accident investigation and aircraft hazard areas in the post- columbia world
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Accident Investigation and
Aircraft Hazard Areas in the Post-Columbia World
Paul D. Wilde, Ph.D., P.E.FAA/AST-4
Columbia Accident Investigator
Introduction
• I was an investigator for the Columbia Accident Investigation Board (CAIB).– At the CAIB, I investigated the technical cause and
the public safety issues.– The implications listed are derived from my CAIB
and other experience.
• Some things have changed since the CAIB, but some thing have not.– Aircraft Hazard Area (AHA) implementation has
evolved substantially (Murray AIAA 2010-1349)– Aircraft and space safety and investigation
paradigms remain vastly different.
Overview of CAIB Findings and Implications for Space Safety
• Space launches are risky
• Past success does not provide future success
• Standards and formal structure can help
• Independent technical authorities are valuable
• Be prepared for accidents
• Understand anomalies
• Don’t short cut formal processes
• Safety vigilance is challenging
Finding Implication
Space Vehicles Are Dangerous
• “Building rockets is hard. Part of the problem is that space travel is in its infancy.” CAIB Vol. 1 page 19
• “Building and launching rockets is a very dangerous business and will continue to be so for the foreseeable future” CAIB Vol. 1 page 19
• Rockets fail catastrophically 10-100 thousand times more often than commercial transport aircraft (per flight).
• Accidents should be expected; prepare plans for emergency response
• Prepare investigation and RTF plans, including interface to media and other orgs
• No presumption of safety: accidents usually stop all flights until cause is found
Evidence Implication
Independent Technical Authorities Are Valuable
• A compliance verification organization independent of operational program cited as key to success for Navy subs and nuclear reactors, and in Air Force launch verification.
• “Organizations that deal with high risk operations must always have a healthy fear of failure - operations must be proved safe rather than the other way around.” CAIB Vol. 1 page 190
• Independent compliance verification enhances the safety of complex technical systems
• Checks and balances promote communication (in-flow of new info, addressing minority opinions)
• Safety takes real effort
Evidence Implication
Public Safety
• Columbia break-up during re-entry clearly could have caused public casualties
• Lack of public casualties due to Columbia break-up was the expected outcome given the sparse population– P>1 serious injury was <50% (~10-30%)– Same accident over a major city expected to
produce a few public casualties
• Hypersonic ops late at night lowers risk– Roofs protect effectively from most debris
• Relatively high probability of failure makes “safe” for public difficult to verify
Risk to Aircraft Flying Near Columbia Break-up
• At the time of Columbia break-up, FAA was unaware of any hazard to aircraft.– TFR issued ~ 45 minutes afterward based on
radar detection of debris, media rept., etc.
• Post CAIB analysis by FAA showed aircraft PI ~ 0.001 to 0.01
• Post CAIB simulation illustrates the issue– Actual aircraft flight locations/trajectories – Blue dots are recovered debris locations – Statistical distribution of debris during fall– The view is from the southeast
• Green lines show County boundaries
Safety of Aircraft Flying Near Space Launch or Re-entry
• To provide safety and efficiency in US NAS, both pre-defined and real-time AHA are used.– AHA for planned debris (jettisoned stages)
– Break-up generally spreads debris over a large area; aircraft PI often exceeds 1E-6
– During exo-atmospheric flight, several minutes between break-up and debris reaching aircraft altitudes.
– Vulnerability of aircraft to such debris strikes is highly uncertain and under investigation.
Sub-models for AHA Development
The last two (vulnerability and impact probability), plus the risk criteria for aircraft, have aspects that are necessarily unique to aircraft hazard
area analysis; all other sub-models are common with the debris risk analysis
Aircraft Grid & Trajectory Approaches to PI Estimate
• Grid approach– Assumes aircraft
continuously present in each grid cell
– Produces conservative results
• Specified trajectory
– Accounts for aircraft azimuth and limited dwell time in each cell
– More realistic PI is 2x to 7x lower
Airbus A300: Struck by a missile at 8,000 ft but landed safely 22 Nov 2003
Aircraft Vulnerability Modeling
See Wilde & Draper AIAA paper 2010-1542
Current Efforts Toward Higher Fidelity Aircraft Vulnerability Models (AVMs)
• FAA sponsored higher fidelity analysis using previously developed tools (e.g. military) and input data
• FAA impact testing to improve skin penetration eq., evaluate• Influence of obliquity, fragment density, distance from support,
etc.• Available results show
– Current penetration equation is conservative – 321-10 AVMs are excessively conservative, esp. for
“catastrophe”
V_Relative_Velocity_Fragment
V_Aircraft
V_Terminal_Velocity_Fragment
Elevation Angle
Public Safety Findings
• NASA should – Implement public risk acceptability policy
– Mitigate public risk from STS flight
– Study debris to improve risk estimates
• Collective public risk from space flight is small compared to civil aircraft operations.– Principle reason is huge number of aircraft
operations relative to launches.
• One in a million risk to individuals is a recognized benchmark for both and others
• Complete report at www.caib.us Vol.II D-16
Understand Anomalies
• O-ring blow-by and foam impacts were previously detected as anomalies
• The cause, effect, and limits of these anomalies were not understood
• “Engineers understood what was happening, but they never understood why.” CAIB Vol. 1 page 196
• Anomalies are often early warnings
• Successes do not prove problem solved or not dangerous
• Examine all data on anomalies separately and as a set
• Provide technical rigor in all requirements, rationales, validations
Evidence Implication
Formal Structure Can Help
• Formal documentation traces what was done to verify requirements were satisfied
• Formal structure can ensure that the burden of proof is on those saying it’s safe
• Formal structure identifies the responsible party
Implication Implication
• Formal standards help define what is an anomaly
• More uncertainty, justifies more attention and more caution
• Formal documents and peer reviews promote better decisions and help inform future generations
Informal Processes Are Not Effective
• Several informal attempts to obtain on-orbit imagery failed
• Lack of ground rules hampered engineering teams that evaluated the issues CAIB Vol. 1 page 200
• Management teams violated their own rules
• “When …analyses are condensed to fit on a…overhead slide, information is inevitably lost.” CAIB Vol. 1 page 191
• Clearly defined roles and rules improve effectiveness
• Design structure to promote communication
• Minority opinions should be addressed
• Communication needs to flow both up and down the chain of command
Evidence Implication
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