY. IN DEFERENCE TO ME, THE FO INCREASED THE DSCNT SPD UP TO OUR PREVIOUSLY ASSIGNED LIMIT SPD TO HASTEN THE DSCNT. IT WASN'T UNTIL WE WERE PASSING FL180 THAT I REALIZED THAT MY MATH COMPUTATIONS HAD NOT TAKEN THE TAILWIND INTO ACCOUNT AND THAT WE WOULD NOT MAKE IT QUITE IN TIME, BUT THAT IT WAS TOO LATE TO DO ANYTHING ABOUT IT. AS IT TURNED OUT, WE CROSSED THE RESTR POINT AT 17400 FT AND WERE LEVEL AT 29.25 NM FROM LANCASTER. WE WERE VERY CLOSE BUT NOT PERFECT. I DID LOOK AT THE TCASII AND VERIFIED THAT THERE WAS NO TFC ANYWHERE AROUND US SO SEPARATION WAS NOT AN ISSUE. IT TOOK A WHILE BUT I FINALLY REALIZED THAT THE FO HAD CONSTRUCTED THE XING WAYPOINT CORRECTLY BUT HAD INSERTED IT AFTER THE NEXT INTXN INSTEAD OF BEFORE IT. THE FMC ASSUMED THAT WE WERE GOING TO FLY TO THE PRE-EXISTING INTXN THEN BACK TO THE XING POINT, WHICH ADDED A NUMBER OF FLYING MI TO THE XING POINT AND LED TO THE DSCNT PROFILE BEING IN ERROR. UNFORTUNATELY THE ERROR WAS CAUSED BY A RELIANCE ON MODERN TECHNOLOGY WHICH IS WONDERFUL BUT RELIES ON CORRECT INPUTS. THE ERROR WAS NOT IMMEDIATELY APPARENT TO ME AND I BEGAN TO QUESTION MY OWN INFO WHICH LED TO MY LACK OF FORCEFULNESS IN INSISTING ON A PWR OFF DSCNT PROFILE UNTIL THE RESTR WAS ASSURED. WE HAD AN EXTENDED CONVERSATION AFTER THE FACT WHERE I SHOWED THE FO HOW TO VERIFY THAT CONSTRUCTED INTXNS COULD BE VERIFIED TO BE PLACED CORRECTLY. WE ALSO DISCUSSED HOW WHEN THERE IS ANY CONFUSION AS TO WHO IS CORRECT, THE APPROPRIATE SOLUTION IS TO FOLLOW THE MOST CONSERVATIVE COURSE OF ACTION. IF IT TURNS OUT THAT IT WAS UNNECESSARY AND WE ARE WAY EARLY ON THE XING PROFILE, NOTHING IS LOST BUT A SMALL AMOUNT OF FUEL. THE OTHER POSSIBILITY IS UNACCEPTABLE. CTLRS COULD HELP MINIMIZE THIS POTENTIAL PROB WHEN DEALING WITH MODERN FMC EQUIPPED ACFT BY GIVING XING RESTRS BASED ON PREDEFINED INTXNS THAT WOULD LIKELY ALREADY BE IN OUR DATABASE. ANY TIME YOU HAVE TO CONSTRUCT A XING POINT, IT TAKES A LOT MORE TIME AND OPENS UP A SIGNIFICANT INCREASED POSSIBILITY FOR ERROR. IT APPEARED IN HINDSIGHT THAT THIS RESTR WAS NOT GIVEN FOR ANY SPECIFIC TFC SIT BUT TO PUT US AT A SPECIFIC ALT FOR XING A SECTOR BOUNDARY. THIS IS A NECESSARY PART OF THE ATC ENVIRONMENT BUT IN THESE CASES IT WOULD BE NICE TO HAVE THE EXPECTED XING RESTR PUBLISHED ON THE STAR, GIVE US A LITTLE MORE WARNING OR TO JUST GIVE US THE DSCNT CLRNC WITH THE WORDS TO EXPEDITE. I AM ONCE AGAIN REMINDED THAT THE POSSIBILITY FOR CONFUSION AND SUBSEQUENT ERROR IS ALWAYS THERE. IF THERE IS ANY CONTINUED DOUBT, TAKE THE MOST CONSERVATIVE OF ALL THE OPTIONS AND DO NOT HESITATE TO ASK ATC FOR HELP OR, IN THIS CASE, TO CLARIFY THE DISTANCE TO THE XING POINT.

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.