Narrative:

In our heavy 737-800 on a warm chicago midway day; the pwb (pilots weather briefing) takeoff data gave us a flaps 25; max thrust takeoff; with a runway 22L stopping margin of 270 ft. It calculated bleeds on. I wanted more takeoff margin; because I don't feel that 270 ft at mdw is adequate. Winds are constantly shifting at mdw; and I have no idea if that 270 ft stopping margin would actually be 500 ft or -50 ft; should we need to reject. The mdw airport is; generally; inadequate; the runways are too short; our aircraft are too heavy and fully loaded; and the weather is often gusty; stormy; snowy; etc. We pilots make it work; because we have to. This is not the place to be playing with 'legal' numbers which provide almost no margin for error or incorrect data input.I called our dispatcher; and they patched me to the chief pilot; who said that; as long as the stopping margin is positive; it's 'legal.' I understand that. But; is it safe? Why are we taking these risks? Have the software engineers ever had to stop a heavy -800 on that short mdw runway; in bad weather; after being on duty for 10 hours? To them; a 2000 ft stopping margin is the same as a 20 ft stopping margin; 'the data says it will work; so won't it?' at the end of the day; we have a tool at our disposal (bleeds off) which will help us improve our stopping margin; should it be needed. Yet; against the guidance in our own flight operations manual; we have removed that tool. It costs us no extra to use bleeds off; yet due to a software program; it's taken away from us.

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Original NASA ASRS Text

Title: B737-800 flight crew reported that takeoff DATA from the Pilots Weather Briefing (PWB) limits the pilots options for a safe Rejected Takeoff.

Narrative: In our heavy 737-800 on a warm Chicago Midway day; the PWB (Pilots Weather Briefing) takeoff data gave us a flaps 25; max thrust takeoff; with a Runway 22L stopping margin of 270 FT. It calculated bleeds ON. I wanted more takeoff margin; because I don't feel that 270 FT at MDW is adequate. Winds are constantly shifting at MDW; and I have no idea if that 270 FT stopping margin would actually be 500 FT or -50 FT; should we need to reject. The MDW airport is; generally; inadequate; the runways are too short; our aircraft are too heavy and fully loaded; and the weather is often gusty; stormy; snowy; etc. We Pilots make it work; because we have to. This is not the place to be playing with 'legal' numbers which provide almost no margin for error or incorrect data input.I called our Dispatcher; and they patched me to the Chief Pilot; who said that; as long as the stopping margin is positive; it's 'legal.' I understand that. But; is it safe? Why are we taking these risks? Have the software engineers ever had to stop a heavy -800 on that short MDW runway; in bad weather; after being on duty for 10 hours? To them; a 2000 FT stopping margin is the same as a 20 FT stopping margin; 'The data says it will work; so won't it?' At the end of the day; we have a tool at our disposal (bleeds OFF) which will help us improve our stopping margin; should it be needed. Yet; against the guidance in our own Flight Operations Manual; we have removed that tool. It costs us no extra to use bleeds OFF; yet due to a software program; it's taken away from us.

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.