- On December 8, 2005, about 1914 central standard time, Southwest Airlines (SWA) flight 1248, a Boeing 737-7H4, N471WN, ran off the departure end of runway 31 center (31C) after landing at Chicago Midway International Airport (MDW), Chicago, Illinois. The airplane rolled through a blast fence, an airport perimeter fence, and onto an adjacent roadway, where it struck an automobile before coming to a stop. A child in the automobile was killed, one automobile occupant received serious injuries, and three other automobile occupants received minor injuries. Eighteen of the 103 airplane occupants (98 passengers, 3 flight attendants, and 2 pilots) received minor injuries, and the airplane was substantially damaged. The airplane was being operated under the provisions of 14 Code of Federal Regulations (CFR) Part 121 and had departed from Baltimore/Washington International Thurgood Marshall Airport (BWI), Baltimore, Maryland, about 1758 eastern standard time. Instrument meteorological conditions prevailed at the time of the accident flight, which operated on an instrument flight rules flight plan.
- The pilots reported that they had thoroughly reviewed the two weather information and dispatch documents they received from dispatch before they left BWI. A third document authorizing the release of the accident flight was prepared but was not delivered to the pilots before departure. This document revised the expected landing winds (from “calm” to “090° at 11 knots”), runway braking action (from “wet-good” to “wet-fair”), and landing runway (from 04R to 31C) based on the changing weather. The pilots stated that they subsequently received updated MDW weather information and runway condition/braking action reports for runway 31C, which was the runway in use at MDW at the time. Post-accident interviews with the pilots and evidence from co*ckpit voice recorder (CVR) data and air traffic control (ATC) communications indicated that the runway 31C braking action reports were mixed, reporting good or fair braking action for the first half of the runway and poor braking action for the second half.
- The pilots also stated (and CVR evidence confirmed) that they reviewed and discussed the company’s new autobrake system procedures while en route from BWI to MDW; the accident landing was the first time either pilot landed using autobrakes.
- About 1833:17, as the airplane was nearing MDW at an assigned altitude of 10,000 feet, ATC issued the pilots instructions to enter a holding pattern. (ATC indicated that the hold was because of runway-clearing snowplow operations at MDW.) About 1844:04, the pilots advised ATC that they were entering the holding pattern at 10,000 feet. The first officer stated that, while in the holding pattern, he entered the updated weather and runway conditions and wind information (090° at 11 knots) in the on board performance computer (OPC) to determine the landing distance required for runway 31C. The reported wind conditions resulted in a computed tailwind component of 8 knots. All SWA 737s are limited to landing with a 10-knot or less tailwind component under all runway surface conditions. Additionally, SWA policies and flight operations manuals indicate that the company does not authorize landings on runways with more than a 5‑knot tailwind component with poor braking action. Post-accident statements and CVR evidence indicated that the accident pilots were aware of these limitations and believed that they would be unable to land at MDW if the braking action was reported poor for the full length of the runway.
Source: NTSB Aircraft Accident Report, AAR-07/06, ¶1.1
Their manual also prohibited landing with more than a 5 knot tailwind when the condition was poor and also stipulated that when the runway had multiple conditions they were to assume the worst condition for the entire runway. The crew was aware of the 5 knot limit but not of the need to consider the entire runway's braking action as poor when part of it was judged fair.
The first officer entered multiple scenarios into the OPC, entering fair and poor pilot braking action reports separately because the OPC was not designed to accept mixed braking action report inputs. Based on the first officer’s inputs, the OPC estimated that the airplane would stop about 560 feet before the departure end of the runway with fair braking action and about 40 feet before the departure end of the runway with poor braking action. The pilots stated that they decided that, consistent with SWA policies, they would divert to one of their alternate destinations (Kansas City or St. Louis, Missouri) if the tailwind component increased to above 10 knots or if pilot braking action reports indicated poor braking action for the full length of the runway. The automatic terminal information service (ATIS) reported a runway visual range (RVR) for runway 31C of about 5,000 feet.
Source: NTSB Aircraft Accident Report, AAR-07/06, ¶1.1
Their performance computer was designed to limit the tailwind entry to 5 knots for a poor runway condition, so the "40 feet before the departure end" prediction could be argued to be invalid.
- About 1854:10, ATC began providing the pilots with radar vectors and descent instructions as they departed the holding pattern for the final approach course for the instrument landing system (ILS) approach to runway 31C. At that time, the RVR was reported as 4,500 feet variable to 5,000 feet, and the ATIS was reporting winds from 100° at 11 knots. About 1903:44, ATC cleared the pilots to intercept the runway 31C localizer. Less than a minute later, ATC cleared them for the approach and advised that the braking action reported for runway 31C was “fair except at the end [it’s] . . . poor.”
- According to the CVR transcript, when the pilots contacted the MDW Air Traffic Control Tower (ATCT) at 1909:53.7, controllers advised them to “continue for [runway] 31C the winds zero nine zero at nine, brakin’ action reported good for the first half, poor for the second half.” About 1912:28, the first officer received a landing clearance from the ATCT. Flight data recorder (FDR) data indicated that the airplane was aligned on the runway centerline as it touched down at an airspeed of about 124 knots. The speed brakes deployed and brake pressure increased within about 1 second. Both pilots described the touchdown as “firm.”
Source: NTSB Aircraft Accident Report, AAR-07/06, ¶1.1
All indications are that the airplane was landed where it needed to be at the correct speed.
The captain stated that he tried to deploy the thrust reversers immediately after touchdown but had difficulty moving the thrust reverser levers to the reverse thrust position. He further stated that he felt the antiskid system cycle after the airplane touched down but then felt it stop cycling and that the airplane seemed to accelerate. He said that he subsequently applied the wheel brakes manually but made no further effort to activate the thrust reversers. He told investigators that he believed that the use of the autobrake system distracted his attention from the thrust reversers after his initial attempt to deploy them.
Source: NTSB Aircraft Accident Report, AAR-07/06, ¶1.1
There was no indication of problems with the thrust reverser levers, the first officer had no difficulty with them, or did the previous captains. The report seems to not attach any credibility to the captains statement that he tried to deploy the reversers.
The first officer said that, when he sensed a decrease in the airplane’s deceleration during the landing sequence, he exclaimed, “brakes, brakes, brakes,” and manually applied the brakes. He stated that he then looked at the throttle console and saw that the thrust reverser levers were still in the stowed position. The first officer moved the captain’s hand away from the thrust reverser levers and, about 15 seconds after touchdown, initiated deployment of the thrust reversers to the maximum reverse setting. FDR evidence confirmed the systems functions described by the pilots and indicated that full thrust reverser deployment occurred about 18 seconds after touchdown.
Source: NTSB Aircraft Accident Report, AAR-07/06, ¶1.1
I think it is quite natural to feel the aircraft isn't decelerating right after touchdown even though it is, because your inner ear can confuse movement forward and down when the down vector ceases. No matter the cause, the flight data recorder shows the airplane was decelerating almost immediately after touchdown.
However, the airplane ran off the departure end of runway 31C and continued through the runway safety area (RSA), a blast fence, a navigational aid antenna, across an airport road, through an airport perimeter fence, and onto an adjacent public roadway. The airplane struck a northbound automobile on that roadway before it came to rest near an intersection located on the northwest corner of the airport.
Source: NTSB Aircraft Accident Report, AAR-07/06, ¶1.1
- The main landing gear touched down about 1,250 feet beyond the runway’s approach threshold. At the time, the airplane’s ground speed was about 131 knots, its airspeed was 124 knots, its heading was 316°, and its vertical acceleration reached about 1.4 Gs.
- The ground spoilers were fully deployed, autobrakes were applied, and vertical acceleration increased to a peak value of about 1.7 Gs within about 1.2 seconds of touchdown.
- About 10 seconds after touchdown, engine fan speed (N1) decreased from about 32 percent at touchdown to about 20 percent, where it remained for about 8 seconds.
- Autobrakes were deactivated about 12 seconds after touchdown, and pilot‑commanded brake pressure increased to 3,000 pounds per square inch (psi).
- The first indication of thrust reverser activity was recorded about 15 seconds after touchdown, with full deployment about 18 seconds after touchdown. N1 reached 80 percent about 9 seconds later (about 27 seconds after touchdown) at a ground speed of about 62 knots.
- The thrust reversers were fully deployed and the brake pressure was 3,000 psi when the nose landing gear departed the runway overrun at a speed of about 53 knots.
- The airplane came to a stop about 500 feet beyond the end of the runway on a heading of about 340° with a collapsed nose landing gear about 8 seconds after it departed the runway overrun.
Source: NTSB Aircraft Accident Report, AAR-07/06, ¶1.11.12
