According to a new study, the risk that RF emissions from carry-on electronic devices will affect avionics, although not high, is still high enough to warrant tougher government regulations

Weather radar

Marker Air traffic beacon control 1

~~

ADF sense A D F s e n i s (forward)

Receiver No. 2

Transmitter No. 2

Transmitter No. 1 altimeters

Receiver No. 1

:E No. 1 DME No. 2

Glide slope (dual) VHF No. 1

Radio

Source McDonnell Douglas

2 6 0018-9235/96/$5 0001996 IEEE IEEE SPECTRUM SEPTEMBER 1996

'ladies and gentlemen, we will begin our safety briefing video momentarily. We would appreciate your attention to this important information. Use of portable electronic devices i s not permitted during taxi, takeoffi and landing. Your crew will let you know when we reach an altitude when you can begin using an approved electronic device. "

he business travelers on the plane continue to tap on their laptop computers, oblivious to the loudspeaker. A T teenager, plugged into his Gameboy with earphones,

shoots down another space alien. A couple sharing a portable CD player daydream to their favorite opera. Eventually, prompt- ed by flight attendants making a final check down the aisles, they all grudgingly turn off what the airline industry calls portable electronic devices (PEDs).

VOIULOC No. 2 7

ADF = automatic direction finde DME = distance-measuring equipment VOR/LOC = VHF omnidirectional range localizer

Except you. You're sitting by a window, across the aisle from the teenager, and somehow, the flight attendant fails to notice that your computer is still running. "Good," you think, "I am right in the middle of drafting this presentation. At least I can finish one more slide. It's a silly rule, anyway. My computer is shielded, I" sure any avionics are shielded, and the aluminum skin of the aircraft is a shield as well. My computer won't do any harm."

Maybe not. Maybe not your computer, in this seat, on this particular aircraft, on this flight. But can a computer or other PED, in the right place, on the right plane, seriously affect the plane's instrumentation or communications? You bet.

"If it were up to me, I would shut PEDs down, period," Finbarr O'Connor, who is electromagnetic compatibility manager of R&B Enterprises, West Conshohocken, Pa., told IEEE Spectrum. R&B Enterprises is a testing, training, and consulting company that specializes in electromagnetic compatibility. "I would feel better if they were not allowed in the passenger compartment at all. They should be stowed. The potential for them to be turned on accidentally is high.

"I have no doubt that PEDs have the potential to affect air- craft avionics."

O'Connor's opinion counts. He is a member of Special Com- mittee 177 (sc-177) of RTCA Inc., Washington, D.C. , a non- profit organization that has been charged with investigating the effect of PEDs on avionics systems, and he chaired that body's Test Procedure Subcommittee.

Interference under scrutiny RTCA, formerly called the Radio Technical Commission for

Aeronautics, recommends standards and offers guidance to the aviation industry. Currently, most airlines in the United States and elsewhere voluntarily follow an RTCA recommendation issued on Sept. 16, 1988, that prohibits the use of PEDs during takeoff and landing. That recommendation was issued mostly to lessen any possibility of interference with aircraft avionics, but also to reduce the chance of passengers being injured by PEDs that might bounce around on a flight and to prevent passengers from being distracted from safety announcements.

A new study that RTCA has been working on marks the orga- nization's third visit to the issue of interference from portable elec- tronics. (The first time was in the '60s.) The SC- 177 committee has made recommendations concerning the use of PEDs, in a draft report now wending its way through a . -

lengthy approval process. Spectrtrm has obtained a copy of this draft, which is being circulated to members of the avi- ation industry. Senior Associate Editor

TEKLA S . PERRY Senior Editor

LlNDA GEPPERT

[l] Navigational instruments on this MD-80 detect radio signals broadcast from ground-based transmitters. The automatic direction finders home in on radio signals in the 17C1750-kHz range. Direction is indicated to the pilot as a magnetic bearing. VHF omnidirectional range finders WOR) detect signals broadcast 360 degrees in azimuth. These signals indicate the azimuthal location of the aircraft relative to the transmitter. Glideslope instruments detect signals transmitted from airports that guide an aircraft's angle of approach to the runway.

PERKY & GEPPERT- DO PORTABLE ELECTRONICS ENDANGER FLIGHT? 27

Until recently, evidence of the danger of PEDs has been almost exclusively anecdotal. But, said OConnor, the RTCA committee has now developed a reporting system. Feedback has been gathered from pilots who were given forms that they could use to report incidents of suspected interference directly to sc- 177.

The committee asked the flight crews to verify the cause and effect relationship between a PED and the apparent interference if they could do so without compromising safety. This they did by watching the affected instruments while the device was turned on and off and then repeating the experiment later, either by moving the device to another location in the plane's cabin or by operating it in another mode.

In addition, the National Aeronautics and Space Admin- istration (NASA), Washington, D.C., maintains a database of safety incidents of all types in its aviation safety report- ing system (ASRS). From January 1986 through June 1996, ASRS collected almost 69 000 incidents, voluntarily submit- ted by members of flight crews. In two searches of the ASRS database (from January 1986 to June 1994 and from June 1994 to June 1996), NASA researchers turned up 62 reports in which such words as "passenger electronics," "laptop," or "tape player" occurred. In 52 of these (slightly less than 0.08 percent of the total), the flight crew suspected that a por- table electronic device operated by a passenger was creat- ing interference to aircraft systems.

Both the RTCA and ASRS reporting systems are volun- tary. Any identifying information, such as the names of crew members, flight numbers, or aircraft registration numbers, is removed from the reports before they are filed. But the an- onymity means that although the reports often leave many questions unanswered, they have to be taken at face value since it is impossible to go back to the crew to get more de- tails of the incident.

The reports vary widely in the amount of correlation between the PED and the interference. The weakest correlations are those in which the flight crews who observed an anomalous instm- ment reading were aware of PEDs on board and suspected them of causing interference.

Roughly half of the reports show a slightly stronger connec- tion, in that abnormal instrument readings returned to normal when the PEDS were turned off. In the strongest reports, the problem reappeared when the device was tumed back on. Ac- cording to the RTCA draft, out of the 33 reports that SC 177 received, six were of the on-off-on variety.

A report selected from the ASRS database illustrates this type of incident. In March 1993, a large passenger aircraft was at cruise altitude just outside the Dallas-Fort Worth In- ternational Airport when the No. 1 compass suddenly pre- cessed I O degrees to the right. The first flight attendant was asked to check whether any passengers were operating elec- tronic devices. She said that a passenger in seat X had just turned on his laptop computer.

The report continues: "I asked that the passenger tum off his laptop computer for a period of 10 minutes, which he did. I then slaved the No. 1 compass, and it returned to normal operation for the 10 minute period. I then asked that the passenger tum on his computer once again. The No. 1 compass immediately pre- cessed 8 degrees to the right. The computer was then tumed off for a 30-minute period during which the No. 1 compass opera- tion was verified as normal."

The report states that it was evident to all on the flight deck that the operation of the laptop computer was adversely affect- ing the operation of the No. 1 compass. It concludes: "I believe that the operation of all passenger-operated electronic devices should be prohibited on airlines until the safe operation of all of these devices can be verified."

Guilt by radiation hat is going on here? The culprit is electromagnet- ic emissions from the PEDs, which interfere with W avionics systems, most commonly radio navigation

and communications [Fig. 11. Co-conspirators are the alu- minum airframe, which can act as a shield, a resonant cavity, or a phased array, and the sensitivities of the avionics. The radia- tion from the devices can couple to the avionics through the antennas, the wiring, or directly into the receiver. According to some experts, avionics should bear some of the responsibil- ity for eliminating the interference problem. Systems should be designed to better withstand interference from PEDs.

In general, manufacturers of affected devices have a responsi- bility to design immunity into their products, according to Bennett Kobb, editor of SpectrumGide: Radio FrequeMcy Allocations in the Uvited States, 30 lbil%-300 GHz and contributing editor to Tele- communications Reports. "There can be substantial differences in the level of interference immunity between what is technically possi- ble, what is cost-effective, and what is reasonable for policy mak- ers to expect from manufacturers," he told Spectrum. Avionics man- ufacturers did not respond to Spectrum's requests for comments.

The PEDs operate at frequencies from a few tens of kilohertz for AM radios to 133 MHz for laptop computers. When the har- monics of these signals are taken into account, the emitted fre- quencies cover almost the entire range of navigation and com- munication frequencies used on the aircraft. The frequency and intensity of the radiation also depend on what mode the device is being operated in.

To complicate matters further, different types of avionics have different sensitivities, according to AI Helfrick, associate profes- sor in the engineering technology department at Embry-Riddle Aeronautical University, Daytona Beach, Fla. A radiation source may cause total destruction of a navigation signal on one chan- nel while nearby channels are completely unaffected. Another type of receiver may be sensitive to the modulation of the signal or to the number of individual radiators.

The frequency bands used by avionics systems dot the elec- tromagnetic spectrum from a few kilohertz to several gigahertz [Fig. 21. At the low end, omega navigation, which is used to determine aircraft position through ground-based transmitters, operates in the frequency range of 10-14 kHz. From 108 MHz to 1 18 MHz is the VHF omnidirectional range finder (VOR), a radio beacon that is used to navigate from point to point.

At 328-335 MHz is the glide slope system (GLS) used dur- ing landings. And above 1 G H z is the distance-measuring equipment (DME), which gauges the space between the air- craft and a ground-based transponder and which is used throughout flight, from takeoff to landing. Also in the spec- trum above 1 G H z are the collision avoidance, global posi- tioning, and weather radar systems.

Most at risk among these systems are those that have anten- nas located at various points outside the skin of the aircraft to pick up the navigation and communication signals. "Those are the instruments that we cannot harden because they are built to receive very small signals," said Dave Walen, manager of elec- tromagnetic effects at Boeing Commercial Airplane Group, Everett, Wash. "We rely on those sensitive receivers to pick up small signals in space. And that is the primary concern we have with carry-on electronic devices."

Once the signals have been picked up by the antennas, they run through coaxial cables to communications or navigation receivers generally located below the floor of the cockpit, ex- plained Todd Degner, manager of avionics engineering for American Air Lines Inc., headquartered in Fort Worth, Texas. The output of those receiver boxes then goes to cockpit indica- tors or to other computers in the airplane, or possibly even both.

Most navigation signals, for example, go to a cockpit indica-

2 8 IEEE SPECTRUM SEPTEMBER 1996

121 The frequency bands occupied by aircraft communication and navigation signals dot the electromagnetic spectrum from a few kilohertz up to several gigahertz. Radiation from today's portable electronic devices can occur almost anywhere in the spectrum below about 1000 MHz.

tor and also to the autopilot computers. The wires that connect the receivers to the indicators or computers are twisted, shield- ed pairs, or twisted, shielded triples, depending on whether the signal is digital or analog.

Often the wires from the antennas to the receivers run along the fuselage inside the aircraft skin, passing less than a meter from a PED-wielding passenger. The thin sheet of nonconduct- ing material that forms the inside of the passenger compartment, typically fiberglass, offers no shielding whatsoever between the radiator and the wiring.

Boeing's Walen confirmed to Spctrum that wires critical to the functioning of the aircraft are generally shielded. And American Airlines' Degner believes that because the cables are so well shielded, most of the interference from PEDS is due to radiation that is picked up by the antennas and then transmitted to the cockpit instruments or the navigational computers.

There is a chance that shielding could be damaged during ser- vicing or could degrade over time. The effectiveness of shielding also depends upon good grounding. This is difficult to maintain over time because of the nature of aluminum's surface chemistry: aluminum oxidizes rapidly in air, thereby increasing the resis- tance of the electrical connection to ground. In that case, inter- fering signals could be picked up directly by the wires.

Even with shielding in mint condition, electromagnetic inter- ference can still couple to the aircraft's navigation or communi- cation systems. For although the aluminum skin of the aircraft forms an excellent electromagnetic shield, it has holes through which the radiation can escape. In commercial aircraft, the biggest holes are the windows.

To further compound the problem, the aircraft's aluminum skin is essentially a resonant cavity. In his presentation on air- craft attenuation at the 13th Digital Avionics Systems Con- ference (held Oct. 30-Nov. 3, 1994, in Phoenix, Ariz.) associ- ate professor Helfrick showed that depending on the frequency emitted by the PED, there are certain locations, or hot spots, in the plane where the device can create standing waves. The result is a signal intensity that is even larger than it would be in free space. ., >

To get to the bottom of the issue, Helfrick had measurements performed on 20 aircraft, from light general aviation aircraft to large transport planes. A spectrum analyzer and a horizontally polarized biconical antenna were placed on the ground 30 to 50

meters from the centerline of the aircraft [Fig. 31. Inside the plane, a signal generator was turned on and moved around until the point of least attenuation was found. Horizontal and vertical polarizations for more than 10 frequencies were measured.

These measurements were then repeated, but without the aircraft present. (In point of fact, the aircraft was towed away and the signal generator was placed on a pole about 3 meters above the ground, where the centerline of the airplane had been.) Helfrick defined the attenuation as the ratio of the received signal with the radiator inside the aircraft to the level received with the aircraft removed.

"Some measurements, particularly frequencies below the VHF navigation band, actually produced gain," Helfrick reported. For these lower frequencies, "The windows become a phased array; you just have to put your radio or whatever into the right spot." Because of these hot spots, signal attenuation can vary greatly from seat to seat.

For frequencies in the VHF navigation band and higher, Helfrick found that the smallest attenuation occurred with the radiator at the window," and for all practical purposes, the atten- uation was 0 dB."

Helfricks measurement technique, however, was rejected by the SC- i 77 committee. In its new investigation, it chose instead to measure the attenuation (or path loss) at the receiver, thus lumping in the attenuation of the antenna and its wiring togeth- er with that of the fuselage.

To emit or not to emit o protect avionics systems from interference, intention- al RF emitters like CB radios, remote control toys, and walkie-talkies are banned outright on commercial air-

craft. Most, but not all, airlines extend the ban to portable radios and television receivers. Although those devices are not inten- tional emitters, the local oscillators of AM radio receivers pro- duce signals in the 1 -MHz range, and of TV sets, up to 800 MHz. FM receivers typically generate signals from 98.7 to 118.7 MHz, covering the entire VHF navigation band.

One incident reported to ASRS involved portable radios. In January 1993, on a flight from Denver, Colo., to Newark, N.J., the aircraft lost all directional gyros [electromechanical de- vices that indicate orientation] at cruise altitude. The captain instructed the flight attendant to go through the cabin and tell

PERRY & CEPPERT- DO PORTABLE ELECTRONICS ENDANGER FLIGHT, 2 9

all passengers to turn off their electronic devices. She reported back that about 25 passengers with portable radios had been lis- tening to a football game and one passenger was using a laptop.

After 5 minutes, the gyros still failed to operate properly. O n rechecking, the flight attendant found that the radios were still on. Using the loudspeaker system, the captain immediately told all passengers using such devices to turn them off because they were affecting the navigation equipment. After 90 seconds, the gyros corrected themselves to the proper heading. But 20 min- utes later, they "began moving off the correct heading by as much as 20-30 degrees."

The captain then picked up the mike and told the passengers that if they did not turn the radios off, he would have the flight attendant go through the cabin and confiscate all radios until they reached Newark. Within 2 minutes, the gyros began to swing back toward the correct heading. No further incidents occurred.

(The use of cellular phones during flight-and that covers any kind of flight, including hot air balloons-is strictly prohib- ited by the U S . Federal Communications Commission [FCC]. Because the cellular telephone system reuses frequencies, an air- borne cellular telephone would be in contact with a number of cells and could easily overload the ground-based cellular net- work, explained Norman Sandler, director of strategic issues for Motorola Inc., Schaumberg, 111. If this restriction did not exist, cellular phones would clearly be banned by the U.S. Federal Aviation Agency as intentional RF emitters.)

But unintentional emitters abound. 'yust about any electronic device you take on an aircraft can

radiate," said John Wade, marketing manager for commercial products of Olin Aerospace Co., Redmond, Wash.

Such electromagnetic interference poses the greatest threat when it is emitted at an operating frequency of one of the avion- ics systems, or when it generates a harmonic at one of those fre- quencies. For example, said Wade, portable compact disk players have an internal clock of 28 MHz. That in itself causes no prob- lem, because the aircraft systems in question operate at frequen- cies above 100 MHz. But that 28-MHz clock produces harmon- ics at 56, 84, and 1 12 MHz-and 1 12 MHz is a VHF navigation channel. Although each harmonic signal is weaker than the orig- inal, under the right circumstances it can cause interference.

Then there is the laptop computer, which is becoming ubiq- uitous among road warriors, as the most active business travelers are called. A computer with a 55-MHz clock has its first har- monic at 1 10 MHz, which is a VOR frequency, and a sixth har- monic at 330 MHz, which is in the glide slope frequency band. Probably this equipment would not be a problem, assuming it came straight from the factory and therefore meets the required FCC class B specification.

But that's assuming a lot. Drop the computer, and it may still work, but its shielding may have been degraded. Let the com- puter age a few years, and the shield also degrades. Open it up to add an upgrade, and all bets are off, explains Bruce Reynolds, senior engineer in the Powerbook group at Apple Computer Inc., Cupertino, Calif.

First, Reynolds said, if the computer is not reassembled prop- erly-say, a grounding path is left unconnected or a piece of metal-coated cardboard is tossed in the trash-the computer will work fine, but its emissions may go way up.

"Like when I work on my car," he said, "sometimes I end up with extra parts, but the car works OK, so I just throw them out."

Adding a modem or other peripherals to a computer can be trouble if they are connected with a cable. Some intemal cards or cables alone are not required to meet FCC specifications and are not tested in all configurations, so integration of the part into the system may cause emissions. By buying a low-cost part or cable that is not shielded, the user could end up with a system with high levels of radiation. And sometimes, the computer's

emissions are out of line from the beginning-a brand new lap- top in the home of one of the authors disrupted the reception of a TV that was 9 meters away.

"Technically," Apple Computer's Reynolds told Spectrum, "we could design a product that would guarantee not to cause prob- lems for aircraft when it's first built, but there is no guarantee that it would continue to meet those requirements."

Getting to the essence hen it comes to getting past the anecdotal reports to some substantial data, one of the biggest prob- lems has been that incidents of interference from

PEDs are all but impossible to reproduce. So over the past three years, the RTCA committee has focused

on three areas, according to John Sheehan, the SC-177 commit- tee chairman. It has devised methods to test PEDS for spurious radiation, devised a method to detect path loss to see how such signals might propagate, and looked at the susceptibility of com- munication and navigation avionics equipment within the airplane to see how susceptible they were to low-level signals.

T h e group tested 37 models of PEDs, in 12 product cate- gories from CD players and radios to laptops and television sets. They even tested one GPS receiver. The test procedure was different from the one called for in the RTCA D O - i 6 0 D document, which is used to certify that installed equipment meets RF radiation specifications. (RTCA DO- 160D super- sedes RTCA D O - l 6 0 C , which was issued in 1989.) This standard, which is stricter than the FCC Part 15, Class B dig- ital-device specification that most consumer products must meet, is required for all equipment installed on an aircraft, including entertainment products.

The main difference between DO- 160D and the SC- 177 com- mittee's test methods is that installed equipment is tested while sit- ting on a ground plane. To more closely simulate their actual oper- ation on the airplane's tray tables or on a passenger's lap, the PEDs were tested on a nonconducting table. The PED measurements

30 IEEE SPECTRUM SEPTEMBER 1996

frequently showed emissions above 60 dBpV/m, and occasionally above 70 dBpV/m, in the frequency range from 100 to 1000 MHz.

By comparison, DO- 160D specifies that installed equipment not exceed 26 dBpV/m in the VOR band between 100 and 160 MHz. The different test procedures used by SC-177 make it hard to compare the PED emissions directly with the DO- 160D specifications because the effect of the absence of a ground plane has to be folded in. The committee did not report any tests of PEDS using installed-equipment procedures that would have made a direct comparison possible.

Said committee member O'Connor: "Over 70 percent of those products tested would have failed the DO- 160D radiat- ed emissions requirements." Not only would they have failed, but, unlike on-board entertainment products, the airline has no control over how the devices are positioned or maintained.

An interesting feature of the PED test results is that portable radios and laptops radiate in the same frequency bands with about the same intensity as each other. Yet radios are banned by most airlines and laptops are not.

While it is possible to manufacture PEDS that will not inter- fere with avionics, it is not easy. Witness what the makers of on-board entertainment devices have to go through to ensure that their products are not a threat to critical aircraft systems. According to Dale Freeman, senior electrical avionics engineer for Northwest Airlines Corp., St. Paul, Minn., and Olin Aero- space's Wade, such on-board electronics (like television sets, videotape players, telephones, and games) are manufactured to meet the RTCA-DO16Oc specification.

After this, equipment is tested for radiated emissions and con- ducted emissions to make sure it meets that specification. Then it is installed in the plane and turned on. Cockpit instniments are turned on as well, to ensure that they show no interference. The automatic landing system is put through its paces and the engines are powered up, to check that there is no interference with the aircraft's circuits.

Each airline must go through this process every time new

equipment is installed on an aircraft, because every plane is dif- ferent (not only are the models of planes different, but every airline specs out its own avionics, seat configurations, and so on). The first time that new electrical equipment is installed on a certain model of aircraft, a flight test is also conducted.

It is also possible to certify certain PEDS for aircraft use. Said Wade: "We've been evangelizing, trying to make laptop manu- facturers aware of the airborne requirements. We hope we will see airborne quality laptops appearing. And then we are devel- oping a standard for laptop power that would allow them to plug into a shielded cable on a plane."

Still, approved laptops and shielded power may not be the ultimate solution. After all, any modification to the computer, even dropping it, may change its electromagnetic profile.

But engineers are looking for other solutions, mulling even such far-out ideas as "smoke alarms" to sniff out dangerous elec- tromagnetic radiation. Working models exist, indicates the RTCA draft, and show promise. "The ability to detect harmful signals should add an additional safety factor," it states.

"Seventy-five years ago, flying was considered dangerous, but people wanted to fly," Wade said. "So we as an industry found a way to make it safe. Now people want to use laptops, and we as an industry can find a way to make that safe."

So if PEDS can generate signals at aircraft navigation and communication frequencies, and tens of thousands of PEDS are operated aboard aircraft daily, why aren't planes crashing all over the place? Because the probability that an individual PED might interfere with an individual avionics system on a specific flight is very low. The RTCA draft report tabulated the probabilities for interference from sources both in and out of the navigation and communication bands for a number of systems and on a number of different aircraft. They looked at coupling through the anten- nas and through the cables and considered both intentional and unintentional radiators.

For the case of in-band measurements, an interference probability of 1 0 ~ 5 was noted for the L1011 instrument land-

PERRY & GEPPERT- DO PORTABLE ELECTRONICS ENDANGER FLIGHT? 31

ing system and the B747 glide slope sys- tem (both used to land the plane). At the low end, the B737 probabilities were below 3 x 1 0 ~ 7 , and the A320 showed no problem for the VOR and GLS systems.

The RTCAs SC- i 77 committee arrived at these numbers using worst-case emis- sion levels from nonscanning-mode PEDS, minimum field strength requirements for each avionics system, and relevant attenu- ation factors.

It never fails he Federal Aviation Administration, Washington, D.C., requires that airplanes, to be certified, must be

designed to be fail-safe to 1 0-9. This means, explained associate professor Helfrick, that a flight-critical system may not fail more than once in every billion flight-hours. Said another way, the likelihood of staying aloft is better than 0.999999999. The agency also sets levels for less serious situations.

The risk of a hazardous effect (one that may seriously injure, or kill, a relatively small fraction of the occupants) must be less than 1 0-7. A major failure, which results in “passenger discomfort” or injury (a gear- up landing, for example) is allowed a prob- ability of up to 1 0-5. Minor effects-that is, those not affecting the air worthiness of the aircraft-are of two types: nuisance and operating limitations, or emergency proce- dures, with probabilities of less than 1 0-2 and 10.3, respectively.

T h e SC- 1 77 committee has assessed the effects of interference on the air- craft and the crew according to the agency’s categories. A flag, fluctuation, or offset on any one instrument while the aircraft is en route is regarded as a nuisance, while interference with multi- ple independent instruments is an opera- tional limitation [Fig. 41.

When the plane is landing or taking off, the seriousness of events goes up a notch, with flag, fluctuation, or offset on one instrument becoming an operational limitation. On two or more independent instruments, it becomes a major event. The only instance of a hazardous event is the equal offset of multiple independent instruments during takeoff or landing.

The committee takes 5 x 10-5 as the

141 A Saab 2000 cockpit presents the pilot with a lot of information. The aircraft’s attitude is shown at the center of the primary flight display, located at the left of the instrument panel. Air speed and vertical air speed are shown on the left and right scales, respectively. Azimuthal position rela- tive to the VOR transmitter appears on the dial at the bottom. Next to the primary display, the mul- tifunction display gives information from the weather radar and traffic alertlcollision avoidance systems. The two center displays allow critical engine and system status to be monitored.

probability of interference by a PED to any one avionics sys- tem, and 2.5 x 10.9 as the probability of interference to two independent instruments. So in all cases, the likelihood of occurrence is less than the upper limit set by the agency. But in some instances, it is greater than the agency‘s lower limit.

The question becomes, said SC-177 committee chair Sheehan, what level of probability the FAA is willing to accept. (That level, conceivably, could be different en route than during critical flight phases.)

What is even less likely is a PED being confirmed as con- tributing to a crash. All the flight data recorder will show are the instrument readings, and not that such readings were being affected by a PED. (The recorder could reveal incon-

sistencies among redundant systems, however.) Still, the odds of a PED interfering with an individual flight

may be going up, for several reasons. In the first place, more and more of them are being used. Second, every generation of lap- top computer uses a higher clock speed, with this year‘s high- end models sporting clock speeds up to 1 33 MHz. At that speed, the fundamental signal-not just harmonics-can threaten avionics. Also to be kept in mind is that the 500-MHz state-of- the-art chips now at the heart of advanced desktop PCs and workstations will soon find their way into laptops as well.

The intensity of the RF power is also an important factor in the likelihood of interference to the avionics. The manufactur- ers of portable electronics are struggling to reduce the power

32 IEEE SPECTRUM SEPTEMBER 1996

consumption of their products, mainly by using lower operat- ing voltages. But they are contending with such factors as larg- er ICs with higher transistor density, higher operating speeds, and the processing demands of more complex software that are pushing RF power generation up. Further, some techniques, such as sleep and snooze modes, reduce average power con- sumption but not peak RF power; and it is peak RF power that is of concern for interference.

New types of portable electronics also are bound to prolifer- ate. SpectrumGuidei Kobb told Spectrum that the use of satellite communications is undergoing a shift. At present, people use satellites indirectly in making long-distance phone calls or view- ing television programs relayed by satellite. But soon, the abili- ty to access satellites directly, both to transmit and receive infor- mation, will be an integral part of laptops and other personal communications assistants.

One such device was described by Peter Karabinis, program director of new products at Ericsson Inc., Research Triangle Park, N.C. At the 1996 IEEE MTT-S International Microwave Symposium held June 17-2 1 in San Francisco, he reported on

the development of dual-mode cellular/satellite hand-held telephone technology. In the satellite mode, the average radiated power of the phones is constrained by require- ments to a limit of 250 m W of average power and 2 W of peak power. The devices will operate in the 1-2- GHz frequency range.

Meanwhile, every generation of aircraft uses less alu- minum and more composites. Said associate professor Helfrick: "You can argue that with an aluminum airplane, no signals will get out to interfere, but with the greater use of composites, the RF holes are getting bigger and bigger in that cage."

Newer aircraft also are moving toward fly-by-wire systems-replacing hydraulic controls with electronic ones. But systems that support a critical function like controlling the aircraft have extensive protection, according to Boeing's Walen. "That includes shielding as well as making the electronics immune to RF, EMI, and lightning."

These systems also have built-in redundancy, according to American Air Lines' Degner. "Fly-by-wire systems have multiple redundant channels and not all of them will fail at once," he said. "Suppose you have a cable that has deteri- orated and someone has a device that radiates a significant amount of energy. The weakest cable would pick that up. But fly-by-wire systems are built such that if you get an anomalous reading on one channel, the system will vote that channel out and log it into the maintenance comput- ers. Then the maintenance crew would be charged with finding that cable. Sometimes this could be a real bear. But eventually they would track it down and change it."

Toward stronger regulations Meanwhile, the RTCA draft makes several recom-

mendations. One is that the current policy of asking passengers to turn off PEDS during taxi, takeoff, and landing (roughly below an altitude of 10 000 ft or 3000 meters), which is now voluntary on the part of the air- line, be made mandatory. Another is that the traveling public be educated about the reason for concern over PEDs and what specific PEDs will cause problems (this, perhaps, would make computer users who have jury- rigged system upgrades think twice about using their computers on board an aircraft.) The report also rec- ommends continued study of emissions from PEDs, especially the newer models that may contain built-in communications devices.

If these recommendations survive the review process under way, they will be forwarded to the Federal Aviation Ad- ministration, and the agency will determine what, if any, regula- tions to issue.

Restricting the use of PEDS while below 10 000 ft (3000 meters) is probably good enough, the experts agree.

An individual airline may, of course, go beyond these recom- mendations, but is unlikely to risk sending PED-laden travelers to its competitors. +

~~~

To probe further Information on avionics, frequencies, and emissions can be found in

Modern Aviation Electronics, 2nd edition, by A.O. Helfrick (Prentice Hall, Englewood Cliffs, N.J., 1994).

RTCA reports may be purchased from the organization at 202-833- 9339. To obtain information from the ASRS database, write t o NASA Aviation Safety Reporting System, Attention: Database Information, Box 189, Moffett Field, Calif. 94035-0189. Or for more information about the database, visit the ASRS Web site at http://www-afo.arc.nasa.gov/ASRS/ASRS.htmI.

PERRY & GEPPERT- DO PORTABLE ELECTRONICS ENDANGER FLIGHT? 33