Jets are most fuel efficient when they have freedom to move around in the sky. Climbing to a higher altitude allows a plane to maintain optimum cruise efficiency as it burns fuel, and when a plane changes altitude it is also able to catch winds that can speed it along. Planes flying over the ocean don't have this option. They're required to maintain a set speed and altitude so that they stay a safe distance from one another while out of range of ground-based radar.

But last month Airbus tested a GPS-based navigation procedure that lets pilots know what is happening in the skies around them without radar. If it goes into effect, it will cut fuel consumption by allowing planes to cruise at their optimal speed and altitude.

Planes flying over land are monitored and tracked by a dense network of ground-based radars. Air traffic controllers use these radars to guide aircraft through congested airspace and keep them a safe distance from one another. Radar's limited range means that aircraft can't be monitored this way over water, which is why the set speed and altitude tracks (known as in-trail procedures) are put into effect.

Airbus' new procedure, called CRISTAL ITP (ITP stands for in-trail procedures) is built on a technology known as Automatic Dependent Surveillance Broadcast (ADS-B). ADS-B does what radar can't – it gives pilots a clear view of what's happening around them during ocean flights. ADS-B-enabled cockpits are tricked out with GPS technology so that pilots can monitor not only their own position, but that of other aircraft in the area. Which means they can change altitude and speed safely, without radar guidance.

Airbus conducted a recent test over Icelandic airspace to see if the technology works. The test flight involved two planes, a Scandinavian Airlines Airbus A330 and an A340 owned by Airbus flying simultaneously through the same airspace. The A330 flew steady at 31,000 feet, while the ADS-B-equipped A340, able to constantly monitor the location of the Scandinavian plane, safely changed altitude and speed several times.

The potential fuel savings of CRISTAL ITP are bigger than you might think. Airbus estimates that a widebody jet could cut its fuel consumption by 374 pounds per transatlantic flight if it were able to shift altitude to reach maximum fuel efficiency. With 700 jets crisscrossing the Atlanticeach day, that means over 260,000 pounds of fuel per day. Do the math, and that means a CO2 cut of 558,000 pounds per day (160,000 pounds of fuel = 23,390 gallons. One gallon jet fuel produces 23.88 lbs CO2 according to Earthlab). Airbus plans to have the CRISTAL ITP procedure up and running over the Atlantic no later than 2010.

Airbus isn't the only one advocating the use of ADS-B technology. America's Air Transport Association can't stop talking about it, saying that replacing the United States' antiquated radar-based navigation with a GPS technology is the only way to prevent U.S. flight delays from getting far worse.

Photo: Swami Stream/Creative Commons 2.0