iFly Blog

 

If you’ve spent much time as a commercial airline passenger, you’ve no doubt noticed the large number of airplanes that operate into and out of each hub airport. Perhaps you’ve wondered just how all those airliners avoid each other, as well as all other forms of air traffic, when airborne. With this post, we’ll explore some of the procedural safeguards in place that help keep aircraft a safe distance apart.

SIDs and STARs

In busy terminal airspace areas, air traffic controllers (ATC) utilize standard instrument departures (SIDs, also known as departure procedures {DPs}) and standard terminal arrival routes (STARs) to streamline the flow of departing and arriving traffic, respectively. Unlike ground-based vehicles, which are largely limited to following roads, aircraft can arrive at a given point from any of the 360̊ around the location. Such chaos would certainly compromise safety and make for an ATC nightmare. With SIDs and STARS, controllers are able to funnel traffic flow in a logical and safety-enhancing manner.

Virtually all hub airports have several SIDs and STARs available to arriving and departing traffic. SIDs/STARs are published in textual (and often graphical too) form and instruct pilots of the headings, courses, & altitudes to fly when operating to/from each hub airport. Depending on the general direction the aircraft is departing to/arriving from, ATC will give that plane a SID/STAR to/from that direction. These procedures also contain transition routes, which allow aircraft to transition over a wider directional range when a safe distance from the airport (and the most congested airspace).

As SIDs/STARs are published, ATC need only inform pilots to “fly               departure/arrival,                     transition.” Pilots then know exactly which headings, courses, & altitudes to fly along their route. By following these routes, aircraft “get in line” behind other traffic, allowing an orderly flow in the most crowded areas.

Preferred IFR Routes

Preferred IFR Routes (IFR meaning instrument flight rules, under which ALL commercial airline flights operate) are very similar to SIDs and STARs. In fact, Preferred IFR Routes are essentially a SID, a STAR, and the cruise portion of a flight all rolled into one. These procedures are common when the departure and arrival airports are located relatively close to one another, as well as for air traffic that transits congested airspace. These routes are, as the name implies, preferred because they streamline traffic and permit an orderly flow of aircraft within that airspace. If you’ve ever flown in the New England region, you’ve almost certainly flown on a Preferred IFR Route.

IFR/VFR Cruising Altitudes

For the cruise stage of flight, when not otherwise directed by ATC, aircraft utilize VFR & IFR cruising altitudes. VFR stands for visual flight rules, and refers to traffic that navigates primarily by visual reference (mainly personal, general aviation aircraft). Cruising altitudes are determined by the magnetic course each aircraft is flying, as well as whether it’s operating under IFR or VFR. For eastbound traffic (0̊ through 179̊) IFR aircraft operate at odd, thousand foot intervals (7000, 9000, etc.) and VFR traffic fly at odd thousand foot intervals + 500 feet (7500, 9500, etc.). For westbound aircraft (180̊ through 359̊), even numbered altitudes are flown (6000, 8000, etc for IFR; and 6500, 8500, etc. for VFR).  This method insures all aircraft will be vertically separated by at least 500 feet while in cruise.

As you can see, the national airspace system has several procedural safeguards in place to keep air traffic safely separated from other aircraft. Bear in mind that these are just the basic, operational methods pilots and ATC use for traffic separation. In a future post, we’ll discuss additional safeguards built into the airspace system. These include ATC separation standards, airspace classes and the associated entry requirements, and the multitude of electronic equipment aircraft use to detect and avoid each other. Until then, breathe easy and know that whenever you fly, numerous safety procedures are working to keep your plane a safe distance from other aircraft.

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