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APPENDIX
D
Improving National Transportation
Statistics: Airline Safely as a Case Study
The annual National Transportation Statistics (NTS) report published by BTS
is a reference publication that compiles a large number of transportation data
series in a single, regularly updated volume. Each year's report contains a profile
of financial, operating, and safety characteristics of each transportation mode-
highway, rail, air, etc. Each year's compendium also has sections on such topics
as safety, with tables and graphs for all of the transportation modes. Most tables
and graphs provide time series of data in 5-year or 10-year intervals for the past 2
or 3 decades, with annual data for the most recent 3 to 4 years. (As in the Statis-
tical Abstract of the United States, which annually provides a large number of
tables on a broad range of subjects, there is no analytical commentary in the NTS
reports.)
Although bringing together a large amount of data in a convenient form, the
usefulness of the NTS reports as reference documents is affected by the scarcity
of explanatory notes, including those that would describe important changes in
definitions of variables across time (see Chapter 3~. Also lacking are explana-
tions that would help users understand the extent to which it is appropriate to
compare data series on particular topics across transportation modes. Finally, the
graphs and charts that are included are not always helpful or appropriate. (The
most recent 1997NTS report includes more tables than previous reports and elimi-
nates all charts and graphs.)
We reviewed the tables and graphs on airline safety from the 1996 NTS as a
case study to identify some of the problems with the NTS reports and ways in
which BTS could improve them, topic by topic, over the next few years. The
publication is valuable; our goal in the case study was to identify areas for im-
provement. The 1997 NTS which became available to the panel only after its
126
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IMPROVING NTS
127
work was finished reflects improvements that anticipate many of our comments;
further improvements can be made, particularly in providing more detailed ex-
planatory notes. (BTS is completing a review of the 1997 NTS to this end.)
Below we present an abbreviated version of our case study from the 1996 NTS.
REVIEW OF 1996 NTS AIRLINE SAFETY STATISTICS:
CONCLUSIONS
The commentary below addresses selected tables and graphs on airline safety
from the 1996 NTS, with suggestions for changes that could help the user make
appropriate comparisons over time and across transportation modes or categories
of a mode. For airlines, categories include major U.S. air carriers, commuter
carriers, on-demand air taxis, and general aviation. The commentary makes a
number of main points:
· There are no graphs of accident and fatality rates across airline categories
that provide data on a comparable basis.
· There are no tables or graphs that break down the components of underly-
ing trends (e.g., growth in aircraft passenger-miles as a function of the number of
flights, distance per flight, and number of passengers per flight) or that draw out
their implications for safety trends.
· Tables on the same topic do not always contain comparable data, and there
is inadequate warning to users when this occurs.
· Graphs are provided for raw counts (e.g., numbers of accidents or fatali-
ties, sometimes with different scales), when such numbers are likely misleading
in the absence of information about exposure (i.e., when the counts are not con-
verted to rates by the use of appropriate denominators a point that is made in the
BTS Transportation Statistics Annual Reports).
· Although sources are cited, there is no information provided about the
underlying data systems or the quality of the data.
REVIEW OF 1996 NTS AIRLINE SAFETY STATISTICS:
COMMENTARY
Air Carrier Profile
The profile section provides numbers of accidents, fatal accidents, and fatali-
ties for scheduled and nonscheduled airlines operating under 14 CFR 121 and for
scheduled commuter airlines and nonscheduled on-demand air taxis operating
under 14 CFR 35. (CFR, which stands for the Code of Federal Regulations, is
nowhere defined.) The profile also provides performance data on aircraft rev-
enue-miles, aircraft revenue-hours, revenue passenger-miles, and revenue pas-
senger emplacements, which could serve as denominators with which to compute
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28
APPENDIX D
accident and fatality rates. However, the performance data are provided for dif-
ferent categories of airlines than are the safety data (e.g., majors, nationals, large
regionals), and there is no explanation of how the categories in the performance
and safety portions of the profile relate, or if indeed they can be related.
Safety Section
Table 28 Fatalities, Injuries, and Accidents by Mode
The data in Table 28 for U.S. air carriers, commuter air carriers, and on-
demand air taxis match the data in the profile; however, the Table 28 definition of
air carriers is "large" carriers operating under 14 CFR 121, which implies some-
thing different from all carriers operating under that set of regulations. Citations
are provided in a separate section; no information is provided about any of the
major data sources or how they might compare across transportation modes.
Figure 9 Fatalities by Transportation Mode, 1960-1994
Figure 9 provides two bar graphs, each showing trends for 4 transportation
modes. Two modes are omitted entirely: motor vehicle traffic and rail-highway
grade crossings. The two graphs differ in scale on the vertical axis, which means
that the reader may incorrectly infer that waterborne transport in the 1970s (bot-
tom graph) was considerably more hazardous than, say, general aviation in the
same time period (top graph). The use of the same scale on the horizontal axis for
single years from 1990 to 1994 as for 5-year intervals from 1960 to 1990 in this
and other graphs may mislead the reader about time trends.
A more useful presentation could be to have a set of line graphs for all of the
modes with appropriate time intervals and a common vertical scale, with a break
in the scale at the top for motor vehicle traffic. However, there is a real question
as to the value of graphing the number of fatalities (or accidents) at all, given
differences in the exposure of the population to risk.
Table 30 and Figure 11 U.S. Air Carrier Accident and Fatal Accident
Rates per Million Aircraft Miles Flown
The data on millions of aircraft miles flown in Table 30 cannot be related to
the profile.
Figure 11 provides two graphs, one on trends in millions of aircraft-miles
flown and another on accident and fatal accident rates per million miles flown. It
could be useful to provide text explaining that changes in aircraft-miles flown are
a function of changes in the number of takeoffs (flights) and changes in the dis-
tance flown per flight. If data exist on these components, it could be useful to
show them together with the trends in total aircraft-miles flown. It could also be
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129
useful to show accident and fatal accident rates per 100,000 takeoffs, as in Table
36 for commuter air carriers, and to explain briefly when one denominator might
be more appropriate to use than another.
Table 31 and Figure 12 U.S. Air Carrier Passenger Fatality Rates per
100 Million Passenger-Miles
The data on fatalities in Table 31 are for scheduled service only and so do not
match the data in Table 28. The data on fatalities can be matched to the data in
the profile, but not so the data on revenue passenger-miles.
Figure 12 contains three graphs for trends in revenue passenger-miles, num-
ber of fatalities, and the passenger fatality rate. The usefulness of the graph on
number of fatalities, given that the vertical scale is so greatly different from that
of the other graphs and that there is no measure of risk exposure, is open to
question (the data are available in Table 31~. Text could usefully be added to
explain that trends in passenger-miles are a function of trends in three factors:
number of takeoffs (flights), distance per flight, and number of passengers per
flight. If data exist on these components, it could be useful to show them, as well
as to show fatality rates for other denominators (e.g., 100,000 passengers) and
briefly explain when one denominator might be more appropriate to use than
another.
Table 32 and Figure 13 U.S. Air Carrier Accidents and Serious Injuries
Figure 13 graphs numbers of accidents and serious injuries, which do not
appear to be useful to show in graphical form, given the absence of denominators.
Table 36 Commuter Air Carrier Accidents, Fatalities, Injuries,
and Accident Rates
Table 36 provides accident and fatal accident rates per million aircraft-miles
flown and per 100,000 departures. No rates are given for fatalities, and no graphs
are shown. It could be useful to show graphs that compare accident and fatality
rates for U.S. air carriers and commuter air carriers on a common basis, if this is
possible.
Table 37 On-Demand Air Taxi Accidents, Fatalities, Injuries,
and Accident Rates
Table 37 provides accident and fatal accident rates per 100,000 aircraft hours
flown. No rates are given for fatalities, and no graphs are shown. It would be
useful if accident and fatality rates could be compared for on-demand air taxis
and other aviation modes (e.g., commuter airlines) on a common basis. If no data
exist for this purpose, it would be useful to point out this fact.
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130
APPENDIX D
Table 38 and Figure 16 General Aviation Accidents, Fatalities, Serious
Injuries, and Fatal Accidents; Table 39 and Figure 17 General Aviation
Fatality and Accident Rates per 100,000 Aircraft-Hours
Figure 16 provides numbers but not rates; its usefulness is open to question.
Table 39 and Figure 17 provide rates for one denominator aircraft-hours flown.
Presumably other rates could be calculated on the basis of the information in the
general aviation profile, which provides information on vehicle-miles and pas-
senger-miles as well as aircraft-hours flown. However, the profile estimates of
hours flown do not always agree with the estimates in Table 39. If possible, it
would be useful to provide graphs that compare accident and fatality rates for
general aviation with the other aviation modes.
Representative terms from entire chapter:
air carriers
