Turn Off That Phone!
For those who've use portable electronic devices aboard airliners: Here's why they're dangerous.
- By John Croft
- Air & Space magazine, September 2004
(Page 2 of 3)
Frontier Airlines’ safety cards give passengers pictures of the dos and don’ts, but not explanations. Passengers are forbidden to use radios, radio-controlled toys, and TVs—ever. (Because they’re tunable over a range of frequencies, TVs and radios could be especially troublesome.) Laptop computers, mobile phones, video games, and CD players may be used when the flight crew says so.
The fact that more than four decades of study has not cleared up the uncertainty that remains is testament to the complexity of the issue. The basic science, however, is irrefutable: An aircraft’s flight navigation and communication systems receive radio signals from ground stations and orbiting satellites through antennas mounted on the aircraft’s exterior. Electronic boxes at various locations inside the aircraft process the signals gathered by the antennas to provide information for cockpit displays, and cables running throughout the aircraft route communications between the antennas, boxes, and cockpit displays. Portable electronic devices can corrupt these avionics networks in two ways. If a passenger talks on a mobile phone or watches a TV inside the aircraft, radio waves emitted from these devices can flow through aircraft windows and leak past door seals. Engineers refer to this as “front door” interference because once outside the aircraft, the radio emissions can be picked up by the craft’s antennas, which then are less able to pick up and send transmissions to and from, say, an air traffic control tower on the ground or an orbiting navigation satellite. And, though less likely, the same emissions from an onboard mobile phone or TV set can mingle with signals passing through cables running under the floor and in the airframe shell; they can even radiate directly into the aircraft’s electronic boxes. Both are examples of “back door” interference.
For front-door interference to occur, however, there must be overlaps between the frequency and power of two radio transmissions. A popular mobile phone, for example, broadcasts its intended signal at a frequency of 1,850 to 1,910 megahertz and a power level of 30 milliwatts. At the same time the phone is emitting its intended broadcast loud and clear, it is also putting out an unintended, or spurious, low-power background buzz of radio signals ranging in frequency from 100 to 2,000 megahertz. It just so happens that the very high frequency radio that air traffic control uses to communicate with cockpit crews broadcasts at frequencies of 118 to 137 megahertz, which falls within the frequency range of the mobile phone’s background buzz. Interference is not likely to occur, however, as long as the VHF transmission is sufficiently stronger than the phone’s background buzz. But the farther the airplane flies from an air traffic control tower, the weaker the tower’s signal is when it reaches the airliner. And if the phone transmits a signal that has the same frequency as the tower’s and is nearly as powerful, the two signals will compete with each other. Result: interference.
That’s one way to explain what happened to Richard Innes, a pilot who flies McDonnell Douglas MD-88s for a major airline. A year and a half ago, Innes was in cruise flight near Indianapolis, Indiana, when static over his headphones made it difficult to speak with his copilot. “It wasn’t hair-raising—more like annoying,” he says. Innes then made what’s known as a “PED announcement” to the cabin, asking passengers to turn off all portable electronic devices. The problem cleared up and Innes was inclined to leave it at that. “I’m not sure what people were operating in the cabin,” he says. “When you’re up in the cockpit, your main focus is flying. You don’t have time to play flight test engineer.”
It’s likely, though, that flight crews will continue to experience PED intrusions as airline passengers seek the convenience of electronics while in transit. However, some companies are developing technology to allow passengers to safely use cell phones during flight. Arinc/Telenor has designed a system to enable cell phone base stations on the ground to communicate with airborne cell phones via a device installed on each airliner. During critical phases of flight (takeoff, approach, and landing), the cell phones would be remotely disabled, and at 30,000 feet and above the phones would be turned on, transmitting at power levels that wouldn’t interfere with airline avionics. Arinc/Telenor expects to have a proof-of-concept demonstrator available later this year, after which testing and certification could begin. It could be a long time, though, before the Arinc/Telenor technology finds its way onto FAA-certified aircraft.
When it comes to electromagnetic phenomena, proving cause and effect is difficult because an unknown number of factors have to be considered, not the least of which is how many people and seats the electronic device’s signal had to pass through before it went through the “front” or “back” door (what engineers refer to as path loss) and whether external sources, such as lightning, ground-based military radars, and television and radio stations have contributed.
The RTCA committee is looking into onboard solutions for protecting avionics: installing window and door shielding that would prevent portable electronic device signals from reaching external antennas, getting manufacturers to build mobile phones and other carry-on devices that won’t interfere with aircraft electronics, and evaluating interference detection systems for the crew. Jay Ely, an RTCA committee member and electromagnetic interference researcher at NASA’s Langley Research Center in Virginia, says the current detector systems have problems involving the handling of false alarms.
There’s much discussion within the industry as to whether the guidelines in place are too strict or too liberal. Ely and his counterparts are of the opinion that more restrictions may be needed. During several weeks of tests on a Boeing 747 and a 737 in 2002, NASA found that an ultra-wide-band transmitter operating within FCC limits in a passenger cabin could wipe out depictions of nearby aircraft on a pilot’s collision avoidance system screen, as well as cause “erratic motion and failure” of the instrument landing system’s horizontal and vertical course guidance indicators, among other unwanted effects.