Signal turbulence: As wireless gadgets multiply, so does the likelihood of interference with aviation systems
Post-gazette.com Monday, April 21, 2003 By Byron Spice, Post-Gazette Science Editor
Something odd was happening as the Boeing 737 made its approach to Chicago's Midway Airport. A cockpit instrument called the course deviation indicator, or CDI, showed the plane was on course, but the pilots, peering through the night sky at the lights below, thought they were too far south. An air traffic controller radioed the same concern.
Then the CDI's vertical needle suddenly swung to the left, showing the plane north of its course. After the captain made a scheduled turn to align the plane with the runway, the CDI needle again indicated the plane was on course. But then the needle swung again, showing the plane too far south.
By this time, the runway was in view and the pilots could see they were too high and too far north to land.
The apparent cause of these electronic gremlins was discovered as the plane circled around for another approach and the captain asked the passengers to make sure they had turned off all electronic devices.
The flight attendants reported that a woman passenger had been talking on her cell phone. When she turned it off, the instruments immediately settled down and the plane landed safely.
This incident, which the captain reported last year to NASA's Aviation Safety Reporting System, is just one example of how portable electronic devices -- everything from cell phones to Game Boys -- can interfere with the electronic navigation and communication systems aboard a modern airliner.
No one has yet blamed an aircraft accident on a malfunctioning laptop computer or an overactive pager, but the profusion of cell phones, laptops, CD/DVD players, game systems and personal digital assistants, or PDAs, that passengers now carry onboard is raising concern about electronic interference with avionic equipment..
"I don't have a sense [electronic interference] is increasing, but I sure see the potential there," said Kent Horton, general manager of avionics engineering for Delta Airlines.
It's not just the sheer number of devices being carried onboard, but their changing capabilties. A particular worry are wireless technologies, which go by such names as Bluetooth, Wi-Fi and a new one called ultrawideband. They allow laptops and PDAs to communicate with each other or connect with the Internet.
"They're so new we don't know a lot about these things," Horton said.
The wireless technologies operate at very low power but, like cell phones, are designed as transmitters, increasing the likelihood of interference. The Federal Aviation Administration has asked the Radio Technical Commission for Aeronautics, a private, not-for-profit group that advises the agency, to convene a special committee this summer to evaluate the new technology and determine whether it poses a threat to safety.
Cell phone use already is prohibited aboard planes and the FAA recommends that use of all portable electronic devices be limited below altitudes of 10,000 feet. But technological changes are making it more difficult to enforce those rules.
"As wireless devices become embedded into other devices, such as laptops, and the antennas for other devices become less conspicuous, it places a greater challenge on our flight crews to identify potential interference sources," said Timothy W. Shaver, program leader for flight avionics engineering at United Airlines.
None of this suggests that a crisis is at hand, emphasized Granger Morgan, head of the Department of Engineering and Public Policy at Carnegie Mellon University.
"Air travel is getting safer and safer," Morgan said. "We've got most of the big things under control." Attention to portable electronic devices, however, could help prevent a small threat from becoming something more. "At the most fundamental level, we need to increase the level of vigilance across the board."
Electronic interference alone might not be a major threat, but combined with other factors, such as bad weather or pilot fatigue, could contribute to accidents, said Bill Strauss, an avionics engineer at the Naval Air Warfare Center in Patuxent River, Md.
It's more than just a safety threat, noted Strauss, who is working toward a doctorate at Carnegie Mellon. Incidents of interference end up costing airlines money, as planes get pulled from service and technicians look in vain for something wrong.
Enforcing restrictions on portable electronic devices can trigger confrontations between passengers and crew.
An airliner was forced to taxi back to the gate at the airport in Caracas, Venezuela, early last year when a passenger refused to turn off his cell phone, at one point slipping into a lavatory to continue a conversation. Last summer, the crew of an MD-80 landing in Atlanta summoned the state police to board the plane after landing when a doctor became abusive when asked to turn off his CD player as the plane descended. And, on a flight from Miami to Chicago O'Hare, a passenger wouldn't turn off her cell phone until the captain came into the cabin to talk with her; after the plane landed, she went into the cockpit and gave the captain an earful.
"She just kept stating that it was very upsetting to her that she was reprimanded for not turning off her cell phone," a flight attendant said in a report filed for NASA's ASRS database.
"The air rage potential right now is probably a worse scenario than any interference events we are aware of," Strauss said.
Some people seem particularly skeptical about the in-flight ban on cell phone use, suggesting it's an airline effort to force passengers to use extra-cost phones located in the seatbacks.
In fact, the prohibition is a Federal Communications Commission rule. An activated cell phone, even when not being used for conversation, sends signals to the nearest base station. High in the air, a cell phone can "light up" base stations in a wide swath beneath the airplane's path and cause headaches for the cell network.
Even without the FCC rule, the FAA and airlines would want to restrict "intentional emitters" such as cell phones. But devices such as laptops, CD players, and the insulin pumps used by diabetics also can emit radio signals, though they are not designed to do so. Dropping a laptop, for instance, can damage it in ways that cause it to emit excessive amounts of electromagnetic radiation.
About 60 percent of all interference caused by portable electronics has been linked in reports to either cell phone or laptop use. But proving a cause-and-effect relationship is daunting.
"A lot of people in the field call it 'black magic,'" Strauss said.
The numbers of possible sources and possible interference paths are virtually impossible to count. Most airliners have 12 to 15 different antennas for various systems, Delta's Horton said, and radio emissions that escape through windows or around doors can reach one or more of these antennas, depending on the device's frequency, power and location.
Strauss said the devices also can cause interference within the plane, infiltrating electronic control boxes or the plane's wiring. A worn bit of electrical insulation, a missing shield, or a grounding wire mistakenly left disconnected may leave aircraft electronics vulnerable.
Even having what appears to be the culprit device in hand doesn't help. Horton said the airline has confiscated cell phones and other items suspected of causing in-flight interference, but has never been able to duplicate the interference.
Other factors no doubt are at work, Horton said. For instance, a passenger's low-power device might not affect voice communications when the plane is near a ground station, where the signals are strongest, but might interfere when the plane is far away from the station and receiving weaker signals.
Strauss suspects that more than one device may be causing interference in some cases. Several low-power devices operating in the passenger cabin can have additive effects, resulting in emissions stronger than any single device could generate, he explained. Turning one of those devices off might alleviate the interference and cause cockpit instruments to return to normal, but trying to recreate the effect using only that one device would be impossible.
Delta has done extensive measurements on the ground of "path loss," noting how a radio emitter might affect various antennas from various locations within different aircraft.
Up to now, though, there haven't been measurements during commercial flights. Strauss and Morgan have obtained a small FAA grant to perform what they say are the first in-flight measurements of the radiofrequency environment. Carrying a spectrum analyzer and an antenna aboard commercial airliners, Strauss will attempt to record the types and amounts of electrical emissions that occur at various points during the flights.
His own analysis of ASRS reports of interference incidents from 1995 through 2001 has convinced Strauss that a lot of cell phones are being switched on during landing approaches. Passengers may be eager to call people on the ground to let them know they have arrived, he reasoned, and may not realize that the cell phone can cause interference when it's turned on, not just when a call is being made or received.
Some of the solutions may simply be procedural, like the rule limiting the use of electronics at altitudes below 10,000 feet. The FAA says the number of interference incidents dropped dramatically when that recommendation was made in 1996.
In addition to direct observation by flight attendants, it may be possible to monitor electronics use by equipping planes with radiofrequency detectors. Handheld detectors could help the crew identify individual devices, particularly those generating excessive radiation, Morgan and Strauss suggest.
Greater cooperation between the FCC and FAA, they added, might result in wireless devices being designed with an override capability so that a centrally transmitted control signal could disable the devices during critical phases of flight.
Byron Spice can be reached at bspice@post-gazette.com or 412-263-1578.