aircraft joins the pattern from a very wide base leg. Altitudes An aerodrome publishes a "circuit height" or "pattern altitude", that is, a nominal level above the field at which pilots are required (recommended in the US, FAA AC90-66A Para. 8c) to fly while in the circuit. Unless otherwise specified, the standard recommended pattern height is 1000 ft AGL (above ground level), although a pattern height of 800 ft AGL is common. Helicopters usually fly the pattern at 500 ft AGL. Extreme caution must be exercised by pilots while flying at or through published traffic pattern altitudes as this might contribute to mid-air collisions. Helicopters Helicopter pilots also prefer to land facing the wind and are often asked to fly a pattern on arrival or departure. Many airfields operate a special pattern for helicopters to take account of their low airspeed. This is usually a mirror image of the fixed-wing pattern, and often at a slightly lower standard height above surface level; as noted above this altitude is usually 500 feet above ground level. However, due to helicopters' unique maneuverability, helicopter pilots often choose not to enter the pattern, and make a direct approach to the helipad or apron they wish to land on. Other patterns If an aircraft intending to land must be delayed, the air traffic control (ATC) may decide to place it in a holding pattern until the airport is prepared to permit the landing. Commercial aircraft on hold will generally fly slow, racetrack-shaped patterns which differ considerably from the airfield traffic pattern that will be commenced once the approval has been given to land. Although an aircraft in a holding pattern may similarly circle the airport, ATC may designate a distant location in which to circle. See also Holding (aeronautics) Teardrop turn Touch-and-go landing References Air traffic control Flight phases