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Urgent call to end near-misses on runways
01 October 2005
NewScientist.com news service
Paul Marks

Only evasive action "bordering on the heroic" and a large slice of luck prevented a clutch of collisions between passenger jets on the runways of some of the US's busiest airports this year.

So says Mark Rosenker, the head of the US air accident investigation body, the National Transportation Safety Board (NTSB). Rosenker claims the technology used to prevent runway collisions at US airports is putting lives at risk, because it is designed in such a way that pilots receive crucial warnings far too late.

Speaking at a conference on airport operations in Dulles, Virginia, on 13 September, he delivered a devastating critique of the measures taken to prevent so-called "runway incursions" that could lead to a collision. These happen at airports when aircraft, or aircraft and ground vehicles breach safe separation distances.

The world's worst ever plane crash was the result of a runway incursion. In 1977, two Boeing 747s collided on the Spanish island of Tenerife, killing 583 people, after a misunderstanding between the crew of a KLM aircraft and air traffic control. The KLM jumbo mistakenly thought it had clearance to take off on a runway where a Pan Am jumbo was already taxiing in heavy fog.

A spate of fatal accidents caused by runway incursions in the US in the late 1980s prompted the NTSB to act. It pressed the US aviation regulator, the Federal Aviation Administration, to improve air traffic control oversight of ground operations, pilot training, airport signs, runway lighting and runway markings.

"The FAA completed action on a number of important objectives to make the ground operation of aircraft safer," concedes Rosenker. "However, these runway incursion incidents continue to occur with alarming frequency." In 2004, the FAA reported 326 runway incursions at US airports, compared with 323 in 2003. Every month, two of these incursions were serious enough to have potentially caused an accident.

Numbers are similar in Europe. In European Union member states there is one runway incursion a day, according to the air traffic control agency Eurocontrol. And here again, serious incursions happen twice a month.

The NTSB says the technology used to detect runway incursions and alert those involved needs to be significantly improved. Under fire is the Airport Movement Area Safety System (AMASS), which the FAA installed at 34 major US airports between 2001 and 2003.

AMASS uses an airport-wide ground radar system to detect the position of aircraft and ground vehicles. A computer compares the position, velocity and acceleration of all aircraft and vehicles and then works out when there is a potential conflict. When it predicts a breach of minimum separation between planes it automatically issues visual and audible alerts to air traffic control. Then it is up to the controller to warn the pilots that they need to take evasive action.

And therein lies the problem. The system relies on the controller getting the alert, deciding what to do and then warning the flight crew. But this takes crucial seconds that the flight crew may not have. The NTSB wants a system that provides a fast, direct warning to the pilots involved, not one mediated by a controller.

Rosenker offers a number of examples to back his case. In August 2004, for instance, an Asiana Airlines Boeing 747 and a Southwest Airlines Boeing 737 nearly collided on a runway at Los Angeles International Airport after air traffic control mistakenly told the 737 to taxi onto the runway in front of the landing 747. Luckily, the Asiana crew saw the 737 ahead of it and had time to abort the landing, flying very low over the 737 instead.

While AMASS did activate a warning in the tower about 10 seconds before the two aircraft would have collided, NTSB simulations show that it would have been too late to make any difference. "The 747 would likely have struck at least the vertical stabiliser of the 737," says Rosenker.

At Philadelphia International Airport last November, a small Mitsubishi MU-2 turboprop commuter plane collided with an aircraft tug on takeoff. The tug had been mistakenly cleared to cross the runway by a controller at the same time as the MU-2 was taking off. "The AMASS alerted but not in time to prevent the collision," Rosenker says. "The MU-2 pilot attempted to avoid the tug, but was unable to do so."

FAA spokesperson Lauren Peduzzi argues that AMASS has so far prevented four serious accidents in the US, and that serious incursions are down 40 per cent since 2001, when the technology was introduced. But the NTSB counters that it sees no signs of this reducing further.

While the US has AMASS, Europe has the Advanced Surface Movement Guidance and Control System (ASMGCS), a similar system that merges data from ground radar and aircraft transponders and passes on the information to controllers rather than flight crew.

Paul Wilson, head of airport traffic management at Eurocontrol in Brussels, also believes a fast, cockpit-based warning system is needed. He says it takes just three to four seconds for an aircraft parked safely on a taxiway to roll onto the runway. If another aircraft is about to land, four seconds would not give a controller much time to alert the planes. "He's got to stop what he's doing, assimilate that information and work out a plan of action. Then the pilot has got to react to what they are told. We need a ground-based collision avoidance system for the pilots."

So Eurocontrol is going to investigate how such a system could work, on behalf of the UN's global aviation regulator, the International Civil Aviation Organisation (ICAO). It will begin drafting its "Action Plan for the Prevention of Runway Incursions" in early 2006. The draft will be circulated around the world for comment by pilots groups, controllers, airlines, airport operators and regulators.

Simple measures such as very clear runway signs and markings will form part of the plan, says Williams. The FAA is already developing a technology called Runway Status Lighting. In RSL, a runway's state of readiness will be obvious from the colour of its lighting. A taxiway holding line lit up in red will mean "don't enter the runway", or red lights on the runway itself will mean "don't take off".

Tests in June at Dallas Fort Worth airport showed RSL could help pilots and tug drivers without increasing the air traffic controllers' workload, says the FAA. The system is automatic and receives data directly from AMASS.

The private sector is also developing technologies that will feed into the Eurocontrol study. Avionics company Honeywell makes the Enhanced Ground Protection and Warning System (EGPWS), which warns pilots when they are flying too low, or are about to hit a mountain. EGPWS has a detailed database of global terrain, including very fine detail for airports. So Honeywell has developed software to augment this, called the Runway Awareness and Alerting System, which gives taxiing pilots a raft of new detail about precisely where they are on the airport surface. Being in the wrong place plays a big factor in the errors that lead to incursions (see "Do you read me?").

There is no shortage of ideas, but action must be taken. Airlines cannot go on resolving near collisions through daring evasive action and pure luck, says Rosenker. "That is not good enough."

From issue 2519 of New Scientist magazine, 01 October 2005, page 22

Do you read me? On the face of it, it seems a pretty simple task for a flight crew to move a plane where an air traffic controller tells them to. But Paul Williams at Eurocontrol says breakdowns in communication cause half of all incidents. "Pilots will get an automatic warning that they are about to enter a runway, but they press ahead in the mistaken belief that they have been given clearance to be there." Alternatively, ambiguous signs or ground markings can leave pilots lost. "Research indicates that pilots in runway incursion incidents always believed they were in the correct place at the time," says Carolyn Evans of the British Airline Pilots Association. She says a number of factors can lead to confusion. Low visibility, as in the Tenerife crash, is a major one. "But another element of incursion risk is the phraseology pilots and controllers use on the radio. Even differences between English and American English can be a problem," she says. The UK's National Air Traffic Services agency in Swanwick, Hampshire, is to review global training procedures for cockpit-to-tower radio communications in a bid to ensure that pilots and controllers are on the same verbal wavelength.

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