As global air traffic expands at record rates, experts warn that near misses on the ground at overcrowded airports are becoming one of the most serious safety concerns in civil aviation.
The danger arises when airports try to alleviate bottlenecks by adding runways. That leads to more taxiways intersecting the runways, raising the risk of accidental incursions — where an aircraft or vehicle becomes a collision hazard by venturing onto a runway being used for takeoffs and landings.
"Runway incursions are right at the top of our agenda," said Gideon Ewers, spokesman for the International Federation of Airline Pilots' Associations.
"They are happening more and more frequently as air traffic increases and older airport designs struggle to cope. Of course most incursions pass without incident, but when they do occur the results are very bad indeed," Ewers said.
The deadliest disaster in aviation history occurred 30 years ago as a result of such an encroachment. The ground collision in 1977 between two fully loaded Boeing 747s in Spain's Canary Islands killed 583 people.
Since then, numerous such accidents have ended in tragedy and experts are now racing to develop systems to prevent even deadlier disasters.
According to Eurocontrol, an average of two incursions take place each day at Europe's 600 civil airports. And in the United States — where reporting standards are different — 182 incidents have been recorded so far in 2007, compared to 158 last year.
The most serious recent accident occurred on Oct. 8, 2001, when a Scandinavian Airline System MD-87 on takeoff smashed into a Cessna Citation which had encroached onto the runway. A total of 118 people died.
More frequent are close calls like one in July at Ft. Lauderdale, Florida, where two jets missed each other by less than 10 meters (30 feet). A United flight with 133 passengers on board missed a turn on the taxiway and entered an active runway where a Delta jet was about to land with 167 passengers.
"In an ideal world you'd have no runway crossings at all," said Paul Wilson, head of Airport Operations at Eurocontrol, Europe's air navigation agency. "But the reality is that as an airport becomes busier, it also introduces more sophisticated guidance systems and procedures to prevent runway incursions."
A Federal Aviation Administration study found that the well-designed Washington Dulles airport in the United States had only four incursions during the period from 1997 to 2000, compared to Los Angeles Airport with a complex layout of multiple intersecting runways and taxiways — which had 29 incursions.
Experts say that when the volume of traffic — projected to double over the next 10-15 years — is taken into account, the potential for near misses and fatal accidents is growing fast.
"It is a problem that affects just about every airport," Wilson said.
The international pilots' union blames poorly designed airports as the primary cause of incursions. High traffic density, complicated operational procedures, nonstandard markings, and poor comprehension of English among cockpit crew add to the risks.
Although low proficiency in English — the standard language of aviation — plays a major role, foreign pilots also complain that air traffic controllers in the United States contribute to the problem by using confusing abbreviations or long and complex instructions.
As a result, the FAA now requires U.S. controllers to provide clear and explicit taxiing instructions to pilots, including the exact route to their designated runway and not merely which runway to use.
In order to minimize future risks, Eurocontrol, FAA and other national air safety agencies are looking into using advanced runway incursion alert systems that detect potential collisions on runways and give advance warning to controllers and pilots.
One such system developed by the National Aerospace Laboratory in the Netherlands and used at Amsterdam's Schiphol Airport alerts controllers to potential collisions by flashing labels on their radar screens together with audio signals. All aircraft and vehicle movements are depicted in real time on an airport map, unlike conventional radar which has a lag of several seconds.
And when Schiphol added a new, sixth runway, multiple runway crossings were specifically avoided, said Bert Ruitenberg, the airport's operational safety expert. Instead, taxiways to the new runway were built around the perimeter of existing runways.
In addition, red lights embedded in the tarmac prevent planes from entering an active runway. Ruitenberg said such stop lights should become standard airport equipment.
"At some airports expansion is driven just by capacity," Ruitenberg said. "But designers will find it is better to plan with safety in mind (because) this could avoid a lot of the problems that may occur afterward."