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separation would have resulted in an average Inter-Arrival Time (IAT) separation of 72
seconds, thus allowing ample time for an aircraft landing rollout before processing a new
arrival. Interestingly enough, a 72-second IAT is still typical today under VMC conditions
at large U.S. airport hubs.
By 1949 or 1950, a three-mile horizontal separation “was arrived at in coordination with
interested representatives of all users of the Washington National Airport. Its choice was
influenced both by considerations of equipment data acquisition, presentation, and
interpretation, and by stress and strain of flying the system and of controlling the system”
[R2.6]. There was precedent in the military. In the mid-1940s “the air forces [had]
established a three-mile radar separation for controlled aircraft based mainly on the idea of
keeping blips on the display from merging. It was a function of the radar beam width and
equipment resolution at 30 miles ... ” [R2.7]. Ultimately, however, “it appeared to be
based on a subjective interpretation of the perceived limitations of equipment rather than
on an actual need for physical separation of that magnitude. The final figure was a
compromise, the controllers suggesting a smaller figure and the pilots a larger one”
[R2.8]. (The wake vortex problem was neither well-known, nor well-understood at the
time.)
In 1953, the provision for five-mile separation for controlled aircraft that are more than 40
miles from the radar site appeared in the Third Edition of Radar Procedures for Airport
Traffic Control Towers. As an aside of some interest, a quote from the United States
Manual of Radar Air Traffic Control Procedures [R2.9] five years later - when the CAA
had just become the Federal Aviation Agency - reveals something of the status of radar
ESTABLISHING SEPARATION STANDARDS
2-3
separation in its early days: “The number of aircraft which will be separated by radar
should be kept to a minimum. By so doing, the controller workload is reduced ... .
Standard nonradar separation shall be provided to any aircraft whenever requested by the
pilot” [R2.9, Section 3.1].
The choice of five miles was almost certainly influenced by the fact that radar target arcs
of more distant targets appear wider on the radar screen, and this is almost certainly why a
larger separation minimum was deemed necessary for targets sufficiently far from the
radar site. However, to many observers there does not appear to have been a precise
rationale, at the time, for the three- and five-mile figures.
In fact, an unpublished, unofficial FAA staff study from the early 1970s flatly asserts, “no
rationale exists for the broadband radar minima” [R2.10]. That study goes on to develop
a rationale pertaining to a “theoretical worst case situation”—one in which radar accuracy
with respect to position, and registration with respect to another radar (mosaicing), were
at their worst expected levels. A 95-percent probability envelope was computed for each
aircraft, and one nautical mile (nmi) was added as “a type of safety valve.” (Note that if
the positioning errors on the two aircraft are independent, the combined probability
envelope is 99.75 percent.)
It should be noted that Terminal Area Instrument Procedures (TERPS) have, for many
years, been based on 99.7 percent for aircraft-to-terrain separation vs. the 95 percent for
aircraft-to-aircraft separation. Protection against aircraft contact with obstacles and
obstructions near airports are dealt with by a series of design standards laid out in FAA
Federal Aviation Regulations Part 77, “Objects Affecting Navigable Airspace,” which
many years of experience have shown to be successful and safe.
The introduction of secondary surveillance radar with its transponder-provided beacon
identifiers and altitudes has not, to date, resulted in a reduction of horizontal or vertical
separation standards in domestic airspace. The RTCA SC-150 on Reduced Vertical
Separation studied the possibility of reducing vertical separation to 1,000 ft. above flight
level 290 (FL 290), but concerns about the altimetry accuracy and altitude-maintaining
ability of systems in some airplanes, and other issues resulted in no recommendation from
the committee.
2.1.2 Target Levels of Safety
One approach to establishing separation standards involves establishment of a “target level
of safety,” based on rational, numerical analysis. Although total safety is a goal, economic
and physical realities require one to accept the possibility of an accident, albeit with a very
small probability of occurrence. The approach was first invoked by the British Air
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a concept paper for separation safety modeling(7)
