N799SW

HISTORY OF FLIGHT

On April 26, 2011, about 1333 central daylight time (CDT), a Boeing 737-7Q8, registration N799SW, operated by Southwest Airlines as flight 1919, exited the left side of the pavement near the departure end of runway 13C after landing at Chicago Midway International Airport (MDW), Chicago, Illinois. The flight was a regularly scheduled passenger flight from Denver International Airport, Denver Colorado, operating under the provisions of 14 Code of Federal Regulations (CFR) Part 121. There were no injuries to the 134 passengers, including 5 lap-held children, or the 5 crewmembers. The airplane sustained minor damage to the left and right engines and the inboard aft flaps.

The crew and operator reported the flight to be routine until nearing the Chicago terminal area. About 1232, the Chicago Approach Control (C90) air traffic controller advised of delays into the Chicago area and that arrivals would be using required navigation performance (RNP) approaches. The crew began to brief the area navigation (RNAV) global positioning system (GPS) Z approach, instead of the RNP approach, and discussed how to set up the flight management system (FMS). At 1238, the flight crew performed the descent checklist, which included setting and cross-checking altimeters and bugs, setting the reference and target airspeeds (to 129 and 145 knots, respectively). The crewmembers stated that they selected autobrakes to “MAX” as indicated by the onboard performance computer (OPC) Landing Output and in accordance with Southwest Airlines Flight Operations Manual (FOM). About 1240, C90 instructed the flight to hold at the SMARS waypoint due to traffic congestion and conflicting traffic patterns with Chicago O’Hare International Airport. While entering and during the hold, the crew discussed fuel and diversion options.

About 1256, the crew received ATIS (automated terminal information service) Oscar, observation time 1251 CDT, wind 190 degrees at 16 knots, gusts to 23 knots, visibility 6 statute miles in light rain and mist. Clouds at 800 feet scattered, ceiling 1,400 feet broken, 2,200 feet overcast, temperature 16ºC, dew point 14ºC, altimeter setting 29.40 inches of mercury. The recording stated that the instrument landing system (ILS) for runway 13C was in use, landing and departing runway 13 (sic), departing runway 22L. The recording also included numerous NOTAMs not pertinent to the incident.

Following the receipt of the weather, the crew began performing landing distance calculations using the OPC. The initial entry included a landing weight of 127,000 pounds, wet runway with good braking action, and thrust reverse at detent 2. According to the Southwest Flight Reference Manual (FRM) and FOM, the OPC approximate stopping margin calculation was based on minimum, medium, and maximum braking and corresponded to three different autobrake settings (2, 3, and MAX). The FOM also provided guidance on approximating the autobrake actions when using manual braking. Each stop margin calculation included a 1,500-foot air distance from threshold to touchdown and an extra 15 percent distance factor. Stop margin was the distance remaining after the aircraft comes to a complete stop, measured from the nose gear to the end of the available runway.

The landing output for maximum braking (autobrake MAX) produced a positive margin of 690 feet. Entering minimum (autobrake 2) or medium braking (autobrake 3) did not provide a positive stopping margin. The crew also performed another calculation with the same inputs except for a 126,000-pound landing weight, and the margin with maximum braking was 720 feet.

About 1314, C90 began accepting RNP-capable aircraft destined to MDW. The incident flight was third in the sequence, following another Southwest 737 and a general aviation Citation jet.

At 1319, the crew reported to C90 that they were level at 6,000 feet, indicating an airspeed of 210 knots and had received information “Oscar.” The controller advised the crew to expect the RNAV (RNP) Y 13C approach and to slow to 170 knots for traffic sequencing. The controller subsequently cleared the flight direct to the Joliet waypoint (geographically coincident with the Joliet very high frequency omnirange (VOR) navigation aid) and stated “at Joliet intercept the final approach course.” The crew acknowledged the instruction but discussed some confusion about it, likely because Joliet does not appear on the RNAV GPS Z approach chart; rather, it is the RNAV (RNP) Y 13C initial approach fix, beginning the initial approach segment, which had a track of approximately 90 degrees to the final approach course. About 1320, after a brief discussion, the crew set flaps to “2” and then further discussed the controller’s instructions. The crew then identified the proper approach procedure chart and reconciled the instructions with the chart. C90 instructed the crew to descend to 2,500 feet after completing the speed reduction to 170 knots. The RNAV (RNP) Y 13C chart noted a minimum altitude of 4,000 feet after Joliet and a mandatory crossing altitude of 4,000 feet at TOYUL waypoint. The crew acknowledged the controller’s instruction and began briefing and configuring the FMS for the RNAV (RNP) Y 13C procedure.

About 1325, the preceding Southwest 737 landed, which was the first arrival on runway 13C in the previous 25 minutes. At 1326:36, the controller stated “Southwest 1919, three miles from TOYUL, cleared for RNAV RNP Y runway one three center approach Midway, maintain 170 knots.” The crew acknowledged and discussed the weather radar display and the nearby rain showers.

At 1328:37, C90 instructed the incident flight to maintain 170 knots until JUPIR waypoint and to contact the MDW airport traffic control tower local controller (LC). The crew acknowledged and began to monitor the LC radio frequency. At this time, the crew was discussing symbology and information on the flight instrument displays regarding the approach procedure.

At 1329:26, the crew selected landing gear down and flaps 15. The preceding Southwest crew was taxiing clear of the runway and reported the runway was wet with fair braking action. LC relayed the braking report to the pilot of the Citation jet. At 1330:22, the LC controller was relieved by another controller, and the braking report was not included in the position relief briefing or passed to the incident crew. However, the incident crew overheard the runway condition report and made new entries into the OPC. Using the wet runway, fair braking action input, with a 126,000-pound landing weight and maximum reverse thrust and braking, the OPC calculated a landing with 210 feet of margin. About the same time, the crew attempted to set flaps to 25 but noted the airspeed was too high for that setting. Shortly after, the crew selected flaps 30 then, almost immediately, flaps 40. The Southwest Airlines B737-700 Normal Checklist “Before Landing” section reads: “Speedbrake - Armed, Green Light; Landing gear- Down, 3 Green; Flaps – [appropriate setting], Green Light.” Neither the phrase “before landing checklist” nor any mention of “speedbrake” were heard on the cockpit voice recording. The Southwest Airlines FOM stated the before landing checklist should be completed no later than 1,000 feet above touchdown zone elevation and after landing flaps are set.

At 1330:44, the incident crew reported passing the JUPIR waypoint inbound and asked LC the direction in which a rain shower was moving. LC responded with an advisory about the traffic they were to follow and landing clearance for runway 13C and advised that the wind was 210 degrees at 9 knots and that the rain was moving to the east-northeast. The crew acknowledged.

Beginning about 1331, the crew further discussed the approach procedure; at 1331:37, the captain stated that the airplane was approaching minimums, which was followed by discussion with the first officer about the altimeter display. This discussion ended with the first officer stating, “I'd set mine wrong.” At this point, the airplane was still about 1,000 feet above the runway (about 1,454 mean sea level). Decision altitude for the approach was 1,065 feet above mean sea level, or 454 feet above the runway touchdown zone.

At 1333:05, the airplane touched down on runway 13C at an airspeed of 136 knots and a groundspeed of 143 knots. The touchdown occurred within 500 feet of the displaced arrival threshold of the runway, leaving approximately 5,600 feet of runway available for rollout. At the same time, LC advised a following airplane that the wind was from 220 degrees at 10 knots, gusting to 17 knots.

Flight data recorder (FDR) information shows that, upon touchdown, the autobrakes indicated “applied” and both main landing gear brake pressures began to increase. After about 1 second, the autobrake parameter indicated “No Auto Brk” while the pilot commanded brake pressure continued to increase (without any fluctuations) up to about 3,000 pounds per square inch (psi), where it remained for the rest of the landing. The crewmembers reported that they applied maximum manual braking shortly after touchdown because of a perceived lack of braking effectiveness; cockpit voice recorder (CVR) evidence is consistent with this report. High manual brake pressure will release the autobrakes and disarm the autobrake system per system design. Anti-skid braking capability is not affected.

About 8 seconds after touchdown, the captain stated “I got no brakes”, and the first officer advised he was also applying brakes. About 15 seconds after touchdown, one of the pilots commented about the power, and, about 10 seconds later, a sound similar to increasing reverse thrust is heard.

FDR data indicate that throughout the final approach, touchdown, and initial rollout, the speedbrake handle remained at 0 degrees (not armed) and the speedbrake “armed” light was not illuminated. The speedbrake handle remained in its down...