Narrative:

Prior to pushback, the calculated opc data was verified by both pilots for a runway 31C departure out of mdw. As we approached the hold short, ground control broadcast that all departures with the exception of ours were on ground hold due to an approaching thunderstorm (our departure heading was away from the approaching storm). Holding aircraft blocked the entrance to the approach end of the runway, therefore, tower instructed us to taxi around to take the runway at intersection K. There was confusion as to whether tower wanted us to enter the runway at intersection K to back taxi for full length or to takeoff from intersection K, yet it was not verbalized with tower, nor between the captain and I. Tower was broadcasting updated wind data that seemed to be changing rapidly due to approaching storm. We (the captain and I) both visually checked windsocks to verify the wind as the data tower was giving didn't appear to be accurate. We determined that the winds weren't as strong as tower was calling them and that we would be ok. As we entered the runway at intersection K, tower cleared us for takeoff, to which I replied we needed a short delay. (The time was needed to complete the takeoff checklist, which was interrupted on the taxi out due to tower conversations and the wind checks.) tower replied that xrwy traffic was on a 4 mi final (this verified in my mind that tower wanted us to expedite and to take off from intersection K vice full length). We made an uneventful takeoff and departure. Because of the rush, takeoff data was not updated on the opc with the new wind data, nor for an intersection takeoff. Post takeoff calculations revealed a stopping margin of only 323 ft, obviously less than optimum and a significant factor to know in the case of a rejected takeoff. Errors: not recalculating the takeoff data with new wind information and less than full runway length. Letting the situation drive our actions and not saying 'stop' or at least addressing questionable items. A high sense of urgency was brought about by the following factors: approaching thunderstorm near the departure end of runway. Congestion on the ground -- numerous aircraft waiting for departure. Modified instructions from tower. Changing wind data from tower. Incomplete checklist prior to taking runway. Xrwy traffic on final. Supplemental information from acn 512109: the tower called the winds when another aircraft asked for a wind check. Since we were doing the checklist, the winds didn't register. As I took the active, I asked for the winds and they were 160 degrees at 11 KTS gusting to 16 KTS. I disregarded the gust factor in my decision making. The winds were changing direction quite a bit due to thunderstorm. Wind socks were calm to 1/2 full. We pulled onto the runway at intersection K and tower told us we were cleared for takeoff with traffic on 4 mi final.

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Original NASA ASRS Text

Title: A B737-700 FLC PERFORMS A DOWNWIND INTXN TKOF WITH WINDS MOST LIKELY OVER THE LEGAL LIMIT WITH APCHING TSTM AND XRWY LNDG TFC AT MDW, IL.

Narrative: PRIOR TO PUSHBACK, THE CALCULATED OPC DATA WAS VERIFIED BY BOTH PLTS FOR A RWY 31C DEP OUT OF MDW. AS WE APCHED THE HOLD SHORT, GND CTL BROADCAST THAT ALL DEPS WITH THE EXCEPTION OF OURS WERE ON GND HOLD DUE TO AN APCHING TSTM (OUR DEP HDG WAS AWAY FROM THE APCHING STORM). HOLDING ACFT BLOCKED THE ENTRANCE TO THE APCH END OF THE RWY, THEREFORE, TWR INSTRUCTED US TO TAXI AROUND TO TAKE THE RWY AT INTXN K. THERE WAS CONFUSION AS TO WHETHER TWR WANTED US TO ENTER THE RWY AT INTXN K TO BACK TAXI FOR FULL LENGTH OR TO TKOF FROM INTXN K, YET IT WAS NOT VERBALIZED WITH TWR, NOR BTWN THE CAPT AND I. TWR WAS BROADCASTING UPDATED WIND DATA THAT SEEMED TO BE CHANGING RAPIDLY DUE TO APCHING STORM. WE (THE CAPT AND I) BOTH VISUALLY CHKED WINDSOCKS TO VERIFY THE WIND AS THE DATA TWR WAS GIVING DIDN'T APPEAR TO BE ACCURATE. WE DETERMINED THAT THE WINDS WEREN'T AS STRONG AS TWR WAS CALLING THEM AND THAT WE WOULD BE OK. AS WE ENTERED THE RWY AT INTXN K, TWR CLRED US FOR TKOF, TO WHICH I REPLIED WE NEEDED A SHORT DELAY. (THE TIME WAS NEEDED TO COMPLETE THE TKOF CHKLIST, WHICH WAS INTERRUPTED ON THE TAXI OUT DUE TO TWR CONVERSATIONS AND THE WIND CHKS.) TWR REPLIED THAT XRWY TFC WAS ON A 4 MI FINAL (THIS VERIFIED IN MY MIND THAT TWR WANTED US TO EXPEDITE AND TO TAKE OFF FROM INTXN K VICE FULL LENGTH). WE MADE AN UNEVENTFUL TKOF AND DEP. BECAUSE OF THE RUSH, TKOF DATA WAS NOT UPDATED ON THE OPC WITH THE NEW WIND DATA, NOR FOR AN INTXN TKOF. POST TKOF CALCULATIONS REVEALED A STOPPING MARGIN OF ONLY 323 FT, OBVIOUSLY LESS THAN OPTIMUM AND A SIGNIFICANT FACTOR TO KNOW IN THE CASE OF A REJECTED TKOF. ERRORS: NOT RECALCULATING THE TKOF DATA WITH NEW WIND INFO AND LESS THAN FULL RWY LENGTH. LETTING THE SIT DRIVE OUR ACTIONS AND NOT SAYING 'STOP' OR AT LEAST ADDRESSING QUESTIONABLE ITEMS. A HIGH SENSE OF URGENCY WAS BROUGHT ABOUT BY THE FOLLOWING FACTORS: APCHING TSTM NEAR THE DEP END OF RWY. CONGESTION ON THE GND -- NUMEROUS ACFT WAITING FOR DEP. MODIFIED INSTRUCTIONS FROM TWR. CHANGING WIND DATA FROM TWR. INCOMPLETE CHKLIST PRIOR TO TAKING RWY. XRWY TFC ON FINAL. SUPPLEMENTAL INFO FROM ACN 512109: THE TWR CALLED THE WINDS WHEN ANOTHER ACFT ASKED FOR A WIND CHK. SINCE WE WERE DOING THE CHKLIST, THE WINDS DIDN'T REGISTER. AS I TOOK THE ACTIVE, I ASKED FOR THE WINDS AND THEY WERE 160 DEGS AT 11 KTS GUSTING TO 16 KTS. I DISREGARDED THE GUST FACTOR IN MY DECISION MAKING. THE WINDS WERE CHANGING DIRECTION QUITE A BIT DUE TO TSTM. WIND SOCKS WERE CALM TO 1/2 FULL. WE PULLED ONTO THE RWY AT INTXN K AND TWR TOLD US WE WERE CLRED FOR TKOF WITH TFC ON 4 MI FINAL.

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.