NTSB Identification: DCA11IA047
Scheduled 14 CFR Part 121: Air Carrier operation of SOUTHWEST AIRLINES CO
Incident occurred Tuesday, April 26, 2011 in Chicago, IL
Probable Cause Approval Date: 06/11/2012
Aircraft: BOEING 737-7Q8, registration: N799SW
Injuries: 139 Uninjured.
NTSB investigators used data provided by various sources and may not have traveled in support of this investigation to prepare this aircraft incident report.
The flight was routine until nearing the Chicago terminal area, where delays due to traffic, weather, and conflicting approaches with O’Hare International Airport resulted in an air traffic controller instructing the flight crew to expect to hold. Shortly afterward, the controller advised the crew that aircraft capable of required navigation performance (RNP) approaches to runway 13C would be accepted to MDW. The flight crewmembers mistakenly loaded and briefed a different procedure, the area navigation (RNAV) global positioning system (GPS) approach, before entering the holding pattern. While in the holding pattern, the flight crew performed a landing distance assessment using the onboard performance computer (OPC). The calculation results showed sufficient runway length for the landing in accordance with the flight manual procedures. Data from the cockpit voice recorder and the OPC indicate that the crew performed the assessment correctly.
After receiving air traffic control (ATC) clearance to leave the holding pattern and begin the approach to MDW, the flight crewmembers discussed confusion about the approach instruction, likely because they had loaded and briefed the wrong approach procedure. The flight crew then identified the proper approach procedure chart. The crew subsequently reprogrammed the flight management system for the correct approach and amended some of the procedure crossing altitudes in order to follow ATC instructions. These activities at this point in the approach resulted in extra workload for the flight crew.
Later, as flight 1919 neared the runway, the flight crew set flaps to 15. The flight crew of a preceding Southwest Airlines 737 arrival reported “fair” braking action on runway 13C to ATC. The air traffic controller did not advise the flight 1919 crew of the braking action report transmitted by the previous arrival; however, the incident crew overheard the report and correctly recalculated the landing distance assessment, which again indicated sufficient runway length available. The incident crew also set the airplane autobrakes appropriately for the conditions.
In addition to discussion regarding the approach procedure automation, the crew had additional operational distractions in the final minutes of the approach. These included a momentary flap overspeed as the first officer attempted to set flaps to 25, assessment of a rain shower passing over the airport, and incorrect settings for minimum altitude reminders. The delay in setting flaps to 25 as the first officer waited for airspeed to decay occurred about the same time that the crew normally should have been executing the Before Landing checklist, which includes the item “speedbrake—armed.” No mention of speedbrakes or the Before Landing checklist is heard on the cockpit voice recording, and data from the flight data recorder (FDR) indicate that the speedbrakes were not armed.
The airplane touched down within 500 feet of the runway threshold. After touchdown, the captain perceived a lack of braking effectiveness and quickly applied full manual brakes. Speedbrakes did not deploy upon touchdown, nor were thrust reversers deployed. About 16 seconds after touchdown, thrust reversers were manually deployed, which also resulted in speedbrake deployment per system design, when the airplane had about 1,500 feet of runway remaining. As the airplane neared the end of the pavement, the captain attempted to turn onto the connecting taxiway but was unable. The airplane struck a taxiway light and rolled about 200 feet into the grass.
FDR data and component examination revealed that all airplane systems operated as expected. The automatic speedbrakes were not armed and, therefore, would not deploy upon touchdown without crew action. Extending the speedbrakes after landing increases aerodynamic drag and reduces lift, which increases the load applied to the main gear tires and makes the wheel brakes more effective. A lack of speedbrake deployment results in severely degraded stopping ability. According to the flight operations manual, braking effectiveness is reduced by as much as 60 percent. The flight crew’s delay in applying reverse thrust also contributed to the amount of runway used.
Simulation studies concluded that the airplane would have stopped with about 900 feet of runway remaining if the speedbrakes had been deployed at touchdown (without reverse thrust) or with about 1,950 feet remaining if both speedbrakes and reverse thrust had been deployed at touchdown, per standard procedures. The calculated braking coefficient of the incident airplane was consistent with a “fair” braking action report, as given by the preceding Southwest Airlines 737 arrival. The braking coefficient is also in accordance with the OPC calculations.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: The flight crew's delayed deployment of the speedbrakes and thrust reversers, resulting in insufficient runway remaining to bring the airplane to a stop.
Contributing to the delay in deployment of these stopping devices was the flight crew's inadequate monitoring of the airplane's configuration after touchdown, likely as a result of being distracted by a perceived lack of wheel braking effectiveness.
Contributing to the incident was the flight crew's omission of the Before Landing checklist, which includes an item to verify speedbrake arming before touchdown, as a result of workload and operational distractions during the approach phase of flight. Full narrative available
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