AIM Text
- At those airports without an operating control tower, a segmented circle visual indicator system, if installed, is designed to provide traffic pattern information.
- The segmented circle system consists of the following components:
- 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.
- 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.
- The landing direction indicator. A tetrahedron 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 indications.
- Landing strip indicators. Installed in pairs as shown in the segmented circle diagram and used to show the alignment of landing strips.
- 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.)
- 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.
- 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.
Source: FAA Aeronautical Information Manual · current edition · paragraph 4-3-4.
The short answer
This AIM section is FAA advisory guidance describing the segmented circle visual indicator system at airports without an operating control tower. It explains the wind direction indicator, landing direction indicator (tetrahedron), landing strip indicators, and traffic pattern indicators that signal pattern direction, supporting safe non-towered operations.
Research Notes
Common Questions
Should I use a tetrahedron to determine wind direction?
No. The AIM cautions pilots against using a tetrahedron to indicate wind direction. The tetrahedron indicates landing direction—its small end points in the direction of landing. Use the wind cone, wind sock, or wind tee for wind; the large end of the wind cone/sock points into the wind, as does the cross bar of the wind tee.
How do I know whether to make left or right turns in the pattern?
Per the AIM, 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. Concern yourself with the indicator for the approach end of the runway to be used; mentally enlarging it makes the base and final approach legs apparent.
Who has right-of-way when two aircraft approach a non-towered airport to land?
The AIM states the aircraft at the lower altitude has the right-of-way over the aircraft at the higher altitude. However, the lower aircraft should not take advantage of another aircraft on final approach to land by cutting in front of, or overtaking, that aircraft.