Mobile phone interference with plane instruments: Myth or reality?

-September 11, 2012

"Please power off your electronic devices like mobile phones, laptops during takeoff and landing as they may interfere with the airplane system." - A common instruction while on board a plane. Some airlines go further asking passengers to keep mobile phones switched off for the entire duration of the flight.

However, it makes one wonder (especially an engineer) how true this could be. If electronic gadgets were able to interfere with airplane communication and navigation systems and could potentially bring down an airplane, you can be sure that the Department of Homeland Security wouldn't allow passengers to board a plane with a mobile phone or iPad, for fear that they could be used by terrorists.

Possible electromagnetic interference to aircraft systems is the most common argument put forth for banning passenger electronic devices on planes. Theoretically, active radio transmitters such as mobile phones, small walkie–talkies, or radio remote–controlled toys may interfere with the aircraft. This may be especially true for older planes using sensitive instruments like older galvanometer based displays.

Technically speaking, the more turns of wires you have around any substance (iron core, carbon core, or simply air core), the more it amplifies the force of a "radio wave's" effect upon any single electron. In other words, the radio waves from a cell phone push electrons along that coil with increasing force thus affecting the measurement.

Galvanometers have a large number of coils, and a very small guage of enameled copper wire, and are extremely sensitive to small electromagnetic stimulus. However these have been replaced by new technologies, which I would assume have good shielding. [Since large number of old planes are still in service, their tolerance to electromagnetic radiation could degrade over time unless repaired and serviced from time to time]. Yet rules that are decades old persist without evidence to support the idea that someone reading an e-book or playing a video game during takeoff or landing today is jeopardizing safety.

Another reason I found that makes the most sense was the fact that when you make a call, at say 10,000 feet, the signal bounces off multiple available cell towers, rather than one at a time. The frequent switching between cells creates significant overhead on the network and may clog up the networks on the ground, which is why the Federal Communications Commission (FCC) not the Federal Aviation Association (FAA) banned cell use on planes.

Since towers might be miles below the aircraft the phone, an additional concern could be that a phone might have to transmit at its maximum power to be received. This will increase the risk of interference with electronic equipment on the aircraft. The FCC did, however, allocate spectra in the 450- and 800-MHz frequency bands for use by equipment designed and tested as "safe for air-to-ground service" and these systems use widely separated ground stations. The 450-MHz service is limited to "general aviation" users, in corporate jets mostly, while the 800-MHz spectrum can be used by airliners as well as for general aviation.

To conclude, the fact is that the radio frequencies that are assigned for aviation use are separate from commercial use. In addition, the wiring and instruments for aircraft are shielded to protect them from interference from commercial wireless devices.

Contrary to the fact, a few airlines do allow mobile phones to be used on aircraft, however with a different system that utilises an on-board base station in the plane which communicates with passengers' own handsets (see figure).

The base station - called a picocell - is low power and creates a network area big enough to encompass the cabin of the plane. The base station routes phone traffic to a satellite, which in turn is connected to mobile networks on the ground. A network control unit on the plane is used to ensure that mobiles in the plane do not connect to any base stations on the ground. It blocks the signal from the ground so that phones cannot connect and remain in an idle state with calls billed through passengers' mobile networks. Since the picocell's antennas within the aircraft would be very close to the passengers and inside the aircraft's metal shell both the picocell's and the phones' output power could be reduced to very low levels reducing the chance for interference.

While researching this topic, I came upon a lot of interesting reasons for restricting mobile phone use on airplanes. Listed below are some of them:

  1. Airlines need passengers under control and the best way to maintain that cattle-car atmosphere might just be with a set of little rules beginning at takeoff.
  2. The barrier is clearly political, not technological. No one in a position of authority wants to change a policy that is later implicated as a contributing factor toward a crash. Therefore, it's a whole lot easier to do nothing and leave the policy as it is, in the name of "caution." (Since old airplanes with analog systems may still be vulnerable to interference, it's best to make the rule consistent.)
  3. The FCC (and not the FAA) bans the use of cell phones using the common 800-MHz frequency, as well as other wireless devices, because of potential interference with the wireless network on the ground. This also clogs the ground network since the signal bounces off of multiple cell towers.
  4. Mobile phones do interfere with airplane communications and navigation networks – trust what they tell you :).
  5. Since the towers might be miles below the aircraft the phone might have to transmit at its maximum power to be received, thereby increasing the risk of interference with electronic equipment on the aircraft. Similar to Point 4.
  6. The airlines might be causing more unnecessary interference on planes by asking people to shut their devices down for take-off and landing and then giving them permission to restart all at the same time. This would increase interference so it's best to restrict mobile phones for the complete duration.
  7. Restrict any device usage that includes a battery.
  8. A few devices, if left on, may not cause any interference. However the case may be different if 50-100 or more devices are left on, chattering away interfering with the plane communications system. Furthermore, there would be no way for the flight crew to easily determine which devices are causing the problem. So best is to restrict usage completely.
  9. If mobile phones are allowed on board, terrorists might use the signal from a cell phone to detonate an onboard bomb.
  10. Airlines support the ban on mobile usage because they do not want passengers to have an alternative to the in-flight phone service. This might have some truth to it since the phone service could be very profitable for the companies involved.
  11. Even though all aircraft wiring is shielded, over time shielding can degrade or get damaged. Unshielded wires exposed to cell phone signals may affect navigation equipment.
  12. Another reason could be to keep passengers aware of the important announcements and safety procedures from pilot and crew, which otherwise could be ignored. In addition, these devices in people's hands could cause injuries during an emergency situation and hence should be required to be switched off during landing and take-off. The idea being that since one could not operate the device, most likely, passengers would keep them away rather than holding them.

Which one do you find most relevant, or rather most funny?

In the end, it is not really an argument whether mobile phones should be allowed. The whole point is what is the exact reason for restricting their use on board?

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