Standard terminal arrival route
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A STAR is an ATC coded IFR arrival route established for application to arriving IFR aircraft destined for certain airports. RNAV STAR/FMSP procedures for arrivals serve the same purpose but are only used by aircraft equipped with FMS or GPS. The purpose of both is to simplify clearance delivery procedures and facilitate transition between en route and instrument approach procedures.
A STAR is a flight route defined and published by the air navigation service provider and which usually covers the phase of a flight that lies between the last point of the route filled in the flight plan and the first point of the approach to the airport, which normally is the IAF (Initial Approach Fix). Hence, a STAR connects the en-route phase with the approach phase of the flight.
A typical STAR consists of a set of starting points, called transitions, and a description of routes (typically via VORs and intersections) from each of these transitions to a point near a destination airport, upon reaching which the aircraft can join an instrument approach (IAP) or be vectored for a final approach by terminal air traffic control. Not all airports have published STARs; however, most relatively large or not easily accessible (for example, in a mountainous area) airports do. Sometimes several airports in the same area share a single STAR; in such case, aircraft destined for any of the airports in such group follow the same arrival route up until reaching the final waypoint, after which they join approaches for their respective destination airports.
While the route segment of the filled flight plan doesn't usually change during the flight itself, the STAR to be flown might change depending on weather, runway or approach in use or air traffic congestion around the airport amongst others. This is the reason why the filled flight plan route doesn't end at the destination airport, but at a point at which a STAR begins, since it's usually assigned to the pilot during the en-route phase of the flight.
Naming conventions for STARs vary by country and region. In Europe, they are often named after the transition waypoint, followed by a digit that is incremented with each revision of the procedure, and a letter designating the runway for which the STAR is intended. In the United States, STARs are named after waypoints, or unique features of the STAR, or geographical features, followed by a digit indicating the STAR revision. A single STAR in the U.S. may serve for multiple runways and transitions; European STARs are more likely to be independently published for each runway and/or transition.
Not all STARs are for IFR flights. Occasionally STARs are published for visual approaches, in which case they specify visible landmarks on the ground and other visual reference points instead of waypoints or radio navigation aids.
STARs can be very detailed (as is often the case in Europe), allowing pilots to go from descent to approach entirely on their own once ATC has cleared them for the arrival, or they can be more general (as is often the case in the United States), providing guidance to the pilot which is then supplemented by instructions from ATC.