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AIM

10/12/17

4

−3−7

Airport Operations

4

−3−4. Visual Indicators at Airports

Without an Operating Control Tower

a.

At those airports without an operating control

tower,

 a segmented circle visual indicator system, if

installed, is designed to provide traffic pattern
information.

REFERENCE

AIM, Paragraph 4

−1−9 , Traffic Advisory Practices at Airports Without

Operating Control Towers

b.

The segmented circle system consists of the

following components: 

1. The segmented circle.

Located in a position

affording maximum visibility to pilots in the air and
on the ground and providing a centralized location for
other elements of the system.

2. The wind direction indicator.

A wind cone,

wind sock, or wind tee installed near the operational
runway to indicate wind direction. The large end of
the wind cone/wind sock points into the wind as does
the large end (cross bar) of the wind tee. In lieu of a
tetrahedron and where a wind sock or wind cone is
collocated with a wind tee, the wind tee may be
manually aligned with the runway in use to indicate
landing direction. These signaling devices may be
located in the center of the segmented circle and may
be lighted for night use. Pilots are cautioned against
using a tetrahedron to indicate wind direction.

3. The landing direction indicator.

A tetrahe-

dron is installed when conditions at the airport
warrant its use. It may be used to indicate the direction
of landings and takeoffs. A tetrahedron may be
located at the center of a segmented circle and may be
lighted for night operations. The small end of the
tetrahedron points in the direction of landing. Pilots
are cautioned against using a tetrahedron for any
purpose other than as an indicator of landing
direction. Further, pilots should use extreme caution
when making runway selection by use of a
tetrahedron in very light or calm wind conditions as
the tetrahedron may not be aligned with the
designated calm

−wind runway. At airports with

control towers, the tetrahedron should only be
referenced when the control tower is not in operation.
Tower instructions supersede tetrahedron indica-
tions.

4. Landing strip indicators.

Installed in pairs

as shown in the segmented circle diagram and used to
show the alignment of landing strips.

5. Traffic pattern indicators.

Arranged in

pairs in conjunction with landing strip indicators and
used to indicate the direction of turns when there is a
variation from the normal left traffic pattern. (If there
is no segmented circle installed at the airport, traffic
pattern indicators may be installed on or near the end
of the runway.)

c.

Preparatory to landing at an airport without a

control tower, or when the control tower is not in
operation, pilots should concern themselves with the
indicator for the approach end of the runway to be
used. When approaching for landing, all turns must
be made to the left unless a traffic pattern indicator
indicates that turns should be made to the right. If the
pilot will mentally enlarge the indicator for the
runway to be used, the base and final approach legs
of the traffic pattern to be flown immediately become
apparent. Similar treatment of the indicator at the
departure end of the runway will clearly indicate the
direction of turn after takeoff.

d.

When two or more aircraft are approaching an

airport for the purpose of landing, the pilot of the
aircraft at the lower altitude has the right

−of−way

over the pilot of the aircraft at the higher altitude.
However, the pilot operating at the lower altitude
should not take advantage of another aircraft, which
is on final approach to land, by cutting in front of, or
overtaking that aircraft.

4

−3−5. Unexpected Maneuvers in the

Airport Traffic Pattern

There have been several incidents in the vicinity of
controlled airports that were caused primarily by
aircraft executing unexpected maneuvers. ATC
service is based upon observed or known traffic and
airport conditions. Controllers establish the sequence
of arriving and departing aircraft by requiring them to
adjust flight as necessary to achieve proper spacing.
These adjustments can only be based on observed
traffic, accurate pilot reports, and anticipated aircraft
maneuvers. Pilots are expected to cooperate so as to
preclude disrupting traffic flows or creating
conflicting patterns. The pilot

−in−command of an

aircraft is directly responsible for and is the final
authority as to the operation of the aircraft. On
occasion it may be necessary for pilots to maneuver
their aircraft to maintain spacing with the traffic they
have been sequenced to follow. The controller can
anticipate minor maneuvering such as shallow “S”
turns. The controller cannot, however, anticipate a

3/29/18

AIM