The FAA is communicating a lot about the NextGen system these days. Their buzz has been relayed by Associated Press, end ended-up in many medias. You probably read that thanks to GPS based ADS-B, the new system will triple ATC capacity, amongst other benefits. Let me be clear: this is rubbish. There are many advantages in using that, but it will not solve the major problems that affect the aviation industry today.

HOW THE SATELLITE SYSTEM WOULD WORK: The NextGen system would draw on GPS satellite technology already used in cars. The GPS-equipped planes would receive and transmit signals. So pilots wouldn’t depend so heavily on controllers and would be able to view where other planes are by simply looking at a cockpit monitor. And satellites would cover all airspace, so planes wouldn’t have to zigzag to stay in range. They’d be able to fly straight, as well as land and take off closer together without risking safety.

Click here to read the wole description.

Land and take-off closer together without risking safety ? Runway capacity is the major cause for delays. A runway can accomodate a limited number of take-offs and landings (a.k.a. movements), and this limitation comes from two factors:

• The time needed to line-up and vacate runways – this can be improved by tangential taxiways, known as high-speed entry / exit taxiways
• Wake turbulence – vortexes generated by aircraft, which represent a fatal danger for other aircraft if they fly too closely

A separation of one minute is required between two movements because of wake turbulence. This separation must be increased to up to three minutes if a light aircraft follows an heavy one. As this problem find its roots in the physics of flight, knowing the position of planes per satellite or per radar will not change anything. Even a magic technology making possible to know the aircraft position by the millimeter, it will improve runway capacity. Improving the en-route part with more direct routes is fine, but this is not where the problem comes from.

Another factor limiting the ATC system capacity is weather. Even widebody airliners can not fly through thunderstorm. When large areas are affected by weather, all planes have to fly through the same holes. And if weather affects a major airport, with limited runway capacity, the situation gets even worse. Here again, better detection will not improve the situation. The present separation in crowded airspace (departure and arrival airspace, except take-off and landing) is set to 3 nautical miles horizontally, and 1000 feet vertically. Air traffic controllers have to make sure that these distances are always respected. The reason behind these distances has not a lot to do with surveillance technology, but more with reaction time. For en-route airspace, they are increased to 5 nautical miles. Let’s see what that means…

If for any reason two airliners flying at 400 knots each fly towards each other at the same altitude, their relative speed is 800 knots, which means the distance will reduce by 5 miles every 22.5 seconds ! This is an extreme example, but is shows how short is the reaction time can be. This usually never happens, because procedures are designed so that aircraft flying in opposite directions are not at the same altitude. Another typical case is when an quicker aircraft has to overtake a slower one. If the speed difference is 100 knots, a five miles distance will be covered in three minutes only. The present separations are designed to give air-traffic controllers and pilots sufficient time to react to an abnormal situation. Reducing the separation would anihilate these possibilities…

The number of aircraft that an air traffic controller is limited, and the number of air traffic controllers is limited. It’s quite frequent that crowded airspace (you know, around the crowded airports) can not be operated at their maximal capacity due to lack of controllers. Here again, improving the accuracy of the detection technology will not change anything. And one more thing… pilots already have cockpit screens displaying the position of others aircraft, and automated system generating alerts when a risk exists…

Using ADS-B will certainly straighten en-route segments. Getting coverage where there is nothing today is an improvement, but there are also procedures that perfectly accomodate non-radar environment, as long as the traffic level is low. The new system will help to prevent ruwnay incursions. The operating costs are also lower than those of radar, beause there is no rotating parts, nor complex mechanical systems. My point is not that the new system is wrong, but the way the FAA is “selling” it is simply wrong.

This post is the second one I wrote on this topic. Click here to read what I posted when the FAA initially announced the NextGen system.