Electro-ramblings

During his time at Imperial Collge, London, Prof. Igor Aleksander used to relate a sorry tale of a flight crew member coming down the aisle of a transatlantic flight with an rf sniffer probe and homing in on his Cambridge Computer Z88 word processor. Having had the Prof switch it off, the probe still registered a high signal strength from the unit. The radiation was traced to a dram chip on a proprietary battery-backed memory card used. The Prof was given the choice of losing half of the book that he was writing or the plane reaching its destination. Goodbye book.

Cheers Underdoug. It makes a good story, and it's such a sensitive subject, but were the dram chips really such a threat?

As someone who is from an avionics background I am reminded about a story from when I was in the RAF and they first used the Chinooks. Everytime the aircrew transmitted on HF the aircraft would drop in altitude. They found that the cable from the HF transmitter to the outside ariel passed next to the autopilot computer. The problem was solved by fitting armoured cable, which was thick enough to stop these internal transmissions.

The mobile phone issue is interesting.
After taking off or before landing close to a busy urban centre (where most large airports may be found) I wonder how many cells a 'phone could be trying to access?
What is the effect of so much choice? Do the base stations compete for service or does the 'phone just pick the first one with adequate signal?
I haven't dared try to find out but I'm very tempted.....

One cell phone is not going to have any influence on an aircrafts electronics, not sure what 350 or more would do.
Phones at 35000 ft do find cell towers - Only last week I saw someone desperately trying to open an overhead locker to silence a ringing phone!
The problem with this is that phones will turn up their transmit power to max to try and reach a tower - so 350 phones all at max. The answer would be to put a mini cell on the plane and then all the transmit powers would be kept at min.
Also I read somewhere that for cell phones in flight, the main problem is the "beeping" in the flight crew headphones caused by nearby cell phones - hence, I believe, the 10,000 ft limit. The same article did also mention that the phone probably needed to be on the flight deck to generate the problem, but maybe a hole plane load behind would have a similar effect

Where do I start? Whilst the blogs section is supposed to be offbeat and somewhat challenging I found the article was more sensationalist journalism than fact.
Personally I have never heard the television comment during a safety briefing, on any airline, anywhere, but that is irrelevant; as I am sure the writer knows, the TV will contain a local oscillator which, no matter how small, will be transmitting. The most common advice is “all electronics, all devices with an on/off switch must be in the off position” The main reason for this is to try to assure that in this ADD affected world, there may be at least a small chance that passengers will pay attention to both the briefing and any subsequent announcements.

The 10K feet is a recommendation from the FAA in regards to maintaining a sterile cockpit environment where outside distractions should kept to a minimum during a critical phase of flight, i.e. getting the plane away from terra firma and out of the low-level clouds and the odd mountain that might be hiding in those clouds.

However the writer makes a fairly basic mistake in his assumption of immunity testing. Whilst it is true that today’s avionics are exceedingly well tested in a number of ways, including RF immunity, that testing has not always been so rigorous. Planes flying today may be 30+ years old and that is the problem, no one was testing for emissions from 802.11 ABGN, WIMAX, Bluetooth, GSM 850,900,1800,1900, UMTS 2100 devices etc.etc. How can assure yourself that that older plane has the same level of integrity against the plethora of possible emissions that a plane today is tested for? In reality you can't and hence the reason why WIFI on board may be available on a brand new Airbus A320 but not on that older Boeing 737-100.

Regarding cell phones in particular, most of them step-up their power output if they fail to find a base station and a cell phone switched on is always operating and transmitting. The only thing ‘asleep’ is the display and user interface.

The penultimate paragraph again shows a lack of clarity in thinking. Whilst it is true that a device designed by a competent engineer and put through a set of rigorous tests (CE, FCC, UL, VDE etc.) will most likely be safe, there is an increasing trend to a lot of cheap, consumer oriented products being designed, tested and manufactured in parts of the world where testing, either during design or production is deemed optional and where putting a CE mark on a device is as simple as peeling a label. There is a flood of poorly designed, substandard electronics coming from China….

If I was writing the article, my conclusion would have been slight more rational…. “Whilst there is very little evidence that consumer electronic devices have interfered with navigational and communications systems on modern aircraft, it would be prudent to mitigate the risks of interference by assuring that all possible transmitters, however miniscule, are turned off during the critical phases of flight.”
Oh and a 10W transmitter is really not that much, check out the power output of the transponder and DME in a large commercial aircraft….

My credentials – 400+ hour instrument pilot, electronics engineer working in Test & Measurement, over one million miles on American Airlines and over 300K miles on EVA Airlines....

My interpretation is that while the plane's electronics _should_ be ok, there is always the potential for interference at close range to a transmitter of any sort. Older planes in particular may not have been tested against GSM-type pulsed transmissions.

We're all familiar with the brrrp... de... brrrp... de... brrrrppp of using a mobile phone too close to audio equipment. Further, two different battery powered clocks I own will reset if a mobile phone is used within a few inches of them, and one will even change language (an undocumented feature). My electronic kitchen scales will give wildly fluctuating readings if a mobile phone is used within a few feet of them. So interference can and does happen.

With mobile phones on planes, there's also the consideration of the network infrastructure - from the air a handset can 'see' a plethora of base stations simultaneously and surely could potentially 'jam' that frequency for a large number of base-stations (frequencies are normally reused over quite short geographical distances). This is very different to the case on the ground where the range is little more than line of sight.

I would guess that the mystery height (perhaps 10000 feet) below which use of electronic equipment is banned has more to do with the time available to recover control of the plane _if_ something went very wrong, than of any change in susceptibility to interference with height.

Further, televisions and radio receivers can leak RF radiation, especially from their local oscillator, acting as a low-power transmitter with a range of a few metres. You can verify this for yourself by taking two portable VHF/FM radios, placing them side by side, and tune them 10.7MHz apart: you will normally find that the one at the higher frequency causes a quiet spot in the tuning of the other at the lower frequency.

Anything which transmits (intentionally or in spurious emissions) on a frequency used for reception of communication by the aircraft cannot, by definition, be totally screened out by design if there's any chance of it reaching the plane's aerials.

Three comments ( already made elsewhere ) :
1. the TV probably has an LO to mix down the TV signal so it could be causing some emc.
2. research shows that in most flights there are several phones etc left on accidently in any case.
3. the altitude limit may be to do with NOT loading networks below the plane with a block of phones moving at very high speed ( cellular netwroks were designed for fast moving cars, not much faster aircraft ).

"televisions are NOT transmitters"

Yes they are, all electronic devices radiate RF signals and could potentially jam the reception of signals in the immediate vicinity.

Basically it's down to the fact that no-one has done any tests on the electronic equipment to prove beyond doubt that they are safe on aircraft so they go for the easy and litigation free option of banning the equipment.

Its a great pity that Joel Young doesn't know anything about EMI or EMC. If he had, he would never have made the statement "For those of us that design electronics, we know that they need to meet very strict electromagnetic emissions and immunity guidelines precisely so they CAN'T interfere with other electronics and so other electronics can't interfere with them."
If you are interested in how to properly deal with EMI/EMC for safety reasons, visit www.theiet.org/factfiles/emc/index.cfm and download their 2008 guide.

I thing Anonymous put it very well about the flight plan.

I connection with the GSM phone.

"Hand over" from one site to another usual takes place with two pieces of information,
1) When the existing signal level is 6dB lower than the nearest site.
2) This will have to last for about 6 seconds.

So I would say that if a phone was in use on take off it would hand over about 6 to 8 times before hitting 300 feet.