Passenger Mortality Risk for Various Worldwide Jet Services, 199099

1
Air Safety End of the Golden Age?



Arnold
Barnett Blackett Memorial Lecture
2
How should we measure aviation safety?
3
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.

6
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 (1000s) 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 1987-96
• No fatal accidents
• Letter Grade A
• Airline 2
• 200,000 flights over 1987-96
• One fatal accident, which kills one passenger out
  of 100
• Letter Grade F

9
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 within
• First World 1 in 6 million
• International Between First
• And 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 0
• International 0
• First World Outside US
• Domestic 0
• International 1 in 2 billion

19
This record is all the more remarkable because of
several successful acts of sabotage in the late
1980s.
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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.

21
Unfortunately, neither of these explanations
is especially convincing.
22
October 2000Fear 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
million International 0 First World Outside
US Domestic 0 International 0
24
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.
29
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).

30
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.)

31
Intriguingly, the hypotheses that dangerous
events varied across airports with either N or N3
did not pass Chi-squared tests.
32
Overall, runway collisions over the next two
decades could take 600 lives among US jet
passengers, and cause 200 serious injuries.
(Mid-range projection)
33
Estimated Runway Collision Death Risk per Flight,
US Domestic Jets 2003-22

• 1 in 25 million
• (Four times the actual risk in the 1990s.)

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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 0 International 0 Fi
rst World Outside US Domestic 0 International
0 (Based on 100 million flights)
35
Is it safe to adopt free-flight?(Operatio
ns Research, Nov 2000)
36

B
A
F
D
E
C
Present Routings A-E-F-B and C-E-F-D Free 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.

39
Consider two planes on a collision course that
have just come within five miles of one another.

• For example

5
3.5
C
3.5
40
Resolution Time As a Function of Angle of
ConvergenceConvergence Angle Time to
Resolve Emergency 180º
18 seconds 90
25 10 30º
70 5
30







(Assume both planes going 500 mph)
41
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.
42
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