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Q: I’ve heard for decades that if we had a modernized air-traffic control system, planes wouldn’t have to fly in “highways in the sky” and could approach airports more efficiently for landing. Is the 3-degree descent profile you’ve talked about in previous columns a byproduct of our current air-traffic system, and if so, would that change if we were able to have more direct flight paths?           

— TC, Washington

A: An ideal air-traffic system would allow every airplane to fly the most efficient route to its destination. We have not developed a traffic separation system that is capable of ensuring separation when so many airplanes would fly so many “random” routes.

In the past, navigating required using ground stations, resulting in “highways” in the sky known as airways. As independent navigation became possible with GPS, the need for airways decreased. However, as the number of airplanes increased, the need to have the traffic flow in a controlled way increased. As a result, airplanes can navigate to any point in space, but we have to control the flow into the airport area.

The 3-degree descent is partially aerodynamic (many jets descend at approximately 3 degrees when at idle power) and partially standardizing the descent path.

As the airplane approaches the runway, it is very important that the approach is stable. Being on the right vertical path, at the right airspeed, in the landing configuration (landing gear down with flaps/slats set for landing) and with the engines powered up are the elements of a stable approach. If any of these conditions are not met by 1,000 feet above the ground, a go-around is initiated (a few operators make this decision at 500 feet).

Q: What is the status of “Free Flight”? Did the events of 9/11 do away with the possibility of pilots not being forced to follow the same flight plans from point A to point B?    

— Matt G., New York

A: Free Flight is not as active as it once was; however, the increasing use of non-ground-based navigation (e.g. GPS, IRS or RNAV) and the proliferation of waypoints have increased the ability to get off the airways and go directly to a waypoint. This has made the concept of free flight a lower priority.

Though it is possible that we may see true free flight one day, I think it is a long way in the future. More likely is the ability to fly from waypoint to waypoint, achieving nearly the same result.

Q: Why are contrails often seen in a cross-hatch or checkerboard configuration?

— Jim Hall, Vancouver, British Columbia

A: The airways often intersect over navigation stations or waypoints. This results in…