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CHAPTER 2
Airfield Separation Rationale
A preliminary task in the study was the identification of 1944, the CAA published the first of four manuals titled Air-
approaches that could be used as a framework for the risk- port Design. This manual had standards for five classes of
assessment methodology. The first step was the gathering of airports, with the classes based on runway lengths required by
information on two basic rationales--that used by the FAA aircraft expected to use the facility.2
and that used by the International Civil Aviation Organization It is important to remember that these manuals were devel-
(ICAO)--to establish their airfield separation standards.1 The oped just prior to and during World War II. At that time, the
bases for the development of both rationales were the ran- United States had a massive war mobilization effort under-
dom deviations of aircraft during operations. Such deviations way, and it is unknown if the manuals were the output of any
are greater for runways and less for taxiways and taxilanes. In intensive research and development effort. In all likelihood,
addition, some incidents may lead to very large deviations the standards were based on the best engineering judgment
(e.g., runway excursions), and safety areas must be planned of the era.
to mitigate the risk of these large deviations. In January 1949, the CAA published the second manual titled
Airport Design, and it provided for eight different airport classes.
These classes were based on the type of service rather than the
FAA Rationale
expected type of aircraft.
Most of the FAA documents reviewed for this study pres- In relation to the historical context of this document, there
ent the separation standards and sometimes identify design are two points worthy of note:
considerations, but they rarely provide detailed information
on the design rationale. For this reason, two engineers were · The standards contained in the document represent the
interviewed. They worked in the FAA airport organization knowledge gained from aircraft operations during World
that was responsible for developing design standards, includ- War II.
ing the separation standards. Also, an attempt was made to · In 1946, the Federal Aid Airport Program (FAAP) was
place several of the documents in the context of the historical enacted to provide federal funds to airport sponsors for cap-
time when they were issued. ital development at their airports, and the program required
In 1940, the Civil Aeronautics Authority issued a document that such development be done in accordance with stan-
entitled Airport Design Information (1940). The manual was dards issued by the CAA.
"prepared for the instruction and guidance of Airport Section
Engineers in their field consultation activities" (Hathi Trust The importance of the FAAP cannot be overemphasized.
Digital Library). It provided standards for four airport classes This carrot and stick approach was successful in achieving
that were based on runway lengths required by aircraft expected uniformity of design within the various airport classes; how-
to use the facility. The Civil Aeronautics Administration (CAA) ever, there were so many classes that it was easy to incor-
published an updated version of this manual in 1941. In April rectly predict an airport's ultimate role in the national airport
system.
1 FAA and ICAO may use different terminology for their standards and recom-
mended practices. Throughout the text, the original terms for each agency were 2The information on these manuals is taken from a paper prepared by Robert
kept. For example, the FAA "runway safety area" function is equivalent to that David in 1973 (David, 1973). At that time, the documents were obtained from
of ICAO's "graded area of the runway strip" plus its "runway end safety area." the FAA library, but they are no longer available.
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Table 1. Minimum clearance standards of airports (ft)
(FAA, 1959).
Runway Centerline Taxiway
Type of Centerline of Parallel Centerline Centerline Taxiway
Service to Taxiway Runways of Parallel to Aircraft Centerline
Centerline for Contact Taxiways Parking to Obstacle
Operations Areas
Secondary 150 300 125 100 75
Local 250 500 200 175 100
Trunk 350 500 275 240 150
Continental 400 700 300 260 175
Intercontinental 450 700 325 280 200
In August 1959, the FAA issued Airport Engineering Data tions was advisory, the airport standards contained in them
Sheet Item 24 as a revision to the table that appeared in the became mandatory when federal aid was used for airport
1949 publication (see Table 1). development. As documents were updated to conform to the
This data sheet reduced the number of airport classes from new publication system, there was a tendency to develop Advi-
eight to five. The data sheet stated, "In order to assure maxi- sory Circulars containing design information for a specific type
mum safety and the economical and efficient use of the air- of airport. For example, AC 150/5300-1, VFR Airports, applied
port site, careful consideration must be given to the clearance to airports that were intended to have operations by general
and separation between the various aircraft operating areas" aviation aircraft during visual meteorological conditions (FAA,
(FAA, 1959, p. 1). It states that the distances established are 1963). AC 150/5300-4A, Utility Airports, was issued to provide
recommendations and further states "increases to these dis- guidance and standards for airports that intended to serve air-
tances may be desirable in some cases, necessary in others" craft weighing 12,500 lb or less (FAA, 1968).
(FAA, 1959, p. 1). It appears that although some classes were During the late 1960s and through the 1970s, there were
consolidated and renamed, only minor changes were made to several Advisory Circulars published on specific aspects of
the actual distances. No specific information is provided as to airport design for airports intended to serve air carriers. Sub-
how these separation standards were defined. jects of these Advisory Circulars included such things as run-
This document was issued 2 months before the "jet" age in way geometry, taxiways, surface gradient and line-of-sight,
U.S. commercial aviation began. In October 1959, nonstop and jet blast.
transatlantic flights with Boeing 707s were initiated between During this period, there were no funds allocated for the
New York's Idlewild Airport (now known as John F. Kennedy research and development of design standards. The runway/
International Airport) and Europe. runway and runway/taxiway separation standards contained
In 1961, the FAA published the third document titled Airport in these publications were based on the experience gained
Design (Federal Aviation Agency, 1961). The revised standards during the post-World War II period, including experience
(see Table 2) were no longer based upon the type of service, but
with the precision of navigational aids such as instrument
rather on the runway length.
landing systems (ILSs), the ability of pilots to stay on center-
In the early 1960s, the FAA initiated the Advisory Circular
line, and air traffic control considerations. In the 1960s, the
publication series. Although the information in these publica-
FAA's Flight Standards organization and the ICAO Obstacle
Clearance Panel (OCP) developed the Collision Risk Model
Table 2. Airport design and clearance (CRM)3 for ILS operations. The CRM was based on actual
recommendations (ft) (Federal Aviation observation of 2,500 aircraft on an ILS precision approach to
Agency, 1961). a runway. Four observations were made for each aircraft's
approach. This model was used to define the area that needed
Runway Taxiway
Centerline Centerline Centerline Taxiway to be protected on an airport when an aircraft was making an
Runway
Length
to of Parallel to Aircraft Centerline ILS approach. The runway/taxiway separation also took into
Taxiway Taxiways Parking to Obstacle account the possibility of an aircraft on landing rollout or
Centerline Areas
1,6003,200 150 100 100 75
takeoff roll veering off the runway. Additional information
3,2014,200 250 200 175 100 on the CRM is provided in Appendix B.
4,2016,000 400 300 250 200
6,0017,500 400 300 250 200 3
FAA developed the CRM (ICAO, 1980) approach with the University of
7,50110,500 400 300 250 200
Oklahoma and input from other countries represented on ICAO's OCP.
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The taxiway/taxiway separations were based on taking the Appendix 9 of AC 150/5300-13 provides the design rationale
most critical aircraft that would be using the taxiways (gener- for separations associated with taxiways and taxilanes, except
ally, the aircraft with the largest wingspan) and placing its for those between a runway and its parallel taxiway (FAA, 1989).
main gear on the edge of the usable taxiway. The separation A number of parameters are contained in this appendix. To
between the taxiway centerlines then could be calculated by maintain airport operational capacity, the taxiway system
adding half the width of each taxiway to twice the length of should be designed so that aircraft can maintain an average
the wingspan that extended beyond the taxiway plus a safety speed of 20 mph. The parameters affecting taxiway separations
factor. Likewise, the taxiway/object separations were based on for other than parallel taxiways are wingspan and wingtip
taking the most critical aircraft that would be using the taxi- clearance, with the need for wingtip clearance being driven
ways and placing its main gear on the edge of the usable taxi- by the fact that pilots of most modern jets cannot see their
way. The separation between the taxiway centerline and the wingtips from the cockpit. Appendix 9 then provides the
object could then be calculated by adding half the width of the following information on separations:
taxiway to the length of the wingspan that extended beyond
the taxiway plus a safety factor. · Taxiway to taxiway centerline (see Figure 1): Separation is
In the 1980s, in response to feedback from the aviation com- calculated based on 1.2 times the wingspan of the most
munity, the FAA undertook an effort to consolidate the numer- demanding aircraft plus 10 ft (wingtip clearance).
ous design Advisory Circulars. In 1983, AC 150/5300-12, Air-
port Design Standards--Transport Airports, was published STWY-TWY = 1.2 WS + 10 ft
(FAA, 1983). This consolidated many of the design standards
where
for transport aircraft into one document. In 1989, the FAA pub-
lished AC 150/5300-13, Airport Design (the fourth document STWY-TWY is taxiway to taxiway centerline separation and
with this title), which consolidated the design standards for all WS is wingspan of the most demanding aircraft.
airports except heliports and sea plane bases into one document
· Taxiway centerline to object (see Figure 2): Separation is
(FAA, 1989). This publication grouped standards according to
the ARC, consisting of a letter and a Roman numeral. The let- calculated based on 0.7 times the wingspan of the most
ter indicates the aircraft approach category and relates to the demanding airplane plus 10 ft (wingtip clearance).
FAA Flight Standards approach speed group of the design air- STWY-OBJ = 0.7 WS + 10 ft
craft (as used in terminal instrument procedures [TERP]). Gen-
erally, runway standards are related to the approach speed. The where
Roman numeral relates to the airport design group and the air-
STWY-OBJ is taxiway centerline to object separation.
craft wingspan of the design aircraft. It is possible to have the
approach category based on one design aircraft and the aircraft · Taxiway object free area (OFA): Width is equal to twice the
design group based on a different design aircraft. taxiway centerline to object separation.
Figure 1. Wingtip clearance, parallel taxiways.
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Figure 2. Wingtip clearance from taxiway.
TWY-OFA = 2 STO TXL-OFA = 2 STXL-OBJ ( single lane )
where TXL-OFA = 2.3 WS + 30 ft ( dual lane )
TWY-OFA is taxiway object free area width and STO is Appendix 9 of AC 150/5300-13 does not indicate how the
taxiway centerline to object separation. safety factors or the wingtip clearances were determined (FAA,
· Taxilane centerline to object (see Figure 3): Separation is 1989). Interviews indicated that these factors were based on
calculated based on 0.6 times the wingspan of the most engineering judgment.
demanding airplane plus 10 ft (wingtip clearance). Reduced As far back as the 1970s, and probably even before then,
clearances are acceptable because taxi speed is very slow there were differences in airport design standards and the cri-
outside the movement area, taxiing is precise, and special teria used to establish instrument approaches. These differ-
operator guidance techniques and devices normally are ences primarily affected the standards associated with runway
present. approaches and separations. Several attempts were made over
the years to resolve these differences. This issue became crit-
STXL-OBJ = 0.6 WS + 10 ical in the early 2000s, when an analysis performed using the
· Taxilane OFA (see Figure 4): Width is equal to twice the CRM indicated that aircraft located on a parallel taxiway 400 ft
taxilane centerline to object separation for a single lane from the runway centerline posed a safety risk to aircraft on
width and 2.3 times the wingspan of the most demanding an instrument approach to that runway when those aircraft
airplane plus 30 ft for a dual lane. executed a missed approach.
Figure 3. Wingtip clearance for taxilanes.
