Nancy Leveson MIT Outline Accident Causation in Complex Systems STAMP New Analysis Methods Hazard Analysis Accident Analysis Security Analysis Does it Work Evaluations Why We Need a New Approach to Safety ID: 583498
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Slide1
Engineering a Safer World
Nancy LevesonMITSlide2
Outline
Accident Causation in Complex Systems: STAMPNew Analysis Methods
Hazard AnalysisAccident AnalysisSecurity AnalysisDoes it Work? EvaluationsSlide3
Why We Need a New Approach to Safety
Traditional safety engineering approaches developed for relatively simple electro-mechanical systemsAccidents in complex, software-intensive systems are changing their nature
Role of humans in systems is changingWe need new ways to deal with safety in complex systems
“
Without changing our patterns of thought, we will
not be able to solve the problems we created
with our current patterns of thought.”
Albert EinsteinSlide4
Accident Causality Models
Underlie all our efforts to engineer for safetyExplain why accidents occurDetermine the way we prevent and investigate accidents
May not be aware you are using one, but you areImposes patterns on accidents “All models are wrong, some models are useful” George BoxSlide5
Traditional Ways to Cope with Complexity
Analytic ReductionStatisticsSlide6
Analytic Reduction
Divide system into distinct parts for analysis Physical aspects
Separate physical components Behavior Events over timeExamine parts separatelyAssumes such separation does not distort phenomenon
Each component or subsystem operates independentlyAnalysis results not distorted when consider components separatelyComponents act the same when examined singly as when playing their part in the wholeEvents not subject to feedback loops and non-linear interactionsSlide7
Chain-of-Events Accident Causality Model
Explains accidents in terms of multiple events, sequenced as a forward chain over time.Simple, direct relationship between events in chain
Events almost always involve component failure, human error, or energy-related eventForms the basis for most safety engineering and reliability engineering analysis: e,g, FTA, PRA, FMECA, Event Trees, etc. and design:
e.g., redundancy, overdesign, safety margins, ….Slide8
Domino “Chain of events” Model
Event-based
Cargo door fails
Causes
Floor collapses
Causes
Hydraulics fail
Causes
Airplane crashes
© Copyright John Thomas 2013
DC-10:Slide9
Chain-of-events exampleSlide10
Accident with No Component Failures
Mars Polar LanderHave to slow down spacecraft to land safelyUse Martian gravity, parachute, descent engines (controlled by software)
Software knows landed because of sensitive sensors on landing legs. Cut off engines when determine have landed.But “noise” (false signals) by sensors generated when parachute opensSoftware not supposed to be operating at that time but software engineers decided to start early to even out load on processorSoftware thought spacecraft had landed and shut down descent enginesSlide11
Types of Accidents
Component Failure AccidentsSingle or multiple component failuresUsually assume random failure
Component Interaction AccidentsArise in interactions among componentsRelated to interactive and dynamic complexityBehavior can no longer be Planned
UnderstoodAnticipatedGuarded againstExacerbated by introduction of computers and softwareSlide12
A
B
C
Unreliable but not unsafe
Unsafe but not unreliable
Unreliable and unsafe
Confusing Safety and Reliability
Preventing Component or Functional
Failures is NOT Enough
Scenarios
Involving failures
Unsafe
scenariosSlide13
Analytic Reduction does not Handle
Component interaction accidentsSystemic factors (affecting all components and barriers)SoftwareHuman behavior (in a non-superficial way)
System design errorsIndirect or non-linear interactions and complexityMigration of systems toward greater risk over timeSlide14
14
Summary
The world of engineering is changing.
If safety engineering does not change with it, it will become more and more irrelevant.
Trying to shoehorn new technology and new levels of complexity into old methods does not workSlide15
Systems Theory
Developed for systems that areToo complex for complete analysisSeparation into (interacting) subsystems distorts the results
The most important properties are emergentToo organized for statisticsToo much underlying structure that distorts the statisticsDeveloped for biology (von Bertalanffy) and engineering (Norbert Weiner)Basis of system engineering and “System Safety”
Slide16
Systems Theory (2)
Focuses on systems taken as a whole, not on parts taken separatelyEmergent properties
Some properties can only be treated adequately in their entirety, taking into account all social and technical aspects “The whole is greater than the sum of the parts”These properties derive from relationships among the parts of the system
How they interact and fit togetherTwo pairs of ideasHierarchy and emergenceCommunication and controlSlide17
Emergent properties
(arise from complex interactions)
Process
Process components interact in
direct and indirect ways
Safety is an emergent propertySlide18
Controller
Controlling emergent properties
(e.g., enforcing safety constraints)
Process
Control Actions
Feedback
Individual component behavior
Component interactions
Process components interact in
direct and indirect waysSlide19
Controller
Controlling emergent properties
(e.g., enforcing safety constraints)
Process
Control Actions
Feedback
Individual component behavior
Component interactions
Process components interact in
direct and indirect ways
Air Traffic Control:
Safety
ThroughputSlide20
Controls/Controllers Enforce Safety Constraints
Power must never be on when access door openTwo aircraft must not violate minimum separation
Aircraft must maintain sufficient lift to remain airbornePublic health system must prevent exposure of public to contaminated water and food productsPressure in a offshore well must be controlledRunway incursions and operations on wrong runways or taxiways must be preventedSlide21
A Broad View of “Control”
Component failures and unsafe interactions may be “controlled” through design
(e.g., redundancy, interlocks, fail-safe design) or through processManufacturing processes and proceduresMaintenance processesOperations
or through social controlsGovernmental or regulatoryCulture InsuranceLaw and the courts
Individual self-interest (incentive structure)Slide22
There may be multiple controllers, processes, and levels of control
(with various types of communication between them)
Each controller enforces
specific constraints, whichtogether enforce the system level constraints (emergent
properties)
Controller
Controller
Controller
Controller
Controller
Physical Process 1
Physical Process 2Slide23
Controlled Process
Process
Model
Control
Actions
Feedback
Role of Process Models in Control
Controllers use a
process model
to determine control actions
Accidents often occur when the process model is incorrect
How could this happen?
Four types of unsafe control actions:
Control commands required for safety are not given
Unsafe ones are given
Potentially safe commands given too early, too late
Control stops too soon or applied too long
Controller
23
(Leveson, 2003); (Leveson, 2011)
Control
AlgorithmSlide24
Example
Safety
Control
StructureSlide25
Potential Accidents and Hazards for Aircraft
Accidents (Losses):Aircraft mid-air collisionUncontrolled collision with terrain
Aircraft collision with something on the groundPassenger injury due to wake turbulence or unsafe movement of aircraftHazards: Controlled aircraft violate minimum separation standards.Aircraft enters an unsafe atmospheric region.
Loss of controlled flight or airframe integrityAircraft enters unsafe attitude (excessive turbulence or pitch/roll/yaw that causes passenger injury but not necessarily aircraft loss).Aircraft enters restricted airspace without permission …Slide26
In-Trail Procedure (ITP)
Enables aircraft to achieve FL changes on a more frequent basis.Designed for oceanic and remote airspaces not covered by radar.
Permits climb and descent using new reduced longitudinal separation standards.Potential BenefitsReduced fuel burn and CO2 emissions via more opportunities to reach the optimum FL or FL with more favorable winds.
Increased safety via more opportunities to leave turbulent FL.But standard separation requirements not met during maneuverSlide27
Example High-Level Control Structure for ITP
CONSTRAINTS:
Enforce minimum separation
Maximize throughput
Minimize fuel burnSlide28Slide29
STAMP:
System-Theoretic Accident Model and Processes
A new accident causality model based on Systems Theory
(vs. Reliability Theory)Slide30
STAMP: Safety as a Control Problem
Safety is an emergent property that arises when system components interact with each other within a larger environmentA set of constraints related to behavior of system components (physical, human, social) enforces that property
Accidents occur when interactions violate those constraints (a lack of appropriate constraints on the interactions)Goal is to control the behavior of the components and systems as a whole to ensure safety constraints are enforced in the operating system. Slide31
Safety as a Dynamic Control Problem
Examples
O-ring did not control propellant gas release by sealing gap in field joint of Challenger Space Shuttle
Software did not adequately control descent speed of Mars Polar Lander
At Texas City, did not control the level of liquids in the ISOM tower;
In Deep Water Horizon, did not control the pressure in the well;
Financial system did not adequately control the use of financial instrumentsSlide32
Safety as a Dynamic Control Problem
Events are the result of the inadequate control
Result from lack of enforcement of safety constraints in system design and operationsSystems are dynamic processes that are continually changing and adapting to achieve their goalsA change in emphasis:“prevent failures”
“enforce safety constraints on system behavior” Slide33
Changes to Analysis Goals
Hazard analysis: Ways that safety constraints might not be enforced (vs. chains of failure events leading to accident)Accident Analysis (investigation)
Why control structure was not adequate to prevent loss (vs. what failures led to loss and who responsible)Security AnalysisPotential weaknesses in security controls (vs. threat analysis)Slide34
Systems ThinkingSlide35
STAMP: Theoretical Causality Model
Accident/Event Analysis
CAST
Hazard Analysis
STPA
System Engineering
(e.g.,
Specification
,
Safety-Guided Design, Design Principles)
Specification Tools
SpecTRM
Risk Management
Operations
Management Principles/
Organizational Design
Identifying Leading
Indicators
Organizational/Cultural
Risk Analysis
Tools
Processes
Regulation
Security Analysis
STPA-SecSlide36
Outline
Accident Causation in Complex Systems: STAMPNew Analysis Methods
Hazard AnalysisAccident AnalysisSecurity Analysis
Does it Work? EvaluationsSlide37
STPA: System-Theoretic Process Analysis
Integrated into system engineeringCan be used from beginning of projectSafety-guided design
Guidance for evaluation and testIncident/accident analysisWorks also on social and organizational aspects of systemsGenerates system and component safety requirements (constraints)Identifies flaws in system design and scenarios leading to violation of a safety requirement (i.e., a hazard)Slide38
ITP Procedure – Step by Step
Check that ITP criteria are met.If ITP is possible, request ATC clearance via CPDLC using
ITP phraseology.Check that there are no blocking aircraft other than Reference Aircraft in the ITP request.Check that ITP request is applicable (i.e. standard request not sufficient) and compliant with ITP phraseology.
Check that ITP criteria are met.If all checks are positive, issue ITP clearance via CPDLC.
Flight Crew
Air Traffic Controller
If ITP criteria are still met, accept ITP
clearance via CPDLC
When ITP clearance is received, check that ITP criteria are still met.
9. Execute ITP clearance without delay
10. Report when established at cleared FL
Involves multiple aircraft, crew, communications (ADS-B, GPS) ,
ATCSlide39
High-Level Control Structure for ITPSlide40
Pilot Responsibilities and Process Model
Responsibilities:Assess whether ITP appropriateCheck if ITP criteria are met
Request ITPReceive ITP approvalRecheck criteriaExecute flight level changeConfirm new flight level to ATCProcess Model
Own ship climb/descend capability ADS-B data for nearby aircraft (velocity, position, orientation)ITP criteria (speed, distance, relative attitude, similar track, data quality)State of ITP request/approvaletc.Slide41
Step 1 for Pilot
Control Action
Not providing causes hazard
Providing causes hazard
Too early/too late, wrong order
Stopped too soon/ applied too long
Execute
ITP
Maneuver
Pilot
Aircraft
Execute ITP
maneuver
A/C status, position, etc.Slide42
Potentially Hazardous Control Actions
by the Flight Crew
Control Action
Not Providing Causes Hazard
Providing Causes Hazard
Wrong Timing/Order
Causes Hazard
Stopped Too Soon/Applied Too Long
Execute ITP
ITP executed when not approved
ITP executed when ITP criteria are not satisfied
ITP executed with incorrect climb rate, final altitude, etc
ITP executed too soon before approval
ITP executed too late after reassessment
ITP aircraft levels off above requested FL
ITP aircraft levels off below requested FL
Abnormal Termination of ITP
FC continues with maneuver in dangerous situation
FC aborts unnecessarily
FC does not follow regional contingency procedures while aborting
Slide43
High Level Constraints on Flight Crew
The flight crew must not execute the ITP when it has not been approved by ATC.The flight crew must not execute an ITP when the ITP criteria are not satisfied.
The flight crew must execute the ITP with correct climb rate, flight levels, Mach number, and other associated performance criteria.The flight crew must not continue the ITP maneuver when it would be dangerous to do so.The flight crew must not abort the ITP unnecessarily. (Rationale: An abort may violate separation minimums)
When performing an abort, the flight crew must follow regional contingency procedures.The flight crew must not execute the ITP before approval by ATC.The flight crew must execute the ITP immediately when approved unless it would be dangerous to do so.The crew shall be given positive notification of arrival at the requested FLSlide44
Potentially Hazardous Control
Actions for ATC
Control Action
Not Providing Causes Hazard
Providing Causes Hazard
Wrong Timing/Order Causes Hazard
Stopped Too Soon or Applied Too Long Causes Hazard
Approve ITP request
Approval given when criteria are not met
Approval given to incorrect aircraft
Approval given too early
Approval given too late
Deny ITP request
Abnormal Termination Instruction
Aircraft should abort but instruction
not given
Abort instruction given when abort is not necessary
Abort instruction given too late
Slide45
High-Level Constraints on ATC
Approval of an ITP request must be given only when the ITP criteria are met.Approval must be given to the requesting aircraft only.Approval must not be given too early or too late [needs to be clarified as to the actual time limits]
An abnormal termination instruction must be given when continuing the ITP would be unsafe.An abnormal termination instruction must not be given when it is not required to maintain safety and would result in a loss of separation.An abnormal termination instruction must be given immediately if an abort is required.Slide46
Steps in STPA
Establish foundation for analysisDefine “accident” for your systemDefine hazardsRewrite hazards as constraints on system designDraw preliminary (high-level) safety control structure
Step 1: Identify potentially unsafe control actions (high-level safety requirements and constraints)Step 2: Determine how each potentially hazardous control action could occurSlide47
STPA Step 2
47
Inadequate Control Algorithm
(Flaws in creation, process changes, incorrect modification or adaptation)
Controller
Process Model
(inconsistent, incomplete, or incorrect)
Control input or external information wrong or missing
Actuator
Inadequate operation
Inappropriate, ineffective, or missing control action
Sensor
Inadequate operation
Inadequate or missing feedback
Feedback Delays
Component failures
Changes over time
Controlled Process
Unidentified or out-of-range disturbance
Controller
Process input missing or wrong
Process output contributes to system hazard
Incorrect or no information provided
Measurement inaccuracies
Feedback delays
Delayed operation
Conflicting control actions
Missing or wrong communication with another controller
ControllerSlide48
Example Causal Analysis
Unsafe control action: Pilot executes maneuver when criteria not metPossible Causes?Thinks criteria met (incorrect process model)Inadequate feedback provided by ITP boxFeedback delayed or corruptedReceives incorrect info from ATC or ADS-B
ATC thinks criteria are met and safe to perform maneuver but it is not…Slide49
Is it Practical?
STPA has been or is being used in a large variety of industriesSpacecraftAircraft Air Traffic Control
UAVs (RPAs)Defense Automobiles (GM, Ford, Nissan?)Medical Devices and Hospital SafetyChemical plantsOil and GasNuclear and Electrical PowerC02
Capture, Transport, and StorageEtc.Slide50
Analysis of the management structure of the space shuttle program (post-Columbia)
Risk management in the development of NASA’s new manned space program (Constellation) NASA Mission control ─ re-planning and changing mission control procedures safelyFood safetySafety in pharmaceutical drug development
Risk analysis of outpatient GI surgery at Beth Israel Deaconess Hospital Analysis and prevention of corporate fraud
Social and Managerial
Is it Practical? (2)Slide51
Does it Work?
Most of these systems are very complex (e.g., the U.S. Missile Defense System)In all cases where a comparison was made:
STPA found the same hazard causes as the old methodsPlus it found more causes than traditional methodsIn some evaluations, found accidents that had occurred that other methods missedCost was orders of magnitude less than the traditional hazard analysis methodsSlide52
Automating STPA (Step 1): John Thomas)
52
Requirements can be derived automatically (with some user guidance) using mathematical foundation
Allows automated completeness/consistency checking
Hazardous Control Actions
Discrete Mathematical Representation
Predicate calculus /state machine structure
Formal (model-based) requirements specification
HazardsSlide53
Others (that we are doing)
Automating Step 2Leading IndicatorsSophisticated Human Factors AnalysisFeature Interactions (Automobiles)
Safety Management SystemsSafety-Guided Development (Design)Slide54Slide55
Conops
Identify:
--
Missing, inconsistent, conflicting information -- Vulnerabilities, risks, tradeoffs -- Potential design or architectural solutions to hazardsDemonstrating on TBO (Trajectory Based Operations)Cody FlemingSlide56Slide57
Others (that we are doing)
Changes in Complex SystemsAir Traffic Control (NextGen)ITPInterval Management (IMS)
UAS (unmanned aircraft systems) in national airspaceWorkplace (Occupational) SafetySome Current ApplicationsHospital Patient SafetyFlight Test (Air Force)Security in aircraft networks
Defense systemsSlide58
Human Factors in Hazard AnalysisSlide59
Adding Human Factors to Hazard AnalysisSlide60
Outline
Accident Causation in Complex Systems: STAMPNew Analysis Methods
Hazard AnalysisAccident AnalysisSecurity Analysis
Does it Work? EvaluationsSlide61
Common Traps in Understanding Accident Causes
Root cause seduction and oversimplificationNarrow views of human errorHindsight bias
Focus finding someone or something to blameSlide62
Root Cause Seduction
Assuming there is a root cause gives us an illusion of control.Usually focus on operator error or technical failuresIgnore systemic and management factors
Leads to a sophisticated “whack a mole” gameFix symptoms but not process that led to those symptomsIn continual fire-fighting modeHaving the same accident over and overSlide63Slide64
Oversimplification of Causes
Almost always there is:Operator “error”Flawed management decision makingFlaws in the physical design of equipment
Safety culture problemsRegulatory deficienciesEtc.Need to determine why safety control structure was ineffective in preventing the loss.Slide65
Blame is the Enemy of Safety
Two possible goals for an accident investigation:Find who to blameUnderstand why occurred so can prevent in future
Blame is a legal or moral concept, not an engineering oneFocus on blame can:Prevent openness during investigationLead to finger pointing and cover upsLead to people not reporting errors and problems before accidentsSlide66
Human Error: Traditional
ViewOperator error is cause of most incidents and accidents
So do something about human involved (fire them, retrain, admonish) Or do something about humans in generalMarginalize them by putting in more automationRigidify their work by creating more rules and proceduresSlide67
Fumbling for his recline button Ted
unwittingly instigates a disasterSlide68
Human Error: Systems View
(Sydney Dekker, Jens Rasmussen, Leveson)
Human error is a symptom, not a causeAll behavior affected by context (system) in which occursTo do something about error, must look at system in which people work:Design of equipmentUsefulness of proceduresExistence of goal conflicts and production pressures
Human error is a sign that a system needs to be redesignedSlide69
Sidney Dekker, 2009
Hindsight BiasSlide70
Overcoming Hindsight Bias
Assume nobody comes to work to do a bad job.Assume were doing reasonable things given the complexities, dilemmas, tradeoffs, and uncertainty surrounding them.
Simply finding and highlighting people’s mistakes explains nothing. Saying what did not do or what should have done does not explain why they did what they did.Investigation reports should explainWhy it made sense for people to do what they did
rather than judging them for what they allegedly did wrong and What changes will reduce likelihood of happening againSlide71
ComAir 5191 (Lexington) Sept. 2006
Analysis using CAST by Paul Nelson,
ComAir pilot and human factors expert
(for report: http://sunnyday.mit.edu/papers/nelson-thesis.pdfSlide72
Identify Hazard and Safety Constraint Violated
Accident: death or injury, hull lossSystem hazard: Operation on wrong runways or taxiways.
System safety constraint: The safety control structure must prevent operations on wrong runways or taxiways Goal: Figure out why the safety control structure did not do thisSlide73
Start with Physical System (Aircraft)
Failures: NoneUnsafe InteractionsTook off on wrong runwayRunway too short for that aircraft to become safely airborne Then add controller of aircraft to determine why on that runwaySlide74
Aircraft
Flight CrewSlide75
Component Analysis in CAST
Safety responsibilities/constraintsUnsafe control actionsWhy?Mental/process model flawsContextual/environmental influencesSlide76
5191 Flight Crew
Safety Requirements and Constraints:
Operate the aircraft in accordance with company procedures, ATC clearances and FAA regulations.Safely taxi the aircraft to the intended departure runway.
Take off safely from the planned runway.Unsafe Control Actions:Taxied to runway 26 instead of continuing to runway 22.
Did not use the airport signage to confirm their position short of the runway.Did not confirm runway heading and compass heading matched.40 second conversation violation of “sterile cockpit”Slide77Slide78
Mental Model Flaws:
Believed they were on runway 22 when the takeoff was initiated.
Thought the taxi route to runway 22 was the same as previously experienced.Believed their airport chart accurately depicted the taxi route to runway 22.Believed high-threat taxi procedures were unnecessary
Believed “lights were out all over the place” so the lack of runway lights was expectedSlide79
Context in Which Decisions Made:
No communication that the taxi route to the departure runway was different than indicated on the airport diagram
No known reason for high-threat taxi proceduresDark outComair had no specified procedures to confirm compass heading with runwaySleep loss fatigue
Runways 22 and 26 looked very similar from that positionComair in bankruptcy, tried to maximize efficiencyDemanded large wage concessions from pilotsEconomic pressures a stressor and frequent topic of conversation for pilotsSlide80
The Airport Diagram
What The Crew had
What the Crew NeededSlide81
Federal Aviation Administration
Comair: Delta Connection
Airport Safety & Standards District Office
LEX ATC Facility
National Flight Data Center
Jeppesen
5191 Flight Crew
Certification, Regulation, Monitoring & Inspection
Procedures, Staffing, Budget
Aircraft Clearance and Monitoring
Charts, NOTAM Data
(except “L”) to Customer
Read backs, Requests
Local NOTAMs
Reports, Project Plans
NOTAM Data
Chart Discrepancies
ATIS & “L” NOTAMs
Operational Reports
ALPA
Safety ALR
Airport
Diagram
Airport Diagram Verification
Optional construction signage
= missing feedback lines
Certification, Inspection, Federal Grants
Composite Flight Data, except “L” NOTAM
Graphical Airport Data
ATO: Terminal Services
Pilot perspective information
Construction information
Blue Grass Airport Authority
Procedures & Standards
Flight release, Charts etc.
NOTAMs except “L”
IOR, ASAP
Reports
Certification & RegulationSlide82
Comair (Delta Connection) Airlines
Safety Requirements and ConstraintsResponsible for safe, timely tranport of passengers within their established route system
Ensure crews have available all necessary information for each flightFacilitate a flight deck environment that enables crew focus on flight safety actions during critical phases of flightDevelop procedures to ensure proper taxi route progression and runway confirmationUnsafe Control Actions
:Internal processes did not provide LEX local NOTAM on the flight release, even though it was faxed to Comair from LEXIn order to advance corporate strategies, tactics were used that fostered work environment stress precluding crew focus ability during critical phases of flight.
Did not develop or train procedures for take off runway confirmation.Slide83
Comair (2)
Process Model Flaws:Trusted the ATIS broadcast would provide local NOTAMs to crews
Believed tactics promoting corporate strategy had no connection to safetyBelieved formal procedures and training emphasis of runway confirmation methods were unnecessary
Context in Which Decisions Made:In bankruptcy.Slide84
Blue Grass Airport Authority (LEX)
Safety Requirements and Constraints: Establish and maintain a facility for the safe arrival and departure of aircraft to service the community. Operate the airport according to FAA certification standards, FAA regulations (FARs) and airport safety bulletin guidelines (ACs).
Ensure taxiway changes are marked in a manner to be clearly understood by aircraft operators. Slide85
Airport Authority
Unsafe Control Actions: Relied solely on FAA guidelines for determining adequate signage during construction. Did not seek FAA acceptable options other than NOTAMs to inform airport users of the known airport chart inaccuracies. Changed taxiway A5 to Alpha without communicating the change by other than minimum signage.
Did not establish feedback pathways to obtain operational safety information from airport users.Slide86
Airport Authority
Process Model Flaws: Believed compliance with FAA guidelines and inspections would equal adequate safety. Believed the NOTAM system would provide understandable information about inconsistencies of published documents.
Believed airport users would provide feedback if they were confused. Context in Which Decisions Made: The last three FAA inspections demonstrated complete compliance with FAA regulations and guidelines. Last minute change from Safety Plans Construction Document phase III implementation plan.Slide87
Airport Safety & Standards Office
Safety Requirements and Constraints: Establish airport design, construction, maintenance, operational and safety standards and issue operational certificates accordingly. Ensure airport improvement project grant compliance and release of grant money accordingly.
Perform airport inspections and surveillance. Enforce compliance if problems found. Review and approve Safety Plans Construction Documents in a timely manner, consistent with safety. Assure all stake holders participate in developing methods to maintain operational safety during construction periods. Slide88
Unsafe Control Actions: The FAA review/acceptance process was inconsistent, accepting the original phase IIIA (Paving and Lighting) Safety Plans Construction Documents and then rejecting them during the transition between phases II and IIIA.
Did not require all stake holders (i.e. a Pilot representative was not present) be part of the meetings where methods of maintaining operational safety during construction were decided. Focused on inaccurate runway length depiction without consideration of taxiway discrepancies.
Did not require methods in addition to NOTAMs to assure safety during periods of construction when difference between LEX Airport physical environment and LEX Airport charts.
Airport Safety & Standards OfficeSlide89
Airport Safety & Standards Office
Process Model Flaws Did not believe pilot input was necessary for development of safe surface movement operations. No recognition of negative effects of changes on safety. Belief that the accepted practice of using NOTAMs to advise crews of charting differences was sufficient for safety.
Context in Which Decisions Made: Priority to keep Airport Facility Directory accurate. Slide90
Standard and Enhanced Hold Short MarkingsSlide91
LEX Controller Operations
Safety Requirements and ConstraintsContinuously monitor all aircraft in the jurisdictional airspace and insure clearance compliance.
Continuously monitor all aircraft and vehicle movement on the airport surface and insure clearance compliance.Clearances will clearly direct aircraft for safe arrivals and departures.Clearances will clearly direct safe aircraft and vehicle surface movement.All Local NOTAMs will be included on the ATIS broadcast.Unsafe Control Actions
Issued non-specific taxi instructions; i.e. “Taxi to runway 22” instead of “Taxi to runway 22 via Alpha, cross runway 26”.Did not monitor and confirm 5191 had taxied to runway 22.Issued takeoff clearance while 5191 was holding short of the wrong runway.Did not include all local NOTAMs on the ATISSlide92
Mental Model Flaws
Hazard of pilot confusion during North end taxi operations was unrecognized.Believed flight 5191 had taxied to runway 22.Did not recognize personal state of fatigue.
Context in Which Decisions MadeSingle controller for the operation of Tower and Radar functions.The controller was functioning at a questionable performance level due to sleep loss fatigueFrom control tower, thresholds of runways 22 and 26 appear to overlapSlide93
LEX Air Traffic Control Facility
Safety Requirements and ConstraintsResponsible for the operation of Class C airspace at LEX airport.
Schedule sufficient controllers to monitor all aircraft with in jurisdictional responsibility; i.e. in the air and on the ground.Unsafe Control ActionsDid not staff Tower and Radar functions separately.Used the fatigue inducing 2-2-1 schedule rotation for controllers.Slide94
LEX Air Traffic Control Facility (2)
Mental Model FlawsBelieved “verbal” guidance requiring 2 controllers was merely a preferred condition.Controllers would manage fatigue resulting from use of the 2-2-1 rotating shift.
Context in Which Decisions MadeRequests for increased staffing were ignored.Overtime budget was insufficient to make up for the reduced staffing.Slide95
Air Traffic Organization: Terminal Services
Safety Requirements and ConstraintsEnsure appropriate ATC Facilities are established to safely and efficiently guide aircraft in and out of airports.
Establish budgets for operation and staffing levels which maintain safety guidelines.Ensure compliance with minimum facility staffing guidelines.Provide duty/rest period policies which ensure safe controller performance functioning ability.Unsafe Control ActionsIssued verbal guidance that Tower and Radar functions were to be separately manned, instead of specifying in official staffing policies.
Did not confirm the minimum 2 controller guidance was being followed.Did not monitor the safety effects of limiting overtime.Slide96
Process Model Flaws
Believed “verbal” guidance (minimum staffing of 2 controllers) was clear.Believed staffing with one controller was rare and if it was unavoidable due to sick calls etc., that the facility would coordinate the with Air Route Traffic Control Center (ARTCC) to control traffic.Believed limiting overtime budget was unrelated to safety.
Believed controller fatigue was rare and a personal matter, up to the individual to evaluate and mitigate.Context in Which Decisions MadeBudget constraints.Air Traffic controller contract negotiations.
FeedbackVerbal communication during quarterly meetings.No feedback pathways for monitoring controller fatigue.Slide97
Federal Aviation Administration
Comair: Delta Connection
Airport Safety & Standards District Office
LEX ATC Facility
National Flight Data Center
Jeppesen
5191 Flight Crew
Certification, Regulation, Monitoring & Inspection
Procedures, Staffing, Budget
Aircraft Clearance and Monitoring
Charts, NOTAM Data
(except “L”) to Customer
Read backs, Requests
Local NOTAMs
Reports, Project Plans
NOTAM Data
Chart Discrepancies
ATIS & “L” NOTAMs
Operational Reports
ALPA
Safety ALR
Airport
Diagram
Airport Diagram Verification
Optional construction signage
= missing feedback lines
Certification, Inspection, Federal Grants
Composite Flight Data, except “L” NOTAM
Graphical Airport Data
ATO: Terminal Services
Pilot perspective information
Construction information
Blue Grass Airport Authority
Procedures & Standards
Flight release, Charts etc.
NOTAMs except “L”
IOR, ASAP
Reports
Certification & RegulationSlide98
Jeppesen
Safety Requirements and Constraints Creation of accurate aviation navigation charts and information data for safe operation of aircraft in the NAS. Assure Airport Charts reflect the most recent NFDC data
Unsafe Control Actions Insufficient analysis of the software which processed incoming NFDC data to assure the original design assumptions matched those of the application. Not making available to the NAS Airport structure the type of information necessary to generate the 10-8 “Yellow Sheet” airport construction chart. Slide99
Jeppesen (2)
Process Model FlawsBelieved Document Control System software always generated notice of received NFDC data requiring analyst evaluation. Any extended airport construction included phase and time data as a normal part of FAA submitted paper work.
Context in Which Decisions MadeThe Document Control System software generated notices of received NFDC data. Preferred Chart provider to airlines. FeedbackCustomer feedback channels are inadequae for providing information about charting inaccuracies. Slide100
National Flight Data Center
Safety Requirements and ConstraintsCollect, collate, validate, store, and disseminateaeronautical information detailing the physical description and operational status of all components of the National Airspace System (NAS). Operate the US NOTAM system to create, validate, publish and disseminate NOTAMS.
Provide safety critical NAS information in a format which is understandable to pilots. NOTAM dissemination methods will ensure pilot operators receive all necessary information. Slide101
Unsafe Control ActionsDid not use the FAA Human Factors Design Guide principles to update the NOTAM text format.
Limited dissemination of local NOTAMs (NOTAM-L). Used multiple and various publications to disseminate NOTAMs, none of which individually contained all NOTAM information.Process Model Flaws:
Believed NOTAM system successfully communicated NAS changes. Context in Which Decisions MadeThe NOTAM systems over 70 year history of operation. Format based on teletypes Coordination: No coordination between FAA human factors branch and the NFDC for use of HF design principle for NOTAM format revision.Slide102
Federal Aviation Administration
Safety Requirements and ConstraintsEstablish and administer the National Aviation Transportation System. Coordinate the internal branches of the FAA, to monitor and enforce compliance with safety guidelines and regulations.
Provide budgets which assure the ability of each branch to operate according to safe policies and procedures. Provide regulations to ensure safety critical operators can function unimpaired. Provide and require components to prevent runway incursions. Slide103
Unsafe Control Actions:
Controller and Crew duty/rest regulations were not updated to be consistent with modern scientific knowledge about fatigue and its causes. Required enhanced taxiway markings at only 15% of air carrier airports: those with greater than 1.5 million passenger enplanements per year. Mental Model Flaws
Enhanced taxiway markings unnecessary except for the largest US airports. Crew/controller duty/rest regulations are safe. Context in Which Decisions Made FAA funding battles with the US congress. Industry pressure to leave duty/rest regulations alone. Slide104
NTSB “Findings”
Probable Cause:FC’s failure to use available cues and aids to identify the airplane’s location on the airport surface during taxiFC’s failure to cross-check and verify that the airplane was on the correct runway before takeoff.
Contributing to the accident were the flight crew’s non-pertinent conversation during taxi, which resulted in a loss of positional awareness, Federal Aviation Administration’s (FAA) failure to require that all runway crossings be authorized only by specific air traffic control (ATC) clearances.Slide105
Communication Links Theoretically in
Place in Uberlingen AccidentSlide106
Communication Links Actually in PlaceSlide107
CAST (Causal Analysis using System Theory)
Identify system hazard violated and the system safety design constraintsConstruct the safety control structure as it was designed to work
Component responsibilities (requirements)Control actions and feedback loopsFor each component, determine if it fulfilled its responsibilities or provided inadequate control.If inadequate control, why? (including changes over time)Context Process Model FlawsSlide108
CAST (2)
For humans, why did it make sense for them to do what they did (to reduce hindsight bias)Examine coordination and communicationConsider dynamics and migration to higher risk
Determine the changes that could eliminate the inadequate control (lack of enforcement of system safety constraints) in the future.Generate recommendationsSlide109
CAST (3)
Continuous ImprovementAssigning responsibility for implementing recommendationsFollow-up to ensure implementedFeedback channels to determine whether changes effective If not, why not?Slide110
Conclusions
The model used in accident or incident analysis determines what we what look for, how we go about looking for “facts”, and what facts we see as relevant.
A linear chain of events promotes looking for something that broke or went wrong in the proximal sequence of events prior to the accident.
In accidents where nothing physically broke, then currently we look for operator error.
Unless we look further, we limit our learning and almost guarantee future accidents related to the same factors.
Goal is to learn how to improve the safety control structure
\Slide111
Evaluating CAST on Real Accidents
Used on many types of accidentsAviationTrainsChemical plants and off-shore oil drillingRoad Tunnels
Medical devices Etc.All CAST analyses so far have identified important causal factors omitted from official accident reportsSlide112
Evaluations (2)
Jon Hickey, US Coast Guard applied to aviation training accidents US Coast Guard currently uses HFACS (based on Swiss Cheese Model)Spate of recent accidents but couldn’t find any common factors
Using CAST, found common systemic factors not identified by HFACSUSCG now in process of adopting CASTDutch Safety Agency using it on a large variety of accidents (aircraft, railroads, traffic accidents, child abuse, medicine, airport runway incursions, etc.)Slide113
Outline
Accident Causation in Complex Systems: STAMPNew Analysis Methods
Hazard AnalysisAccident AnalysisSecurity AnalysisDoes it Work? EvaluationsSlide114
Strategy vs. Tactics
Primarily focus on tacticsCyber security often framed as battle between adversaries and defenders (tactics)Requires correctly identifying attackers motives, capabilities, targetingCan reframe problem in terms of strategy
Identify and control system vulnerabilities (vs. reacting to potential threats)Top-down vs. bottom-up tactics approachTactics tackled laterSlide115
Integrated Approach to Safety and Security:
Safety: prevent losses due to unintentional actions by
benevolent actorsSecurity: prevent losses due to intentional actions by malevolent actorsKey difference is intent
Common goal: loss preventionEnsure that critical functions and services provided by networks and services are maintainedNew paradigm for safety will work for security tooMay have to add new causes, but rest of process is the sameA top-down, system engineering approach to designing safety and security into systemsSlide116
STPA-Sec Allows us to Address Security “Left of Design” [Bill Young]
Concept
Requirements
Design
Build
Operate
System Engineering Phases
Cost of Fix
Low
High
Attack
Response
System
Security
Requirements
Secure
Systems
Engineering
Cyber
Security
“Bolt-on”
Secure
Systems
Thinking
Abstract Systems
Physical Systems
Build security into system like safetySlide117
Real World Evaluation of STPA-Sec to Date
Demonstrated ability to identify previously unknown vulnerabilities in a global DoD missionCreated model based on actual planning documents
Demonstrated ability to identify high-level vulnerabilities in early system concept documentsRequired security constraints missing Demonstrated ability to improve ability of network defenders to assure a real-world space surveillance missionReal mission, Real mission owner, Real networkDefenders able to more precisely identify what to defend & why (e.g. set of servers
integrity of a single file)Defenders able to provide traceability allowing non-cyber experts to better understand mission impact of cyber disruptions
117Slide118
It’s still hungry … and I’ve been stuffing worms into it all day.Slide119
Reason Swiss Cheese