On 1 December 2011, a Boeing 737-700 being operated by Southwest AL on a scheduled passenger flight from Minneapolis-St. Paul to Chicago Midway was cleared to cross runway 31R in normal daylight visibility after landing on runway 31C at the same time as a Gama Charters Learjet 45 was taking off from runway 31R on a scheduled passenger flight to Savannah GA. Following a late sighting of the Learjet by the 737 crew and their immediate avoiding action, the two aircraft came into close proximity as the Learjet passed nearly overhead the 737. None of the respective 79 and 6 occupants of the two aircraft were injured.
An Investigation was carried out by the National Transportation Safety Board (USA) (NTSB). It was noted that in the period prior to the investigated event, runway 31C had been in use for take off and landing and intersecting runway 22L for take off. Whilst the traffic load at the time of the event had been unexceptional, attempts to clear traffic ahead of an imminent VIP arrival were beginning to be made and these had led to the introduction of 31R as an additional departure runway.
Chicago Midway (KMDW) Airport Diagram - click to enlarge
It was found that the TWR controller responsible for both runways 31C and 31R had issued conflicting clearances for occupation of runway 31R which had led directly to the conflict. The Issue of the 737 crossing clearance had been followed by the Learjet take off clearance. The stopped position of the 737 was found to have been beyond the runway 31R holding point but just short of the runway itself. Actual separation between the two aircraft was determined from recorded ATC data to have been 287 feet horizontally and 62 feet vertically. The Investigation found the controller involved “could not explain his action”.
It was found that:
- the 737 First Officer acting as PNF had visually checked the runway about to be crossed and, having thereby seen another aircraft approaching at speed, had immediately called the aircraft commander to stop the aircraft just as actual runway entry was about to occur.
- the Learjet crew had not noticed the 737 approaching the runway and although they had been on frequency when the 737 crossing clearance had been issued, they had not heard it and so did not detect a conflict when their take off clearance was subsequently issued.
- although other controllers in the VCR were in the vicinity of the position from which the conflicting clearances had been issued and had realised the possibility of a conflict, none had effectively intervened.
- those VCR controllers who were aware of the possibility of a conflict and/or the subsequent fact initially assumed that it was a result of the 737 failing to stop at the limit of its clearance.
- initial attempts by the 737 crew to report the incident to the controller who had issued both clearances were unsuccessful.
- the three previous runway 31C arrivals, which had taxied in with runway 31R not active, had received identical taxi in clearances to the 737 involved in the investigated conflict.
It was concluded that ATC SOPs designed to prevent conflicting clearance issue from occurring had not been followed. In this context, the ineffective relationship between the designated and assisting TWR controllers was especially highlighted. Locally specified duties for the latter were noted to include giving first priority to advising the designated TWR controller of safety issues related to TIPH (Taxi into position and Hold), same runway and intersecting runway separation and to ensuring runway crossing coordination was completed properly.
It was noted that although ASDE-X radar, a ground movement safety system that included conflict detection and warning capability, the system had not sounded an alarm during the conflict.
The NTSB determined the Probable Cause of the event to be:
“The Tower local controller did not ensure that the runway was clear of conflict before directing the B737 to cross the runway and other air traffic control personnel did not effectively intervene when the separation between the two airplanes became questionable.”
Information recorded on the Investigation Docket included the finding that the investigated error was the third ‘Operational Error’ by the Controller involved within six months. The previous two events had been lining up an aircraft for take off on a runway with an aircraft on approach and a take off clearance issued whilst another aircraft was crossing the same runway.
The Final Report OPS12IA167B was released on 11 July 2012. No Safety Recommendations were made.