Commercial, Military

Avionics System Design: How to Prevent Incidents from PEDS

By Walter Shawlee 2 | February 1, 2000
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More e-mail has come my way on the issues associated with personal electronic devices (PEDs) in aircraft than any other topic. I am indebted to the many readers who provided me with insight into both the political and technical issues involved with PEDs as well as details of their own experiences.

From those experiences and from our own tests, it becomes clear that this issue is becoming more significant in terms of flight safety. And to judge by consumer electronics trends, this issue will require firm guidelines soon to avoid incidents.

The general fault mechanism appears consistent with our observations: It essentially is a "denial of service" problem, when passenger equipment emissions block a required radio channel’s use.

The issue goes beyond the 747-size aircraft and affects smaller aircraft, too, which are used as feeder/shuttle services to large hubs. These aircraft have much closer avionics system coupling and close antenna proximity to the passengers.

Any rule changes must insure that the new standards work in all commuter situations, where unsuspecting passengers may be put at risk by other passengers’ routine use of PEDs.

I think a practical approach is for airlines to advise that they will permit limited PED operation for one year, subject to individual inspection and rejection of items if problems are suspected. After that period, they will require that any device carried on-board be suitably certified for low-emission aircraft operation. This correctly shifts the onus for device compliance to the consumer hardware makers, and allows passengers a way to have safe "on board" operation if they wish to have it. The Federal Aviation Administration would determine the penalties for shipping non-compliant "certified" goods.

A clear-cut change in government level policy is required urgently to avoid uncontrolled proliferation of potentially interfering devices and to create pressure for industry design and construction of less troublesome equipment.

A technical standard order (TSO) for PEDs would be required, which is certainly overdue. A global policy adds complexity but cannot be avoided. Without this level of protection from passengers, operators must turn to some aircraft hardening (which may be prudent in any case), probably to include RF shielded windows and apertures (via mesh screen or conductive coatings). Their aircraft would also need supplemental outer shielding of all antenna coax feeds and possibly an "over-wrap" of exposed wiring with a shielding film or metalized interior panels, plus some antenna relocation.

An RF "cabin sniffer" may also become a common device on board (as smoke alarms are now), to indicate unwanted emissions from passengers. Such a device would not be difficult to make, or costly. Their use also will help clarify to passengers the problem of using their unsatisfactory items on board and provide detection of problem devices anywhere in the world.

Many people have asked for a way of quickly and inexpensively determining if a device is potentially problematic. The simplest solution is to use a handheld VHF scanner, one foot away, and cover 108-156Mhz. If the suspect device trips the squelch on the scanner and can be heard on any channel, it should not be used on board, as it is a potential problem emitter. A shielded box is helpful for tests on the ground where many interfering signals may exist. In flight, the box would be cumbersome but likewise not essential, as few conflicting signals exist inside the cabin. Removing the antenna or using a smaller antenna on the scanner allows very close pinpointing of devices.

Don’t forget that discussion is now underway for wireless e-mail, two-way paging, "bluetooth"-based technology, wireless laptop Internet connections, and so forth. These will lead to overt and intentional transmission inside the aircraft, with possibly disastrous effects if not considered and planned for now.

One area just barely touched on but equally significant is the widespread use of new neodymium and samarium cobalt magnets in speakers and headphones. They have very high magnetic fields relative to their physical sizes. Many pilot reports have been made of strange and unpredictable interference with compass systems. These may be due to these devices on-board affecting standby compasses and flux valves. Safe compass distances of one foot are needed to keep the interference under control. This is much more significant in smaller aircraft, as items stowed in baggage and carried on board may become quite problematic—even if they are not turned on—due to this effect.

The reality is that we simply cannot be quite so cavalier about the mode of interference caused by passenger’s possessions. It can represent signals squarely in the design range of the onboard systems, and the problem cannot be prevented by the aircraft itself without some heroic preventative measures. When we can talk to on-board radios by operating a laptop or handheld game, this situation moves out of the potential threat category and into the "please pay attention" category. I think it is clearly time to give this area some serious examination, as our own test results were quite a wake up call to us.

Walter Shawlee welcomes comments and may be reached by e-mail at [email protected]

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