Radiocommunications Agency  EMC Awareness

Aviation/Aerospace

Why cellphones are banned during airflights

Video Walkman interferes with critical instruments

Why cellphones are banned during airflights

Description

In October 2002, a set of avionic equipment was tested under controlled conditions in a test chamber for susceptibility to cellphone interference. General avionic equipment, representative of earlier analogue and digital technologies, was used. The equipment, comprising a VHF communication transceiver, a VOR/ILS navigation receiver and associated indicators, together with a gyro-stabilised remote reading compass system, was assembled to create an integrated system.

The tests covered the cellphone transmission frequencies of 412MHz (Tetra), 940MHz (GSM900) and 1719MHz (GSM1800), including simultaneous exposure to 940 and 1719MHz. The applied interference field strengths were up to 50 volts/metre for a single frequency, and 35 volts/metre for dual frequencies.

The following anomalies were seen at interference levels above 30 volts/metre, a level that can be produced by a cellphone operating at maximum power and located 30cms from the victim equipment or its wiring harness.

• Compass froze or overshot actual magnetic bearing.

• Instability of indicators.

• Digital VOR (VHF Omnidirectional Ranging, an aeronautical navigation aid using the VHF spectrum) navigation bearing display errors up to 5 degrees.

• VOR navigation To/From indicator reversal.

• VOR and ILS (Instrument Landing System, an aeronautical navigation aid using the VHF spectrum) course deviation indicator errors with and without a failure flag.

• Reduced sensitivity of the ILS Localiser receiver.

• Background noise on audio outputs.

Most anomalies were observed at 1719MHz.

The results of the tests endorse current policy that restricts the use of cellphones in aircraft.

The CAA will remind operators about the specific risk from cellphone usage on the flight deck, and recommend that confirmation be obtained from passengers at check-in that cellphones in their luggage have been switched off.

Commentary

Cellphones are an example of a Passenger Electronic Device (PED) that pose particular problems for aircraft because of the relatively high powers of their RF emissions compared to other PEDs (such as a video walkman or laptop computer, discussed elsewhere).

Much of the electronic equipment for the avionics instruments is located under the floor of the passenger compartment, which may not provide any shielding. Certain seats in the passenger cabin of an aircraft will be closer to certain avionic equipment, and/or may be closer to antennas located outside the aircraft. So cellphones may be more liable to cause interference when they are used by people sitting in those seats.

Whereas the weak emissions from most PEDs are most likely to cause interference when an emitted frequency coincides with a frequency already being used by radiocommunications or navigation systems, the more powerful emissions from a cellphone are capable of causing interference in electronic circuits, if the phone is close enough, through demodulation in non-linear devices such as transistors, diodes, surge protectors, ICs, etc.

Many aircraft flying today use avionics navigation and communication systems designed well before cellphones became commonplace, and as a result they may lack the necessary immunity.

References and links

Effects of Interference from Cellular Telephones on Aircraft Avionic Equipment, CAA Paper 2003/3, http://www.caa.co.uk/docs/33/CAPAP2003_03.PDF. The CAA is the UK’s Civil Aviation Authority.

For the above report and many others on aircraft interference issues, including those relating to cellphones, go to the CAA homepage at http://www.caa.co.uk and click on ‘Search’, then type ‘interference’ into the search box (also try searching by ‘cellphone’ or ‘PED’).

“Phones in flight can be fatal”, IEE Review, June 2003, page 16, from sales@iee.org.uk, http://www.iee.org/shop or http://www.iee.org.uk/Library.

Items numbered 447036, 460415, 467979, 487546, 495128, 504194 and 524699 in the NASA Aviation Safety Reporting System (ASRS) report dated 1st May 2002. Download the report from: http://asrs.arc.nasa.gov/report_sets/ped.pdf.

Other examples of cellphone interference with aircraft can be found in the “Banana Skins compendium”, via a link from www.compliance-club.com or at: http://www.compliance-club.com/archive1/Bananaskins.htm, especially (at the time of writing) numbers: 112, 113 and 187.

Links to Mitigation techniques

Clearly, ensuring that all cellphones carried onto an aircraft by a passenger, or in their hold baggage, are switched off is a good mitigation technique. But guaranteeing that nobody forgets or uses their cellphone during take-off, flight or landing requires the fitting of ‘cellphone detectors’ that sound an alarm if they detect the characteristic transmissions from cellphones in stand-by mode or in use. At least one company makes such detectors. It would clearly be better to check hold baggage in this way before it is loaded into the hold of the aircraft.

Designing avionics systems that are less susceptible to interference, without adding too much weight in additional shielding and filtering, requires the full range of EMC-by-design techniques.

	Installation	Design & Development	Resources
Circuit design for reducing demodulation
PCB layout for EMC
Shielding of areas and volumes
Shielding of cables
Filtering with CM cable-mounted chokes
Filtering

Video Walkman interferes with critical instruments

Description

Following take-off from ‘BDL’ airport in a DC9 in July 1999, whilst climbing to cruising altitude, the captain’s radar altimeter gave an error message (‘flagged’) and the traffic and ground proximity alert systems subsequently showed ‘FAIL’. These three critical instrument malfunctions were independently verified by two other flight crew.

The problem continued throughout the climb to 35,000 feet when it became possible for a flight attendant to check the passenger cabin to see if any passenger portable electronic devices were in use. She discovered that a Video Walkman was in use in seat XX. After it was switched off, the problems cleared up.

Commentary

Radiated emissions from digital processing products (such as computers, portable video/DVD players, etc.) occur at discrete frequencies – the harmonics of its internal ‘clock’ oscillators. If these emitted frequencies coincide with the frequencies used by various navigational instruments, such as radar altimeter, they could cause interference. The emissions could come directly from the product’s printed circuit board or the devices mounted on it, but are mostly emitted by leads attached to the product (such as headphone leads) acting as ‘unintentional antennas’. Closer to the product, the emitted fields are more intense.

Much of the electronic equipment for the avionics instruments is located under the floor of the passenger compartment, which may not provide any shielding. Certain seats in the passenger cabin of an aircraft will be closer to certain avionic equipment, and/or may be closer to antennas located outside the aircraft. So portable electronic devices (PEDs) may be more liable to cause interference when they are used by people sitting in those seats.

One would usually expect equipment that meets emissions standards such as required for CE, FCC or VCCI marking not to cause interference, but not all equipment complies with these standards, and sometimes a ‘bad batch’ could get manufactured which had higher emissions than expected.

References and links

NASA Aviation Safety Reporting System (ASRS) report dated 1st May 2002, download from: http://asrs.arc.nasa.gov/report_sets/ped.pdf. This example is No. 443329 in the ASRS report.

There are many other examples of suspected Passenger Electronic Device interference problems in the ASRS report, and there are also numerous examples and useful links in the next reference.

“Electromagnetic Interference with Aircraft Systems: why worry?” article ref: RVS-J-97-03 by Peter B. Ladkin and colleagues, University of Bielefeld – Faculty of Technology, Networks and Distributed Systems, Research group of Prof. Peter B. Ladkin, Ph.D., at: http://www.rvs.uni-bielefeld.de/publications/Incidents/DOCS/Research/Rvs/Article/EMI.html

“Avionics and Portable Electronics: Trouble in the Air” by Albert Helfrick, presented at the Aircraft Electronics Association Annual Convention, Nashville, TN, 1996. Avionics News Magazine, September 1996. Available from http://bluecoat.eurocontrol.fr, Public Reports.

Links to Mitigation Techniques

	Installation	Design & Development	Resources
Circuit design for emissions control
Filtering with CM cable-mounted chokes
Filtering
Shielding of area and volume
Shielding of cables