Date: 201403
Local time of day: 0601-1200
Location: MDW.Airport, IL
Aircraft: B737-800
Flight plan: IFR
Flight phase: Landing
Aircraft component: Laptop Computer (performance; planning; etc.)
Component problem: Design
Reporter organization: Air Carrier
Reporter function: Captain
Human factors: Situational Awareness
Anomaly: No Specific Anomaly Occurred All Types
Detector: Person Flight Crew
When detected: Routine Inspection
Result: General None Reported / Taken
Contributing factors: Aircraft; Company Policy; Procedure
Primary problem: Ambiguous

Narrative: Our autobrake policy is unsafe when combined with the calculated landing data; which is empirically inaccurate to a significant degree.  When landing a fully-loaded B737-800 on this short runway; the output was bracketed '-180' for the stopping margin on autobrakes 3.  Because our autobrake policy is; in my opinion; exceptionally unsafe; I was not allowed to select 3; and was required to either pull the switch out over the guard and use Max which would make a violent stop on the dry runway; or set 0. I felt forced to use 0; since the last time this happened (and I reported it) I used MAX and the airplane stopped so quickly I used less than 1/2 of the runway. Choosing zero was not the safest choice.  Pilots every day are forced to choose between either 'far too much braking' at max; or 'zero braking' at zero because of our poor autobrake policy.  Perhaps 1 out of 3 landings on short runways force the Pilots to sigh and turn the switch off. In this case; the landing output showed me going off the end of the runway by 200 FT with autobrakes 3 and having only a 1;300 foot margin at autobrakes max.  As always; I normally; manually braked the airplane using a medium effort and easily slowed to 60 KTS well before the end.  The calculated number is incorrect.  It is wrong.  It is not accurate.  The procedure that requires us to use inaccurate data to turn off autobrakes when we are at the heaviest allowing autobrakes to add the greatest margin of safety is; in itself; unsafe.  Why does it make sense to anyone to have Crew after Crew sigh; reach up and turn off the autobrakes when we are close to our max landing weight?  If the calculation provided even close to accurate landing data; then the policy to not use bracketed autobrake selection would make some sense (but not really; I want the brakes on as soon as I touch down; like every other operator allows).  But the calculation clearly does not come anywhere close to providing accurate data (I think it's off by between 35% to 45%).  Can we please address this?  I'm sending the actual data to you via email; but I do not want to ever land a fully-loaded -800 on a short runway with autobrakes off.  It isn't the safe answer.  And we all know it; but we're still landing without autobrakes when we need them the most.




Synopsis: B737 Captain laments that his company will not allow use of an autobrake setting that produces a negative stopping margin when landing performance is calculated; requiring either 0 (manual braking) or max; even though the the calculations are wildly pessimistic. When landing on an unrestricted runway; medium autobrakes will easily stop the aircraft at maximum landing weight in 4;000 FT while the calculations show that much more than 5;000 FT would be required.