Air Safety: End of the Golden Age? Arnold Barnett Blackett Memorial Lecture.
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Transcript of Air Safety: End of the Golden Age? Arnold Barnett Blackett Memorial Lecture.
![Page 1: Air Safety: End of the Golden Age? Arnold Barnett Blackett Memorial Lecture.](https://reader030.fdocuments.in/reader030/viewer/2022032611/56649ed55503460f94be5658/html5/thumbnails/1.jpg)
Air Safety: End of the Golden Age?
Arnold Barnett
Blackett Memorial Lecture
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How should we measure aviation safety?
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“NTSB studies show that, from 1993 through 1996, scheduled US carriers averaged only 0.2 fatal accidents per 100,000 flight hours, less than half the fatal accidents rate for the four-year period a decade earlier.
--Wall Street Journal, 8/11/97
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Two problems with the statistic fatal accidents per 100,000 flight hours:
The numerator and
the denominator!
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• The generic term “fatal accident” blurs the distinction between a crash that kills one passenger out of 300 and another that kills 300 out of 300.
• Measuring activity by “flying hours” misses the point that most accidents occur on landing or takeoff.
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Airline Report Cards(Air Travelers Association)
Score = 100 - 10,000 (Z/N)
where Z = number of fatal crashes over 1987-96
N = number of flights over 1987-96 (1000’s)
Letter Grades: A: 90 -100 B: 80-89.9
C: 70-79.9 D: 60-69.9 F: below 60
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This grading system:
• Again uses the generic term “fatal accident”
• Is quite arbitrary, and sometimes assigns vastly different grades because of statistically meaningless differences
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Example:• Airline #1
200,000 flights over 1987-96No fatal accidents
Letter Grade: A
• Airline #2200,000 flights over 1987-96
One fatal accident, which kills one passenger out of 100
Letter Grade: F
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What about hull losses per 100,000 departures?
(This is a popular one.)
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Consider two hull losses this year:• Southwest Airlines, Boeing 737, Burbank, CA
Passengers on board: 137 Passengers killed: 0
• Alaska Airlines, MD-80, off Los Angeles
Passengers on Board: 83 Passengers Killed: 83
No difference?
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Measure of Safety Performance Over a Past Period:
Death Risk Per Randomly Chosen Flight
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Question: If a person chooses a flight at
random from among those of interest (e.g. UK domestic jet flights over the period 1990-95), what is the probability that he will not survive it?
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This death risk per flight statistic has conceptual advantages compared to the other statistics just discussed.
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What Conceptual Advantages?
• Ignores length and duration of flight, which are virtually unrelated to mortality risk
• Weights each crash by the percentage of passengers killed
• Easy to calculate and understand
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First-World Domestic Jet Services
Death Risk per Flight, 1990-99:
1 in 13 million
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At a mortality risk of 1 in 13 million per flight, a passenger who took one flight per day would on average travel for 36,000 years before dying in a plane crash.
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Passenger Mortality Risk for Various World-wide Jet Services, 1990-99
Type of Service Death Risk per Flight
First-World Domestic 1 in 13 million
International withinFirst World 1 in 6 million
International Between FirstAnd Developing Worlds 1 in 1 million
Within Developing World 1 in 500,000
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Passenger Mortality Risk Arising from Criminal/Terrorist Acts, Scheduled First-World Jet Services Over 1990-99
Type of Service Death Risk per Flight
US:Domestic 0International 0
First World Outside US:Domestic 0International 1 in 2 billion
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This record is all the more remarkable because of several successful acts of sabotage in the late 1980’s.
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Two Possible Reasons for the Quiet Decade:
• The desire to do harm to First-World air travelers genuinely diminished.
• Improved security measures deterred some potential attacks and foiled others.
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Unfortunately, neither of these explanations is especially convincing.
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October 2000:
“Fear of terrorist attacks after the explosion of violence in the Middle East hammered global airline shares Friday.”
--Reuters
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Passenger Mortality Risk Arising from Runway Collisions, Scheduled First-World Jet Services Over 1990-99
Type of Service Death Risk per Flight
US:Domestic 1 in 100 millionInternational 0
First World Outside US:Domestic 0International 0
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We were asked to investigate the following question:
How great a threat do US runway accidents pose to domestic airport operations in the next two decades?
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If there are N aircraft operations at a given airport in a given year, then:
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To a first approximation, one might expect that the risk of a runway accident would vary with N2.
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Why?
1) The number of flights that could (theoretically) collide is (N2-N)/2.
2) The Quadratic Model is conceptually attractive.
3)N2 is widely used in airspace collision-risk models.
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But, to the extent possible, it is desirable to go beyond merely stating conjectures, and to test hypotheses and “approximations” against empirical evidence.
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A most interesting data set
The 40 US runway incursions in 1997 that:
(1) were judged by experts to have “extremely high” accident potential
and(2) took place under known conditions of
reduced visibility (night, sunrise/sunset).
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The N2-hypothesis passed a Chi-squared statistical test with flying colors.
(The test was based on the spread of the 40 dangerous events across US airports.)
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Intriguingly, the hypotheses that dangerous events varied across airports with either N or N3 did not pass Chi-squared tests.
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Overall, runway collisions over the next two decades could take 600 lives among US jet passengers, and cause 200 serious injuries.
(Mid-range projection)
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Estimated Runway Collision Death Risk per Flight, US Domestic Jets 2003-22:
1 in 25 million
(Four times the actual risk in the 1990’s.)
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Passenger Mortality Risk Arising from Mid-Air Collisions, Scheduled First-World Jet Services Over 1990-99
Type of Service Death Risk per Flight
US:
Domestic 0International 0
First World Outside US:
Domestic 0International 0
(Based on 100 million flights)
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Is it safe to adopt free-flight?
(Operations Research, Nov 2000)
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A B
C
DE
F
Present Routings: A-E-F-B and C-E-F-DFree Flight Routings: A-B and C-D
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Under certain assumptions, free-flight would:
• Reduce the likelihood of path intersections
• Tend to reduce the crossing angles of paths that intersect
Why is the latter point important?
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Because:
At present, emergency warnings go off in air-traffic control towers when two planes come within five miles of one another, regardless of the angle at which they are converging.
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Consider two planes on a collision course that have just come within five miles of one
another.
For example:5
C
3.5
3.5
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“Resolution Time” As a Function of Angle of Convergence
Convergence Angle Time to Resolve Emergency
180º 18 seconds 90° 25 10 30º 70 5 30°
(Assume both planes going 500 mph)
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Because of fewer path crossings and longer times to resolve emergencies, the geometric consequences of free-flight might act to reduce mid-air collision risk.
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However:
All these apparent benefits of free-flight could be more than outweighed by a decline in “situational awareness” on the part of air traffic controllers.
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How Does It All Add Up?
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Aviation Safety: Time To Stop Worrying?
Arnold Barnett