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Letter: Bluetooth not a problem, but GSM may be in the air ...

Reader Angus Pinkerton says: "I understand your frustration about the rather vague justifications for the prohibition on the use of wireless equipment in aeroplanes. Whilst I have good reason to believe that Bluetooth transmissions are very unlikely to cause any problems, I do not have the same view about mobile telephones."

Hi Guy,

There are two main reasons that the use of mobile phones in aircraft is not permitted.

Firstly that there is a risk of interference to aircraft systems from the transmissions. This (previously theoretical) risk was confirmed by experiments carried out by the UK CAA in February 2000, when they carried out simulated mobile phone transmissions inside two aircraft.

All important electronic equipment installed on aircraft has to be designed and tested to ensure that it will not be adversely affected by a variety of extreme operating conditions, including radio frequency interference. Prior to 1984 the standard used for these tests was based on the expectation that interference was most likely from outside the aircraft, and was set at a level of 0.1 Volts/metre. There are still a significant number of aircraft flying whose types were certified before 1984 (even though the actual aircraft may have been built well after this date) and which are (and can continue to be) fitted with equipment only required to meet this standard. Calculations show that under ideal conditions the transmissions of standard (2Watt) mobile telephones can generate 0.1 V/m levels of interference at a range of 100 metres.

Although since 1984 the required standards have been raised substantially, and would now be expected to reject interference at the level of 5 V/m, the inside of a civil aircraft is anything but an ideal environment. Reflections from the metallic structure can contribute to both cancellation of the signal and considerable re-enforcement.

For this reason the CAA carried out the research described in the report "UK CAA Report on Interference Levels In Aircraft at Radio Frequencies used by Portable Telephones" published in May 2000.

The basic conclusion was that transmissions from mobile telephones used inside an aircraft are able to significantly exceed the levels that older equipment is required to reject. This does not mean that the equipment WILL be affected, but it does mean that we know that it could, and we certainly don't know that it won't. (Further research is underway to establish whether the measured levels of interference are likely to have significant effects.)

The second reason why using mobile phones in aircraft is not a good idea is that the GSM mobile phone network was not designed with airborne handsets in mind. Two aspects of the system contribute to interference which is likely to adversely affect other users of the system on the ground much more than a passenger in the aircraft. The two aspects are the speed at which the aircraft moves, and the "line of sight" range that an aircraft has.

GSM was originally specified to handle phones travelling at up to 250km/h (which is much slower than most aircraft). Although because they are flying above the cells, an aircraft's velocity towards or away from the transmitter will often be much less than this. However the aerials on the cell transmitters are designed to focus most of their power parallel to the ground, and it is thus quite likely that more distant transmitters will provide more signal than those directly underneath the aircraft. In this case the speed of the aircraft is again more important.

The altitude of an aircraft means that many more mobile phone masts are in "line of sight" than would be expected for a phone on the ground (probably all of those within the normal range of around 40km). This creates considerable opportunities for interference with adjacent cells and will be likely to reduce the effective capacity of the mobile phone infrastructure. Some information about these aspects is contained in the GSM Association FAQ's.

As far as I am aware no one has yet proved that interference has caused a significant problem aboard a civil airliner (although there is quite a bit of circumstantial evidence from Pilots' reports of anomalous equipment behaviour) but I would remind you that until the "Columbia" space shuttle disaster no one had proved that damage to the shuttle's tiles had caused a major problem either.

Best regards,

Angus Pinkerton.

Guy Kewney responds:

Angus,

Thanks for writing! - and I'm posting your letter as a contribution to the debate.

Interesting feedback. And in particular, thanks for the URL of the report ... but I did hope people would see that I'm not actually calling for the use of GSM phones on planes! :-)

What I'm saying is that we need to assess the risks sensibly, not reflexively. And the risks of GSM phones aren't really important, because (as I remarked in my piece) using a phone at 30,000 feet will irritate the heck out of the phone carriers on the ground. (I've been amazed how many readers have written to explain this to me as if I hadn't mentioned it!).

What matters, to me, is that we don't get steamrollered into a ban on WiFi on planes.

The evidence for WiFi isn't trivial; Boeing itself is planning a WiFi service as part of future planes, Lufthansa has already trialled a system, and Britannia (amongst others) is seeking CAA permission to run WiFi in aircraft. But nonetheless, a significant body of airline pilots is lobbying to have ALL PCs banned from flight cabins, in case the built-in WiFi in notebooks today causes some unimaginable effect.

If there is an effect, we should be able to work out what it is, and take steps to eliminate it. Simply saying: "I don't know what the effect is, but it might be bad!" is absurd.

I take your point about the Shuttle. But I hope you'll accept that the 100 flights that the Shuttle has done in total don't match the millions of flights done by civil aircraft? And my point was that intentionally, or accidentally, I know for a fact that a great number of those flights took off with "live" GSM phones on board! I really think the experiment is over ...

Cheers, and again, thanks for writing ...

Guy Kewney