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2.0 ESTABLISHING SEPARATION STANDARDS
2.1 SOME HISTORY OF THE DEVELOPMENT OF SEPARATION
STANDARDS
There appears to be no official history of the development of separation standards.
However some authors, using source materials and institutional memory, provide us with
some insight into the past. Section 2.1.1 is excerpted essentially verbatim from References
R2.1, R2.2. Another noteworthy source is Reference R2.3.
2.1.1 Early Separation Standards
The early efforts to urge government to enter air safety regulation, of which separation
standards are an important part, were not initiated by the government. Aviation historian
Nick Komons [R2.4] wrote in 1978, “An increasing number of people were coming to the
conclusion in the early 1920s that aviation could not develop into a viable transportation
mode without Federal Safety Regulation. These were not concerned citizens seeking
safety for safety’s sake. These were people within the aviation community - aircraft
manufacturers and operators, and others, who, in one way or another, depended, at least
in part, on aviation for a livelihood.”
Agitation for Federal air safety legislation began more than six years before Calvin
Coolidge signed the Air Commerce Act into law in 1926. In 1921, Herbert Hoover wrote,
“It is interesting to note that this is the only industry that favors having itself regulated by
the government.” A Congressional report at the time noted, “Congress has been
denounced unsparingly for passing legislation regulating and controlling business...it is
rather startling, to say the least to have an industry...asking and urging legislation putting
this business completely under Federal control.”
Before radar of any kind was used for air traffic control, separation depended on deadreckoning
and pilot reports. The controller, using flight strips to “see” his targets,
separated aircraft by feeding them into certain routes with time separation, knowing that
known aircraft speeds over the route distance would keep them apart. Pilot reports by
radio, when available, were used to update positions. This was called procedural control.
Of necessity, separation distances were quite large since little was known about winds
aloft and the exact positions, speeds, and directions of the aircraft. Lateral separations on
preestablished routes that might intersect were primarily achieved through altitude
separation or by longitudinal procedural separation. Visual Flight Rules (VFR) or Visual
Meteorological Conditions (VMC) rules, in which the pilot was responsible for
maintaining separation by visual contact with other traffic, were widely used.
The earliest standards for air traffic control separation between aircraft, usually for
longitudinal spacing, were entirely based on time separation, using best estimates of the
aircraft capabilities and environmental vagaries. Early uses of navigation aids, starting
with light beacons and later radio beacons, still used time as the basic separation tool.
SEPARATION SAFETY MODELING
2-2
With the introduction of more sophisticated navigation aids, particularly the VHF
Omnidirectional Range (VOR), computations of probable displacement from desired paths
were introduced. The FAA, in its air route and separation computations, based them on
(and sometimes still uses) a concept of “system use error.” It created airway (route) width
designations, on a “95 percent containment probability” basis, based on a root-sum-square
(RSS) combination of ground station error (at the greatest usable distance from the
facility), airborne navigation system and display error, and a pilotage factor. It assumed
that the error distributions were normal.
“The first radar-equipped control tower for civilian flying was unveiled at the Indianapolis
Airport” in 1946 by the Civil Aeronautics Administration (CAA), a forerunner of the
modern Federal Aviation Administration (FAA) [R2.5]. Shortly thereafter, the Radar
Procedures Manual specified three-mile lateral separation (paragraph 3.611): “the aircraft
may be turned toward the desired course by the radar controller and given headings which
will keep it at least three miles laterally from all holding aircraft until past the pattern”
[R2.6]. At the same time, a two-mile longitudinal separation for aircraft on final approach
was allowed (paragraph 3.516). This value may have been initially based, not so much on
radar accuracy (which was considered very good), but on the desire to establish a twomile
separation between arrivals to balance runway occupancy times and interarrival
separations. Based on the approach speeds of the DC-3s, then in wide use, a two-mile
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a concept paper for separation safety modeling(6)
