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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