Please make a final check that your mobile phone and other electronic devices are switched off before fastening your seatbelt...

We all know we're supposed to turn off our mobile phones before boarding an aircraft. The concern is that the phone's emissions could interfere with the aircraft's vital systems, this being considered a bad thing. People however, being as they are, often re-interpret the instruction to suit themselves, along the lines of:

• Leave everything on but turn off the ringer and don't make any calls.

• Leave it on, feign forgetfulness and apologise profusely when it rings mid-flight.

• Leave it on by accident in spite of all the warnings.

• Leave it on, use it in full view of the cabin staff, and kick up a fuss when they object.¹

Studies in the US, whereby equipment capable of detecting phone emissions has been hidden in overhead lockers, have revealed that a disturbing number of passengers leave their phones on throughout the flight. Most of us have probably heard the characteristic jingle of a mobile being switched on during the eerie silence just before the plane touches down. Presumably the plane is low enough by that time that it would just bounce if anything serious went wrong.

The Risk

So, is putting the lives of hundreds of people at risk for the sake of a phone call the absolute pinnacle of human arrogance? Are some passengers capable of predicting the complex interplay of electromagnetic interference in real time and mitigating against it? Or do they have a point; is the risk really non-existent?

Well no, they don't have a point; the risk is real. Mobile phones have the potential to interfere with the operation of sensitive electronic equipment. That much is known. Modern aircraft are jammed full of sensitive electronic equipment², much of which is 'safety-critical³' including navigation, aircraft control and communications equipment. In fact, most commercial airliners are now flown almost exclusively by computer. There is an extensive list of laboratory studies showing that mobile phones can and do interfere with the operation of this equipment. There are also frightening lists compiled by the Civil Aviation Authority (CAA) in the UK and Federal Aviation Administration (FAA) in the US, containing reports of cell phone-related 'incidents' on board real aircraft. That much is also known.

You might think that, given the horrific consequences, the slightest possibility of an air crash would far outweigh any benefit to be had from making a few in-flight phone calls. You might think that most people would be happy for aircraft to remain the last mode of transport to be utterly free of inane cellphone chatter. You might think that while trapped within a metal tube you don't really need a mobile phone, and that fixed seat-mounted conventional telephones connected to the ground network via the aircraft's own communications systems, as pioneered for some time by certain airlines, would suffice.

You would be wrong. There is a lot of momentum within the airline industry aimed at allowing in-flight calls. Business people could use this previously 'lost' time to do business, holidaymakers could ring ahead to say their flight is late, and drunken stag parties could have endless fun texting each other over the most attractive stewardess. People are doing it anyway, there is currently no technical way to stop them, and there is massive profit to be made from it. In addition, it has never been proven conclusively that any deaths have resulted from cellphone interference. Consequently, there are several schemes in progress aimed at enabling passengers to use their mobiles.

The Technical Stuff

All electrical and electronic devices emit electromagnetic radiation to a degree. This radiation is capable of being picked up by any other device within range. Whether the radiation is capable of actually interfering with the operation of the second device is dependant on the frequency and amplitude (strength) of the signal. A strong enough interference signal could manifest itself as noise over a voice channel leading to a misinterpretation by the pilot say, or jam a navigation receiver so the plane doesn't know where it is, or it flip the state of 1s and 0s in a digital system causing it to behave erratically, give false warnings, or crash altogether.

For example, it has been found that a GSM⁴ and CDMA⁵ phone in close proximity will interfere with each other creating a signal at an entirely different frequency – a mutual interference product – that is very close to the band used by the aircraft's GPS and Distance Measuring Equipment (DME). A particular cellphone model, equipped with a built-in GPS receiver, was found to consistently cause an aircraft's GPS receiver to lose its signal entirely when switched on.

The study and prevention of such interference is called Electromagnetic Compatibility, EMC, and it is a notoriously difficult field. The number of combinations of devices, relative locations, operating modes and frequencies that have the potential to interfere with other systems is essentially infinite. It is impossible to test them all, and almost impossible to predict the effects. Mobile phones are particularly bad as they don’t just emit random unwanted noise; they are proper, intentional radio transmitters.

Of course aircraft systems are tested for immunity to interference, and systems that have entered service since the early 1990s have a higher threshold than older ones. But modern mobiles are still capable of transmitting at higher power than even these new systems are tested against. That situation may change in time, but there is a finite limit to what it is economically viable to achieve. Higher levels of immunity generally require heavier shielding, and by the time you have added this to hundreds of miles of wiring as well as the equipment boxes themselves, the aircraft has become much heavier, using more fuel and with a reduced range.

Just How Risky Is It?

That depends on whom you believe, but it is widely acknowledged that the risk of cellphone interference endangering an aircraft is very, very low. Aircraft are designed to cope with equipment failures – critical systems have back-ups for example - and the flight crew can often take over when systems fail. But in aviation very, very low is not good enough. Given the huge number of people flying these days, a million-to-one incident would be almost certain to happen in the space of just a few days – and the number of lives lost could be in the hundreds.

A NASA study revealed 86 malfunction incidents on aircraft had been linked to passengers using electronic devices inside the cabin - more than a quarter of these were attributed to mobile phones. The CAA attributed 35 incidents between 1996 and 2002 to mobile phone use. These incidents were all 'caught' in time. However, many air crashes are a direct result of pilot confusion, often precipitated by erroneous or conflicting information supplied by air traffic control or instruments. Temporary failures caused by interference are particularly confusing as it may not be obvious that the equipment has 'failed', and it may return to correct operation as soon as the interference is removed. The two nightmare scenarios are:

• During take-off or landing, where many of the systems (landing gear, engines, flaps, brakes, instrument landing systems, radar, collision avoidance amongst others) need to be 100% operational and there is no spare time to spend working around a problem.

• Where a critical system malfunctions, giving erroneous results, but appears to be working correctly.

Has an Aircraft Actually Crashed Because of Mobile Phone Emissions?

Well, the most accurate answer is that we don't really know, but the short answer is that it probably has. On the night of Friday 6 February, 2003, a Piper Navajo Chieftain light aircraft was on approach to Christchurch, New Zealand. It was dark, and the weather was poor, so the landing was using instruments only. Some 2km short of the runway, the plane flew into a tree, killing the pilot and seven passengers. Only two survived.

Later analysis revealed that the plane had been below the correct glide path for the runway, and that the instruments were telling the pilot to descend. The instruments had malfunctioned and the pilot had, understandably, continued to follow their instructions, being unaware of the malfunction. It transpired that the pilot had made a call on his mobile just before the glide path signal was acquired. The call ceased when the plane crashed. Although the final report was inconclusive, no evidence was found to support any other theory for the crash.

More Problems

Much of the objection to the use of mobile phones on aircraft has nothing to do with safety, but relates to the potential effect on the phone network. A cellphone works by registering itself with nearby base stations, a number of which form the periphery of a cell. It does this regardless of whether it is in use.

As the number of lines available in a particular cell is limited, the cells are sized according to the predicted number of simultaneous users. Inner-city cells are smaller than rural ones, based on the likelihood that there will be a greater demand for lines. Consequently, there are more cells per unit area in cities than elsewhere. It is also worth noting that the line-of-sight link from a mobile phone to a particular base station in a city is likely to be obstructed by buildings.

An aircraft could be carrying 500 cellphones. While passing directly over a city and thus unhindered by buildings, these phones could be in the line-of-sight of hundreds of base stations and could try to register with all of them. This would impose a temporary but extreme load on the network. The speed of the plane passing over the small inner-city cells would also result in an unusually rapid handover from cell to cell, possibly far in excess of the network's design limits.

Ironically, aircraft nearly always fly low over cities on approach, as this is where the airports are located. For this reason, the use of cell phones on aircraft is currently banned by the US Federal Communication Commission (FCC) as well as the FAA.

Yet another concern that has come to light in recent years is the possibility of a terrorist using a mobile phone to remotely trigger a bomb located in the passenger compartment or baggage hold.

Picocells

Aircraft manufacturer Airbus has carried out extensive tests on board an A320, using an on-board mobile base station called a 'picocell'. Airbus estimates that the system could be introduced in 2006. The idea is that the aircraft becomes its own 'cell', and phones will connect to this in preference to ground-based stations, thereby limiting their output power to only that required to communicate with the picocell and reducing the possibility of interference correspondingly. Calls are then routed via satellite to the ground network.

There are problems though: it is not clear how the picocell would prevent phones from attempting to connect to nearby ground stations whose signal would be naturally stronger; if the picocell was to fail you'd have a plane-load of phones all going to maximum power at once in an attempt to seek out a new cell; and a mobile cell may have adverse effects on the ground network.

Certainly, shielding an aircraft to contain the emissions is unlikely to be an option – the shielding material would probably add too much weight. In fact, modern aircraft built from carbon composites are less well shielded than the traditional aluminium-tube designs.

But the airlines are forging ahead regardless, falling over themselves to gain every tiny advantage in this extremely competitive business. Those planning or investigating picocells include American Airlines, United, TAP Portugal, BMI and Lufthansa. The European Commission has lent some support for the technology, citing improved security as justification. Crucially, the recommendations still forbid mobile use below cruising altitude, thereby getting round most of the picocell problems. TAP Air Portugal and BMI have announced a trial service using the OnAir system developed by Airbus and Dutch company SITA, to take place late in 2006. In this case, phones must still be switched off below 10,000ft. All these trials and technologies are, however, subject to approval by the aviation authorities in every country in which they intend to operate.

The authorities themselves, effectively led by the FAA in the US, currently have no intention of lifting the ban. The FAA has hinted that approval would, as a minimum, require every single potential model of phone to be tested for safety on each type of aircraft. But commercial interests have a habit of winning through eventually, given enough funding and momentum. These companies would not be spending money on this technology if they didn’t believe it could be made safe and approved by the authorities. Clearly the aviation business does not take risk lightly, but it seems that use of mobile phones on aircraft is inevitable. The question is, would you want to take that risk?