On 26 April 2011 a Boeing 737-700 being operated by Southwest AL on a scheduled passenger flight from Denver to Chicago O’Hare failed to stop within the paved area of runway 13C at alternate Chicago Midway in normal ground visibility and finally stopped just under 70 metres from the end of the runway in grass to the left side of the installed Engineered Materials Arresting System. The aircraft incurred only minor damage and none of the 139 occupants were injured. Passengers were eventually disembarked using steps and bussed to the Terminal.
An Investigation was carried out by the National Transportation Safety Board (USA) (NTSB). Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR) data was available for the Investigation. It was noted that the aircraft commander, qualified as a ‘Check Airman’ had been acting as PF and that both pilots were experienced on the aircraft type.
Both engines were damaged as a result of the excursion - the right engine as a result of the ingestion of a taxiway light and the left engine which suffered bending of two fan blades.
It was noted that the flight had been routine until nearing the Chicago terminal area, when it became apparent that there were significant delays at the intended destination as a result of which ATC then advised that aircraft able to make an RNAV RNP ‘Y’ approach to Runway 13C at Chicago Midway would be so cleared. This option was accepted but the crew then mistakenly loaded and briefed a different procedure, the RNAV GPS approach. The onboard Electronic Flight Bag was used to check the landing performance which was confirmed as sufficient. During the approach, a discussion between the pilots led to the realisation that they had loaded and briefed the wrong approach procedure and the error was corrected. This action at this point in the approach was considered to have caused significant extra workload. A preceding company Boeing 737 was heard to report to ATC that runway 13C was “wet with fair braking action”. Although ATC did not repeat this report to them, the flight crew had noted it anyway and correctly recalculated the LDR (which again indicated sufficient distance was available, this time a margin of 210 feet) and set the autobrake appropriately for the reported conditions.
A momentary flap overspeed occurred as the First Officer attempted to set flaps to 25 and it was noted that the following delay whilst waiting for the airspeed to decay occurred about the same time that the crew would normally have carried out the ‘Before Landing’ Checklist, which includes a check of speed brakes armed. No mention of ‘speed brakes’ or the ‘Before Landing Checklist’ was heard on the CVR playback and FDR data indicates that the speed brakes were not armed during the approach.
Touchdown occurred with 40º of flap set within 500 feet of the runway threshold - with subsequent calculations indicating that the actual tailwind component had been 7 knots. Once on the grooved concrete runway, the PF perceived a lack of braking effectiveness and quickly applied full manual brakes. Since they had not been armed, the speed brakes did not deploy at touchdown and the thrust reversers were not selected either. After ‘about 16 seconds’, the thrust reversers were manually deployed and this action led to the automatic deployment of the unarmed speed brakes. At this point, about 460 metres of the 1860 metre landing distance remained. As the aircraft neared the end of the paved surface, the PF attempted to turn onto the connecting taxiway but was unable to do so and the aircraft struck a taxiway light before leaving the paved surface at about 30 knots ground speed and rolling onto grass.
It was noted that the calculated braking coefficient of the incident aircraft was consistent with the reported “fair” braking action. However, the delay in deployment of the speed brakes had resulted in severely degraded braking effectiveness and poor deceleration was further aided by the delay in thrust reverser deployment on what was a relatively short runway. The Investigation noted that “simulation studies concluded that the airplane would have stopped with about 900 feet (275 metres) of runway remaining if the speed brakes had been deployed at touchdown without reverse thrust or with about 1,950 feet (595 metres) remaining if both speed brakes and reverse thrust had been deployed at touchdown”.
In respect of the Operator’s SOPs, it was noted that these included the requirement for the ’Landing Check List’ to be completed not later than 1000 feet above the TDZ elevation. In respect of the Operator’s use of RNP approaches it was noted that after receiving regulatory approval to fly them in April 2010, they had been introduced from 11 January 2011.
In respect of Federal Aviation Administration (FAA) guidance to Operators in AC 120-71A ‘Standard Operating Procedures for Flight Deck Crew Members’, it was noted that the SOP Template included as Appendix 1 to this Circular includes the recommendation to ensure that “Auto spoiler and auto brake systems armed and confirmed armed by both pilots, in accordance with manufacturer's recommended procedures (or equivalent approved company procedures)” as well as a recommendation that the PM should make a “no spoilers” if spoiler deployment does not occur. The Investigation also noted that this Circular also explicitly recommends a procedure for the PM to monitor thrust reverser deployment and advise the PF of thrust reverser status. The Board noted that their Investigations of runway overrun incidents and accidents have continued to find that in respect of thrust reverser callouts “not all air carriers have implemented procedures that are consistent with this guidance”.
No comment was made in the Report of the Investigation on the aircraft commander’s failure to take advantage of the 58 metre long EMAS bed installed at the end of the runway as an alternative to a longer excursion on grass to one side of it.
The Probable Cause of the accident was determined by the NTSB as:
“ the flight crew's delayed deployment of the speed brakes and thrust reversers, resulting in insufficient runway remaining to bring the airplane to a stop”.
The Board also noted that:
- 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 speed brake arming before touchdown as a result of workload and operational distractions during the approach phase of flight.
The Final Report of the Investigation DCA11IA047 was approved by the NTSB on 11 June 2012. No Safety Recommendations were made.