Gentlemen let me offer a new theory about a different type of RFI. Before I do that let me affirm what Ken Javor has written about cell phones.
During the nineties it was not uncommon to see a handset in the center chair back of the seat in front of the flyers of several major airlines. These were airline cell phones that operated in the same 800 to 900 MHz band that regular cell phones use. Before everyone jumps on me let me hasten to add these were NOT the same frequencies as terrestrial cell phones. They were in a different sub band for aircraft. My point is simply they were 25 watts, not the milliwatts normally radiated by terrestrial cell phones. If 25 watts in the same cell phone band doesn't bother the aircraft systems I see little chance a terrestrial cell cell phone would ever cause harmful interference. Ken is correct. They prohibit airborne usage because it severely disrupts terrestrial cell phone usage. i.e. your dropped cell call may have been caused by someone using a cell phone overhead. Can we all agree cell phones are a special case that does lend itself to this discussion at hand. New Theory: This is another special case that I think does lend itself to the discussion at hand. In the domain of electronic warfare there is a common jamming technique where the received radar signal is delayed and retransmitted to the bad guys radar. This throws off the range calculation. The fact the delayed retransmitted signal is much stronger than the reflected signal allows the 'capture' of the radar receiver causing further disruption. Without going into a lot of detail I think you will agree these systems can cause missiles to go crashing into the ground in life or death scenarios. Now how does noise canceling work? The offending acoustical signal is converted to an electrical signal which is delayed and fed back to the earphone such that there is phase cancellation making the offending sound appear to go away. Here is a system that will automatically sync up to an external stimulus. Hold that thought. All commercial aircraft have transponders which retransmit radar signals. They attach critical aircraft information, like altitude, to the retransmitted signal. This is how the ground controller knows aircraft altitude. Now, let's suppose the noise canceling headset (RFI receptor) intercepts the transponders signal. By now some of you may be saying that is apples and oranges. We are talking about an acoustical PED picking up microwave. True but the transistors in the noise canceling headset may have gain bandwidth beyond 10 GHz and the transponders is a high power signal. Microwave detectors abound in such circuitry. If you run the numbers I think you will agree that is possible. Now think of the aircraft transponders as a RFI receptor. All digital circuits are broad band emitters. Because of the close proximity of the aircraft transponders and the noise canceling headset it may be possible to capture the transponder's receiver. This sets up the scenario where transponder and the noise canceling headset talk to each other. Ping-Pong transponders if you will. This has the potential of causing problems for both the aircraft and the ground controllers. Hopefully I'm not chicken little here but I do see a potential for harmful interference that is not likely to be picked up normal test methods. To the best of my knowledge we don't do any synchronous transpond testing. It seems to me we have created a new category of PEDs called transponders. (Just my dollars worth. One dollar equals eight bits.) Fred Townsend DC to Light Ken Javor wrote: In turn: 1) Cell phones should not cause a problem on aircraft unless there are aircraft radios in-band. The prohibition against airborne cell phone usage protects the cell system, not the aircraft. Thus, United's policy is a rational response. 2) While the terminal area may well be the most severe electromagnetic environment in terms of spectral usage and density, that is not at all the same thing as the worst case environment for rfi - interference to radios. Rfi is a signal-to-noise ratio (SNR) issue, and with a strong signal from the tower, a high ambient can be tolerated. 3) See (1). 4) A PED-caused serious rfi problem requires the confluence of many factors. The PED has to be emitting an offending signal at the same frequency to which an aircraft COMM or NAV radio is tuned, and the function controlled by the reception has to be critical. The PED has to be located such that the very low-level emission efficiently exits the aircraft and couples to the external aircraft antenna at a level commensurate with that >from the ground-based station. Either the PED signal has to couple at a magnitude higher than the ground-based station if the emission is clock-related (unmodulated) or the emission has to have a modulation characteristic similar to that of the desired signal, and even then the coupled signal must be of the same order of magnitude as the ground-based station to cause a problem. When you put all this together, the probability of an individual PED causing an aircraft mishap is vanishingly small. When you multiply that small number by the large number of flights and PEDs on each flight, the probability of a problem somewhere at some time increases, but is still small. This is entirely commensurate with the FAA response - if there were even a 1% chance in any given year that PED use would drop an aircraft out of the sky all PEDs would be consigned to checked bags, and be checked for battery removal besides - consider that we now have to leave full-sized toothpaste tubes in checked luggage only. Ken Javor From: "Hopkins, Michael J." <mailto:[email protected]> <[email protected]> List-Post: [email protected] Date: Fri, 16 Feb 2007 12:04:31 -0500 To: "Ken Javor" <mailto:[email protected]> <[email protected]>, <mailto:[email protected]> <[email protected]> Subject: RE: Noise canceling headsets Interesting thread --- couple of comments: 1. United now allows cell phone usage on the ground prior to closing the door to leave the gate and allows cells phones to be used while taxiing to the gate after landing. Not aware of any reported problems. 2. According to avionics manufacturers, the terminal area is considered to be the most severe environment for an aircraft, with numerous radiators all around --- other aircraft comms, ground personnel, radars, weather and terminal radars, etc... 3. Numerous reports have been published regarding cell phones being used in aircraft. (On a recent flight, a flight attendant announced "would the person who left their cell phone in the lavatory please push the call button". Don't know of any reports of adverse effects, although there may have been some. 4. I fly frequently -- many people are using noise canceling headsets during taxi and takeoff (including me), and I've seen many people continue to use ipods and other electronic products (Blackberry's??) during that time. Michael Hopkins Manager, Customer Technical Center Process Instruments Division Thermo Fisher Scientific One Lowell Research Center Lowell, MA 01852 Tel: +1 978 275 0800 ext. 334 Fax: +1 978 275 0850 [email protected] From: [email protected] [mailto:[email protected]] On Behalf Of Ken Javor Sent: Friday, February 16, 2007 11:29 AM To: [email protected] Subject: Re: Noise canceling headsets This is not a dissension from the group consensus that personal electronic devices (PEDs) need to be turned off during critical phases of flight, but it is a closer examination of what is really happening. The original event that triggered the action relative to PED operation was the final approach of a commercial air transport under autopilot control and reacting to a VOR or similar ground-based navigation signal. The PED interfered with the ground-based signal, and that caused the aircraft to "think" it was in the wrong place. The aircraft lurched in response to the autopilot commands, and the pilot wrested control away >from the autopilot and landed safely. The important point is that the aircraft was some distance from the airfield, therefore the level of the navigation signal received would have been attenuated by the distance, as compared to voice commands from the tower during taxi operations. The likelihood of PEDs interfering with voice commands during taxi operations seems remote at best. The real reason for prohibiting PED operation during taxi operations seems to be that it would be difficult to allow PED operation on the ground and then require them to be turned off just prior to takeoff. Compliance would be much lower. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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