Faulty radar serving Logan leaves thousands stranded

A malfunctioning radar system serving Logan International Airport caused flight cancellations and delays of several hours yesterday, stranding thousands of passengers on a holiday weekend and adding to the woes of an airport that has logged several runway incidents in the past few months.

The Federal Aviation Administration said last evening that the radar could be fixed within hours or it could take several days, which could mean even bigger headaches for travelers and their airlines.

Radar screens at a regional aviation center in Merrimack, N.H., were showing controllers ''false targets," or objects that didn't exist, setting off the system's ''conflict alert" feature warning of potential collisions, FAA spokeswoman Arlene Murray said. The false readings began at about 10:30 a.m. Sunday, stopped after a while, and then resumed at 7:30 a.m. yesterday.

Murray emphasized that safety had not been compromised and said air traffic controllers had switched to a backup FAA radar system in Nashua, N.H. The system is slower than the one at Logan, though, and so it prompted controllers to increase the space between planes going in and out of Logan from 3 miles to 5.

''The air traffic operation continues to run smoothly, even though we are experiencing delays in the system," Murray said.

The slowdown cut capacity at Logan roughly in half, limiting the facility to one runway for takeoffs and another for landings.

The airport averaged about 30 departures and 24 arrivals per hour yesterday evening, according to the Massachusetts Port Authority, which owns and operates Logan. The maximum capacity when the radar is working properly -- and without weather problems -- is 120 flights per hour, said MassPort spokeswoman Georgeane Tacelli. She added that there were no weather-related delays yesterday at Logan.

That meant thousands of passengers were left waiting in airport lobbies or on tarmacs from Bangor to Fort Lauderdale.

''I am angry, very angry," said Mary Ellen Lewis, 27, a Burlington, Mass., resident stuck in Philadelphia after visiting family over the weekend. ''We will try to fly to Manchester [N.H.]. If not, I guess we will have to try and find a hotel."

The FAA said the full effect of the radar malfunction may not be known until today, when data about delays and cancellations would be analyzed by the agency.

In 2000, the radar fell over at Logan, causing the cancellation of more than 500 flights and disrupting air traffic along the East Coast. The radar was fixed 2 1/2 days later after a replacement was flown in.

In this case, technicians aren't sure which part of the radar system is malfunctioning, Murray said, and they may have to send out the faulty radar data to a technical center in Atlantic City, N.J., for analysis, a process that could take several days.

Before yesterday's problems, federal aviation officials were already planning to send a team of about a dozen specialists to Logan this week to study what caused a spate of runway incidents during the past year.

There have been 16 reported incidents since October 2004, more than double the total for the previous three-year period. They included one close call that officials say was the closest at any US airport recently. On June 9, an Aer Lingus Airbus A330 and a US Airways Boeing 737, carrying a combined 381 passengers and crew members, came within 170 feet and a few seconds of colliding as they sped down intersecting runways at more than 160 miles per hour. Logan is the nation's 17th busiest airport, with 1,250 daily arrivals and departures.

When flights take off from Logan, pilots are first guided by controllers at the airport, then handed off to a regional facility in Merrimack, N.H., and finally to Nashua, N.H., one of 22 larger control centers across the United States, which typically handles flights above 16,000 feet. The opposite procedure applies to arriving flights.

The faulty radar at Logan, identified by FAA officials as an Airport Surveillance Radar model 9, or ASR-9, feeds data gathered within a 60-nautical mile radius to the regional center in New Hampshire.

Specialists say the faulty radar system at Logan is made up of two separate radars, a primary antenna that transmits a signal to detect when planes have entered its airspace and where they are located, and the second, which sends out signals to aircraft transponders to learn their identities and confirm their locations.

Data from both the dishes are combined on a single screen for air traffic controllers, showing a blip along with a transponder code showing which flight it is. FAA officials said they did not know what was causing controllers' screens to show planes that weren't there, whether it was one or both of the dishes or a computer that interprets information from those dishes.

Specialists said false readings are extremely rare and surmised that either a newly constructed building or a software glitch could have caused them.

''I think the odds on a radar going down and causing delays is down around 1 percent," said Raymond LaFrey, a member of the FAA's research and development advisory council and former researcher at Massachusetts Institute of Technology, which helped develop radar software at Logan to avert false readings.

The ASR-9 system has not been without problems. In 2001, FAA investigators found weaknesses in the towers supporting 23 of the radars, including cases at Logan and at John F. Kennedy Airport in New York in which the radars toppled. At the time, the system was used to track planes at 134 major US airports and some military facilities. In 2000 at O'Hare International Airport in Chicago, air traffic controllers reported a problem with ghost airplanes picked up by the ASR-9 radar there.

The ASR-9 was originally made by Westinghouse Electric Corp., which was purchased in 1996 by Northrop Grumman Corp., the Los Angeles defense contractor. A Northrop spokesman said yesterday that the company wouldn't discuss operational issues or the problems at Logan.

The system was an upgrade over past radars because of its ability to show details of moving airplanes and is based on work originally done at MIT's Lincoln Laboratory. It was first fielded in 1989. Concerns that the technology was out of date led to software upgrades over the years, though reliability worries have remained.

Specialists said that travelers should not be alarmed by yesterday's radar failure in Boston.

''This is not a safety issue, it's a nuisance issue, it's an economic issue," LaFrey said. ''If they could not trust the radar, they would absolutely just not use it."

Charles Radin and Ross Kerber of the Globe staff contributed to this report. Radin reported from Philadelphia. Donovan Slack can be reached at dslack@globe.com.