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Just read quote from a recently-retired airline pilot about FAA rules that don't make sense:

Some FAA rules don’t make sense to us either.
Like the fact that when we’re at 39,000 feet going 400 miles an hour, in a plane that could hit turbulence at any minute, (flight attendants) can walk around and serve hot coffee and Chateaubriand. But when we’re on the ground on a flat piece of asphalt going five to ten miles an hour, they've got to be buckled in like they’re at NASCAR.

I'm sure there is a lawyer out there that could explain this in a way that would actually make you believe it makes sense.

It makes about as much sense as the arcane electronics rules. If the plane could actually be affected by your laptop's wifi adapter, you shouldn't be in that plane.

Electronics can and do interfere with radios (com and nav) when they are malfunctioning. Here's a real life example from about a year ago:

An RTR (transceiver) site that I'm responsible for had seemingly random interference problems on one Tower frequency. We brought in direction finding equipment and eventually happened on the source of all the problems.

An employee of a business on the airfield worked random days/hours loading aircraft. Prior to his shift starting, he sat in his car for about 20 minutes and smoked a cigar while listening to the radio. Turned out that his unmodified stock car radio system was malfunctioning and putting out spurious signals. They mixed with a TRACON frequency which created a new signal that landed on one of the tower frequencies. He could be parked up to 1/4 mile away from the RTR site, and still cause the problem.

Now imagine a malfunctioning piece of passenger electronics on an aircraft. Maybe it got dropped one time too many, yet still works fine for whatever its purpose is. Maybe it's also putting out some odd frequency that mixes with the glideslope signal and skews it just a bit.

There's probably ten thousand reasons why that won't happen. But all it takes is once at exactly the wrong time to cause a really bad day for several hundred people.

Personally, I like being able to use my iPad below 10,000' now. But professionally, it makes me ponder things that I'd rather not.

That happened because FM radio bands are very close to ATC and NAVAID bands. FM goes up to 107.9, whereas NAVAIDs like VORs start at 108.0, and ATC coms start at 118.0. It's all very close together. Cell phones and Wifi, on the other hand, are very far away in the bandwidth. Cellular bands for US carriers start at 800MHz and go up to 2.5GHz, and Wifi is around that too. So you'd have a tough time making me believe those particular devices could be a problem. If anything, the announcement should be, "please turn off your 80's boomboxes."

Unless I'm missing something...

EDIT: Your location is KMEM? Is that where this happened? I'm from the Jackson, TN area.

Yup... I work at MEM.

Intermodulation is a very odd thing, and it can be very difficult to track down the source. We got very lucky tracking down the car radio in just a couple of weeks!

When you look at harmonics, the signals don't have to be anywhere near the frequency they end up interfering with. Here's a web page with an example of an IMD calculator. http://www.arcticpeak.com/radiopages/in ... oducts.htm

Great care is taken to select a site and conduct extensive RF studies before an FAA radio site is commissioned. Airliners in flight (with random passenger electronics on board) don't have that luxury.

Hi,

I'm a HAM radio operator and that is one thing you learn about when you go to study for your exam to get your radio license. If you are actually learning the material vs just trying to memorize answers for the test you will be taught all kinds of information about harmonics. Basically it's a transmission on a direct multiple of the base frequency and it can happen over many multiples of that frequency. It happens if the transmitter is not transmitting a clean sine wave for the signal. Will it go as high as the GHz bands used in WiFi and cell phones well that really depends on the wattage of the signal. Cell phones and Wifi devices are extremely low power so it seems unlikely to me that they would cause harmonic interference at that many iterations away from the aviation frequencies.

That is very odd that a receiver was causing interference but I suppose there is a signal output in the mixer in order for it to work right so if that was off a bit I could see it happening. That must have been fun if it only happened 20 minutes at a time and only once in a while.

Rich

Hey Rich... K2ETC here!

Yeah, that was not a fun problem. The Tower wasn't pleased that we took so long to track it down, since most of the time it happened during a push.

We were actually able to clearly see the spike from his car radio (on our spectrum analyzer) from 3/4 miles away. The guy that owned the car was very helpful while we researched what was going on. Oh, yeah... he was also very surprised when a black SUV with tinted windows, antennas everywhere, and government plates pulled up behind his parked car!

Gavin

Boy am I behind at my socializing... haha..

Yeah I bet that was a little chilling for him. Radio equipment does weird things.

K2ETC de N8XMX 73!