Expert: D. Norkus - 4/8/2010

Question
hello good evening.i'm aymen from tunisia;i am an air traffic controller.please can answer me about this question:how can accident investigation help an air traffic controller.thank's allot.

Answer
The findings of accident reports are useful to air traffic control as they can show where there are deficiencies in the system or situations that can lead controllers to become more likely to make mistakes. These problems can then be corrected if they are listed as causal or contributing factors in aviation accidents or incidents. As a result of some tragic- and preventable- accidents, many changes have occurred in ATC procedures and airspace, and technologies such as terrain collision & avoidance systems/TCAS have been developed.

Here are some fatal accidents where the actions of air traffic control directly played a role in the disaster:

1) The mid-air collision of British Airways 476 and Inex-Adria 550 over Zagreb in 1976 killing 176.

The Index-Adria wing impacted the middle of the BA fuselage while both aircraft were at FL330 above the  Zagreb VOR (a navigation aid).  A contributing cause in this accident was "improper ATC operation". An illustration of the two aircraft on their courses-
http://aviation-safety.net/photos/displayphoto.php?id=19760910-1&vnr=1&kind=G

The investigation revealed that ill-handled coordination for Inex-Adria's climb through a sector of airspace, the removal of that aircrafts transponder code before a new one was assigned, the last minute instruction to maintain altitude instead of expediting up in altitude for just a few moments and the use of Serbo-Croation instead of English to the Index-Adria flight (the BA crew couldn't understand the controllers frantic last ditch call to the Index-Adra crew).

Further reading-
http://en.wikipedia.org/wiki/1976_Zagreb_mid-air_collision

The U.K. AAIB accident report:
http://www.aaib.gov.uk/sites/aaib/publications/formal_reports/5_1977_g_awzt.cfm

2) The mid-air collision of Pacific Southwest Airlines 182 and Cessna N7711G over San Diego in 1978 killing 144.

The Pacific Southwest Airlines 727 was entering the downwind leg and descending while overtaking Cessna 11G which was climbing and departing the traffic pattern. At 2600 feet the right wing of the 727 impacted the Cessna and both aircraft crashed into the neighborhood of North Park.  The primary cause was the failure of the jet crew to see and avoid but ATC could have helped to prevent it. Secondary contributing causes in this accident were the failure of the controller to advise PSA 182 of the direction of movement of the Cessna traffic and the improper resolution of the conflict alert (for an imminent collision) by the controller" . An animation of the two aircraft on their courses-
http://en.wikipedia.org/wiki/File:Psa182.gif

The investigation revealed that air traffic control was alerted with an automated conflict alert 19 seconds before the collision but ATC did not relay this information to PSA 182 because the controller believed the airliner had the Cessna in sight and it would be a close pass between the aircraft. Such alerts were "commonplace even when there was no actual conflict", so he didn't feel the need to remind PSA of their traffic since they had it in sight. Only moments before the PSA first officer had radioed, "Okay we've got that other twelve [o'clock traffic]".

As a result of this accident, the NTSB recommended that a Terminal Radar Service Area/TRSA (which has now been replaced with class B airspace) be instituted around San Diego's Lindbergh Field to provide for the better separation of aircraft. They also recommended an immediate review of control procedures for all busy airport terminal areas. The PSA aircraft had also been cleared down to an altitude well below that which should have been the minimum while on downwind leg. All southbound turbojet aircraft executing a VFR or visual approach to Lindbergh Field "shall be instructed to maintain at or above 4,000 feet" to avoid overlapping airspace from nearby Montgomery Field (where the Cessna originated and to where it was returning). Yet some approach controllers commonly cleared aircraft inbound for Lindbergh for a visual approach and monitored their altitude. Since PSA was just outside of this area, they were given an altitude that allowed them to pass through the assigned altitude to which the Cessna was climbing. This lack of separation cushion in altitude assignments while operating under VFR played a role in my opinion, although it wasn't listed as a contributing cause.

The NTSB accident report-
http://www.psa-history.org/museum/NTSBAAR7905.pdf


3) The runway collision of Skywest Airlines 5569 and USair 1493 in Los Angeles/LAX in 1991 killing 34.

The Skywest 5569 had been cleared to position and hold (line up and wait) on runway 24L at an intersection in the touchdown zone of the runway. There was an issue with a misplaced flight progress strip that allowed an aircraft to be misidentified and the controller also became distracted by an issue with another aircraft that had just landed and forget Skywest was holding and cleared the USair 737 to land. The time of day was dusk and the lights of the Skywest flight blended in with the runway environment so the approaching 737 did not see the aircraft on the runway until it was too late: "USAir 1493 touched down near the runway threshold and as the nose was being lowered, the first officer noticed the SkyWest plane on the runway and applied maximum breaking. USAir 1493 shortly thereafter slammed into the Skywest aircraft. Both aircraft skidded down the runway with the Metroliner crushed beneath the 737's fuselage." - http://en.wikipedia.org/wiki/USAir_1493

Diagram of aircraft position and collision point-
http://aviation-safety.net/photos/displayphoto.php?id=19910201-0&vnr=3&kind=G

The investigation revealed that the cause was: "Failure of the Los Angeles Air Traffic Facility Management to implement procedures that provided redundancy comparable to the requirements contained in the National Operational Position Standards and the failure of the FAA ATS to provide adequate policy direction and oversight to its ATC facility managers. These failures created an environment in the Los Angeles ATC tower that ultimately led to the failure of the Local Controller 2 (LC2) to maintain awareness of the traffic situation, culminating in the inappropriate clearances and the subsequent collision of the USAir and SkyWest aircraft. Contributing to the cause of the accident was the failure of the FAA to provide effective quality assurance of the ATC system."

The controller who cleared both aircraft to use the same runway testified before the NTSB and accepted blame for causing the crash. This accident lead directly to the use of the NTSB's recommendation of using different runways for takeoffs and landings at LAX. Runway 24L is used for takeoffs, while runway 24R for landings.

The NTSB docket-
http://ntsb.gov/ntsb/GenPDF.asp?id=DCA91MA018A&rpt=fi

This ought to give you some insight as to how accident investigations can improve and affect air traffic control.

Regards,
Dottie