Narrative:

At gate and while performing the before push checklist; we had the performance computer give us a bad recommendation. Utilizing 'optimum flap' in our performance computer resulted in an improper recommendation of flaps 15 with a stopping margin of only 316 ft. When manually selecting flaps 25 gave us a stopping margin of 1640 ft. Funny thing is that it even recommended 'bleeds off' or another flap setting! Naturally; we were puzzled at why it didn't recommend flaps 25 in the first place.since we don't perform intersection takeoffs unless 2000 ft or better stopping margin is available; it seems academic to assume that we would like something more than 316' for an 'optimum' setting when more is available with a different setting. Please don't take the comments; which follow as personal; they are not and only used here for emphasis. So; here goes.why does the performance computer force us to hunt for an acceptable stopping margin? The use of 'optimum flap' failed to deliver on this event and it knew it as evidenced by the screen flashing with recommendation for a different flap setting. Imagine what the lawyers would do to us if we had accepted 316 ft of stopping margin when 1640 ft was available and something went wrong. Here is the data present/used during our event: a/C type: -800 tow: 168;400; (release) tow: 166;558 (loading schedule). Bos runway 9 temp: 19C; altimeter: 30.12 wind: 100/5.my takeaway here is that something is wrong with how our performance computer chooses a flap setting when 'optimum flap' is selected. I found it interesting to note that dispatch utilized a flap setting of 25 when calculating data for our release out of bos; but that inside the cockpit our performance computer was perfectly happy with flaps 15 and a stopping margin of only 316 ft. Please note that 'loading and planning' were highlighted as areas in this event due to the fact that nothing else looked appropriate given the circumstances.

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Original NASA ASRS Text

Title: A B737-800 Captain questioned the aircraft performance computer erroneously calculating an optimum flaps 15 runway takeoff stopping margin of only 316 FT when manually selecting flaps 25 increased the distance to 1;640 FT.

Narrative: At gate and while performing the Before Push Checklist; we had the performance computer give us a bad recommendation. Utilizing 'Optimum Flap' in our performance computer resulted in an improper recommendation of flaps 15 with a stopping margin of only 316 FT. When manually selecting flaps 25 gave us a stopping margin of 1640 FT. Funny thing is that it even recommended 'bleeds off' or another flap setting! Naturally; we were puzzled at why it didn't RECOMMEND flaps 25 in the first place.Since we don't perform intersection takeoffs unless 2000 FT or better stopping margin is available; it seems academic to assume that we would like something more than 316' for an 'Optimum' setting when more is available with a different setting. Please don't take the comments; which follow as personal; they are not and only used here for emphasis. So; here goes.Why does the performance computer force us to hunt for an acceptable stopping margin? The use of 'optimum flap' failed to deliver on this event and it knew it as evidenced by the screen flashing with recommendation for a different flap setting. Imagine what the lawyers would do to us if we had accepted 316 FT of stopping margin when 1640 FT was available and something went wrong. Here is the data present/used during our event: A/C type: -800 TOW: 168;400; (Release) TOW: 166;558 (Loading Schedule). BOS Runway 9 Temp: 19C; Altimeter: 30.12 Wind: 100/5.My takeaway here is that something is wrong with how our performance computer chooses a flap setting when 'Optimum Flap' is selected. I found it interesting to note that Dispatch utilized a flap setting of 25 when calculating data for our Release out of BOS; but that inside the cockpit our performance computer was perfectly happy with flaps 15 and a stopping margin of only 316 FT. Please note that 'loading and planning' were highlighted as areas in this event due to the fact that nothing else looked appropriate given the circumstances.

Data retrieved from NASA's ASRS site 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.