B733 / SW4, Los Angeles CA USA, 1991
From SKYbrary Wiki
|On 1 February 1991, a Boeing 737-300 had just made a normal visibility night touchdown on Los Angeles runway 24L in accordance with its clearance when its crew saw another aircraft stationary ahead of them on the same runway. Avoidance was impossible in the time available and a high speed collision and post-impact fire destroyed both aircraft and killed 34 of their 101 occupants and injured 30 others. The other aircraft was subsequently found to have been a Fairchild Metroliner cleared to line up and wait by the same controller who had then cleared the 737 to land.|
|Actual or Potential
|Fire Smoke and Fumes, Human Factors, Runway Incursion|
|Flight Conditions||On Ground - Normal Visibility|
|Type of Flight||Public Transport (Passenger)|
|Origin||Port Columbus International|
|Intended Destination||Los Angeles|
|Take off Commenced||Yes|
|Type of Flight||Public Transport (Passenger)|
|Take off Commenced||No|
|Location - Airport|
Inadequate ATC Procedures,
Ineffective Regulatory Oversight
|Tag(s)||Post Crash Fire|
|Tag(s)||ATC clearance error,|
Procedural non compliance
|Damage or injury||Yes|
|Aircraft damage||Hull loss|
|Fatalities||Many occupants (34)|
|Causal Factor Group(s)|
|Group(s)||Air Traffic Management|
Air Traffic Management,
On 1 February 1991, a Boeing 737-300 (N388US) being operated by USAir on a scheduled passenger flight from Syracuse NY to San Francisco via Washington DC, Columbus OH and Los Angeles as US 1493 landed on runway 24L at Los Angeles on arrival from Columbus in normal night visibility and almost immediately collided with a Fairchild Metroliner (N683AV) being operated by Skywest on scheduled passenger flight from Los Angeles to Palmdale CA as SKW 5569 which was stationary at an intersection ahead. After a high speed impact, the two severely damaged aircraft departed the left side of the runway in contact with each other and eventually stopped on the parallel taxiway after hitting and damaging an unoccupied satellite fire station when already on fire. Both aircraft were destroyed by the impact and post crash fire but an emergency evacuation of the 737 was made. All 12 occupants of the Metroliner were killed as were 22 of the 89 occupants of the 737, including the aircraft Captain. Of the other 67 occupants of the 737, 13 sustained serious injuries, 17 sustained minor injuries and 37 were uninjured.
An Investigation was carried out by the NTSB. The FDR and CVR from the 737 were recovered and successfully replayed but the CVR tape was in poor condition due to wear in service and some of the recorded data were of poor quality. The Metroliner was not required to be fitted with a CVR or an FDR and although a new Federal Aviation Administration (FAA) requirement for it to be fitted with a CVR was due to become effective from 11 October 1991, it had, although already purchased, not yet been fitted because the FAA had declined to allow an exemption to the associated MMEL requirement prior to the mandatory carriage date that the aircraft must not take off if it became unserviceable.
It was noted that the flight crew in control at the time of the accident had taken over the aircraft at the Columbus stop. The 48 year-old 737 Captain, who was killed in the accident, had accumulated approximately 16,300 hours total flight time which included 4,300 hours on the 737. He had been upgraded to Captain on the 737 5½ years earlier having been employed as a pilot by USAir and one of its forerunner airlines for the past 22 years. The 32 year-old 737 First Officer, who survived the accident after being rescued by AFRS personnel, had been a USAir pilot for a little over 2 years and had approximately 4,316 hours total flying time which included 982 hours on the 737. He was PF for the accident landing. It was found that the Captain had been a long term user of a prescription drug prohibited for flight crew to treat a "gastrointestinal problem" without reporting the condition or drug use to his AME. However, no conclusion directly relevant to the cause of the accident was attributed to this. The Runway Controller involved had qualified as a Los Angeles TWR Full Performance-Level (FPL) Controller just over 7 weeks prior to the accident after her previous 7 years service at smaller FAA airports.
The Investigation was able to establish that both aircraft had received, acknowledged and complied with conflicting clearances issued by the same runway controller just over one minute apart. The 737 had been cleared to land after the Metroliner had previously been cleared to line up and wait at an intersection after she had "forgotten about" the earlier clearance and then misidentified the aircraft involved as another Metroliner also taxiing for departure. The Investigation found that in accordance with the requirements of the 'Technical Appraisal Program', the controller's performance in position had been assessed by her supervisor after initial experience following qualification and they had "identified deficiencies that were indicative of weaknesses in her performance". These were documented and included two "Critical Training Indicators" (CTIs) - "a loss of awareness of aircraft separation" and "the misidentification of an aircraft by use of an incorrect call sign". The Investigation noted that the two previously-identified CTIs "were again evident in the controller's performance on the night of the accident", suggesting that they were not addressed and remedied after they were initially documented. The Supervisor's subsequent testimony at the Investigation Public Hearing "indicated that although he completed the evaluation and discussed these items with the controller, he did not initiate any other remedial action" and it was noted that, in response to further questioning, he had also indicated that "he did not have a clear understanding of the TAP".
In terms of the context for the controller's error, the Investigation noted that the fact that GND Controllers at Los Angeles did not generate flight progress strips for outbound traffic increased runway controllers' workload. They had to determine flight crew intentions and then "rely on memory and observations of aircraft moving on the ground" to identify and track their progress. It was considered that "if a controller is unable to recall such details or unable to observe or recognize an aircraft, however briefly, the possibility of error is greatly increased".
It was considered that staffing in the Control Tower on the night of the accident was "adequate", that the air traffic volume in the area during the timeframe of the accident was "moderate" and that the workload was "normal". However, the Investigation concluded that "facility procedures in place at (Los Angeles) on the date of the accident that did not allow for lapses in judgment and decision making and removed human performance redundancies" had been relevant to the controller’s error. They had been "required to assume full responsibility for strip marking and position determination, in addition to departure and arrival sequencing" and "as a result, these duties, in addition to working a combined position (helicopter control) and performing the coordination responsibilities to operate that position, created a situation that was abnormally burdensome for them to respond to successfully".
The fact that the ASDE system installed at Los Angeles was inoperative at the time of the accident was noted. It was stated that the NTSB "remains concerned that the ASDE at the Los Angeles Tower has an extensive history of failure and believes that special efforts must be made to ensure that this equipment is maintained to the highest state of operational readiness". The Investigation could not determine if the use of the ASDE would have prevented this accident. It was considered that, given the sequence of events prior to the collision, even if the controller involved "had included a normal scan of the ASDE in her activities, she would not have had a reason for scanning the ASDE specifically in the area of (the taxiway from which the Metroliner had entered the runway) if she had forgotten about the aircraft or if she believed the aircraft was on (the parallel taxiway en route to a full length line up)". Since the prevailing visibility was such that the ASDE was only required to "be used as a tool to confirm visual observations […] the controller's primary focus (would have been) on the visual observation of the airport environment".
The late sighting of the Metroliner by the 737 pilots was also considered. It was noted that the clearance to the Metroliner to line up had been given on the same frequency as the 737 landing clearance when the 737 was already on that frequency but had gone unnoticed. The surviving First Officer stated that no obstruction had been visually evident on the runway prior to touchdown but that as he had lowered the nose of the aircraft onto the runway, he had seen an aircraft "on the runway immediately in front of and below him" and remembered seeing that the landing lights of his aircraft had been reflected off the propellers of it. He stated that there had been insufficient time for any evasive action and "that the collision occurred simultaneous to his airplane's nose wheel contacting the runway" and was accompanied by an explosion and the nose of his aircraft dropping and fire breaking out. It was found that the failure of the 737 pilots to sight the (upper) anti-collision beacon of the Metroliner was likely to have been due to obstruction of their line of sight to it below 100 feet agl by the Metroliner's rudder cap which obstructs the beacon when the aircraft is viewed from the rear. It was found that the applicable FAA Regulations permitted some obstruction of anti-collision beacons by the structure of an aircraft and that the obstruction on the Metroliner involved "was within the allowable criteria". It was also noted that, although it was not possible to determine whether the installation of an anti-collision light in compliance with generally applicable certification standards which had been enhanced since the type certification of the Metroliner would have had any effect on the collision outcome, it was "reasonable to conclude" that any increase in its external lighting would have enhanced the possibility of its earlier detection by the 737 flight crew.
Experiments in similar conditions to those at the time of the accident showed that the white tail navigation light on the aircraft would have been ineffective for the detection of such a relatively small aircraft from a distance because it would have been "virtually indistinguishable" from the (white) runway centreline lighting. It was also noted during the same experiments that the likelihood of detection of an aircraft on the runway ahead by the crew aircraft approaching to land can be increased if the first aircraft is displaced laterally from the runway centreline lighting by approximately a metre.
It was found that the Metroliner's strobe lights had not been on at the time of the collision because Company SOP required that they should only be switched on by the Captain after take-off clearance had been received and the last four items of the Before Take-off Checklist have been accomplished and that Checklist called as complete. It was considered that the use of strobe lighting by aircraft occupying an active runway would also "ease the controllers' memory load by assisting them in locating, identifying, and segregating aircraft" on such a runway. However, an FAA lighting specialist testified to the Investigation Public Hearing that "the federal standards for aircraft external lighting are primarily intended to serve in-flight conspicuity needs and that no effort has been made by the FAA to address the issue of conspicuity of aircraft on airport surfaces".
Aspects of the evacuation of the 737 were considered. Four of the six exits were used, the Rl forward service door, both overwing exits and the R2 service door. Almost all those who escaped used either the right overwing exit or the R2 exit. The L1 exit was damaged by the secondary impact with the unoccupied building which finally brought the aircraft to a stop and the L2 exit was unusable due to fire outside which began after the collision and whilst the aircraft was still moving. All of the surviving passengers interviewed stated that the cabin had "filled with thick black smoke within seconds of the impact with the building" and passengers who made their way to the rear of the cabin to the R2 exit "reported using the emergency floor path lighting" to get there. It was found that "the propagation of the fire in the cabin was accelerated by the release of oxygen from the flight crew oxygen system that was damaged in the initial collision sequence on the runway" and that this had "significantly reduced the time available for a successful emergency evacuation" of all on board. Many of those who did not manage to escape in the time available were found near the overwing exits and had been "overcome when the cabin fire intensified". It was concluded that "the emergency response of the Los Angeles Department of Airports for this accident was timely and effective" but noted that prior to the accident, the FAA had been slow to respond effectively to known issues which were liable to limit the rate of evacuation through Type 3 (overwing) exits, some of which had been in evidence during the 737 evacuation.
The subject of fire retardant cabin furnishings was considered in respect of the 737 in terms of the speed at which toxic smoke/fumes had spread and prevented some occupants from exiting the aircraft before being overcome. It was noted that the because the aircraft had been manufactured prior to the introduction of enhanced requirements applicable to newer aircraft, it had only been required to be upgraded to the new standard when a "general retrofit" of the cabin was carried out, with no specific end date for such action specified.
It was determined that the Probable Cause of the Accident was "the 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 Air Traffic Service to provide adequate policy direction and oversight to its air traffic control facility managers". These failures created an environment in the Los Angeles Air Traffic Control 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.
A Contributory Factor was identified as "failure of the FAA to provide effective quality assurance of the ATC system".
Safety Action taken by the FAA shortly after the investigated accident was noted as having been a general instruction to all terminal ATC facilities as follows:
- Do not authorize aircraft to taxi into position and held at an intersection between sunset and sunrise. Additionally, do not authorize an aircraft to taxi into position and hold at any time when the intersection is not visible from the tower. These procedures shall be implemented at. 7:00 a.m. local on February 16, 1991.
At the conclusion of the Investigation, eighteen Safety Recommendations were made as follows:
- that the Federal Aviation Administration should modify Air Traffic Control procedures at the Los Angeles International Airport to:
- segregate arrivals and departures to specific runways;
- provide redundancies as intended in the National Operational Position Standards in the control tower. [A-91-104]
- that the Federal Aviation Administration should undertake a thorough risk-based evaluation of air traffic control procedures at the Los Angeles International Airport, evaluate whether changes are required, and implement necessary changes. The evaluation should consider at least the following issues:
- Runway intersection take-offs;
- Position-and-hold clearances;
- Displaced runway thresholds;
- Hazards associated with runway crossing traffic;
- Local Assist Controller;
- Airport Surface Detection Equipment use and maintenance. [A-91-105]
- that the Federal Aviation Administration should include in the Office of Safety Quality Assurance the authority and resources to:
- independently evaluate air traffic control facility compliance with FAA directives and;
- audit facility evaluations performed by the Office of Air Traffic System Effectiveness to determine that noted deficiencies are corrected. [A-91-106]
- that the Federal Aviation Administration should retain the National Operational Position Standards as a separate, independent order and:
- direct the FAA's Human Factors and Air Traffic Service staffs to perform a combined review of the order to determine the adequacy of redundancies and incorporate any resultant recommendations into the National Order;
- expedite the development of Chapters 5 through 10 of the National Order. [A-91-107]
- that the Federal Aviation Administration should provide Air Traffic Control Supervisors with formal training to improve their understanding of the intent, objectives and administration of the Technical Appraisal Program. [A-91-108]
- that the Federal Aviation Administration should require that interim evaluations of controller performance, such as those of the Technical Appraisal Program, be retained for 2 years and utilized in conjunction with other performance appraisals to track the performance and training needs of air traffic controllers. [A-91-I09]
- that the Federal Aviation Administration should conduct a one-time examination of the airport lighting at all U.S. tower-controlled airports to eliminate or reduce restrictions to visibility from the control tower to the runways and other traffic movement areas. [A-91-110]
- that the Federal Aviation Administration should redefine the airplane certification coverage compliance standards for anti-collision light installations to ensure that the anti-collision light(s) of an aircraft in position on a runway are clearly visible to the pilot of another aircraft preparing to land or take off on that runway. [A-91-111]
- that the Federal Aviation Administration should evaluate and implement, as appropriate, suitable means for enhancing the conspicuity of aircraft on airport surfaces during night or periods of reduced visibility. Include in this effort, measures such as the displacement of an aircraft away from the runway centerline, where applicable, and the use of conspicuity enhancements, such as high-intensity strobe lighting and logo lighting by aircraft on active runways and encourage operators of airplanes certificated prior to September 1, 1977 to upgrade their airplanes to the present higher intensity standards for anti-collision light installations. [A-91-112]
- that the Federal Aviation Administration should direct the general aviation community and the airlines to take steps to ensure that pilot training programs, including cockpit resource management training and flight operations procedures, place sufficient emphasis on the need for pilots to maintain vigilance in monitoring air traffic control radio communication frequencies for potential traffic conflicts with their aircraft, especially when on active runways and/or when conducting a final approach to a landing. [A-91-113]
- that the Federal Aviation Administration should incorporate into the Airman's Information Manual language that will alert pilots to the need for vigilance in monitoring air traffic frequencies for traffic conflict situations which may affect the safety of their flight. [A-91-114]
- that the Federal Aviation Administration should develop for inclusion in the Airman's Information Manual and the Air Traffic Control Handbook (7110.65F) specific phraseology to be used by pilots when requesting an intersection departure and specific phraseology to be used by controllers when issuing a position-and-hold clearance for an intersection departure. [A-91-115]
- that the Federal Aviation Administration should prohibit the use, after a specified date, of cabin materials in all transport category airplanes that do not comply with the improved fire safety standards contained in 14 CFR 25.853. [A-91-116]
- that the Federal Aviation Administration should direct the Emergency Evacuation Subcommittee of the Aviation Rulemaking Advisory Committee to examine flight attendant emergency procedures regarding the "2nd choice" exit assignments to ensure that such assignments provide for use of the nearest appropriate exit point. [A-91-117]
- that the Federal Aviation Administration should issue an Air Carrier Operations Bulletin directing Principal Operations Inspectors to emphasize that during a crash sequence flight attendants must remain properly restrained and seated in their crew seats until the airplane has come to a complete stop. [A-91-118]
- that the Federal Aviation Administration should establish a comprehensive educational program to alert pilots to the potential adverse effects on flight crew performance that may arise from the misuse of prescribed and over-the-counter medication. [A-91-119]
- that the Federal Aviation Administration should conduct a directed safety investigation of the Sunstrand Model AV-557 CVR to determine the necessary modifications to ensure that the switching mechanism in the unit is able to withstand recording tape anomalies and variations in tape capacity that can be expected to appear during the normal service life of the tape. [A-91-120]
- that the Federal Aviation Administration should disseminate information regarding the circumstances of this accident and the findings of the Safety Board's investigation to the pilot community through operations bulletins and safety seminars, such as the "Wings Pilot Proficiency Program." [A-91-121]
In addition, two previously-issued Safety Recommendations still outstanding at the time the Investigation was completed were also formally reiterated:
- that the Federal Aviation Administration should require 14 CFR Part 121 operators to develop and use Cockpit Resource Management programs in their training methodology by a specified date. [A-89-124]
- that the Federal Aviation Administration should require that scheduled 14 CFR Part 135 operators develop and use Cockpit Resource Management programs in their training methodology by a specified date. [A-90-135]
The Final Report of the Investigation was adopted by the Board on 3 March 1992.