In the past three years, there has been a re-evaluation of the industry attitude about potential causes and effects of Electromagnetic Interference (EMI) transmissions due to passenger "carry on" electronic devices. A Federal Advisory Committee, the Radio Technical Commission for Aeronautics (RTCA), has been commissioned to re- examine the cause and effect of EMI via Special Committee 177 (SC 177). Their report of findings was due in July 1994. However, it may be some time before the RTCA committee recommends new procedures for addressing EMI which makes it appropriate now to review reported events.
Although the number of events that have been reported to be EMI related are extremely low compared to other reportable operational occurrences, the potential consequences of EMI are of concern. A search of our Safety Information System (SIS) reveals 97 such events since 1983. A large number of erratic Omega indications believed to be due to EMI were experienced during 1984 and 1985 which resulted in the formation of committees to analyse EMI.
Studies indicated that emissions from the early type of hand-held recording units, radios and hearing aids were believed not to be a threat to the onboard navigational equipment. Software changes were made, nonetheless, and the number of reported EMI events decreased. By 1990, however, the number of people boarding aeroplanes with electronic devices had grown significantly and the low-voltage operation of modern aircraft digital electronics were potentially more susceptible to EMI. A look at the data during the last ten years indicates that the most likely time to experience EMI emissions is during cruise flight. This may be misleading, however. During the last three years, 43 percent of the reported events occurred in cruise flight while an almost equal percentage of events occurred in the climb and approach phases.
When EMI is suspected or known it is important to identify the specific type of equipment which is causing the interference. In the United States cellularphones are licensed by the Federal Communications Commission for land-mobile operation only. If used in flight, a cellular phone can disrupt aircraft equipment and could disrupt ground users over a large area. Of particular note: during the last three years the number of events relating to computers, compact disc players, and phones has dramatically increased and these devices have been found to more likely cause interference with systems which control the flight of the aircraft.
Recognising an apparent instrument or autopilot malfunction to be EMI related may be difficult or impossible in many situations. In some reported events the aircraft was offcourse but indications in the cockpit displayed on course. Air traffic controllers had to bring the course deviations to the attention of the crews. It is believed that there are EMI events happening that are not recognised as related to EMI and therefore not reported.
Events are on the rise
All phases of flight are exposed (not just cruise)
Often there will be more than one device on a flight
Passengers will turn on a device even after told to turn it off
Passengers will conceal usage of some devices (phones,computers)
Passengers will turn devices on just after take-off and just prior to landing
Phones are a critical problem
Specific device type and location should be recorded and reported by the crew
When the emitting EMI device is shut off, the aircraft systems return to normal operation.
(In the case of positioning errors a course change may be necessary)
Flight attendants should be briefed to recognise possible EMI devices
Craig S Richesin
McDonnell Douglas - Flight Operations Dept
NUMBER / ORIGIN / DATE : 002 / E-mail / 250697
(Also B737 Tech - Ed)
In my experience the first sign of EMI aboard a B737 is a Symbol Generator failure of either pilots' EFI. Fortunately I haven't had BOTH go tits up at the same time. In the 3 SG failures I've had in flight (usually at night while up to my armpits in thunderies), its because some clod has been listening to a CD player. After swapping over to the useable SG,ding for the girls and ask them to look for who's operating an electronic device. Follow this up with a rather aggressive PA after they get back to you. This will hopefully stop other undetected devices.Should the SG failure be of EMI cause, you can bet you'll get your SG back after you reverse the drill. On the last SG failure I had, 3 Hong Kongers were yacking on their 'ha-rrones', another was trying to connect his laptop to the internet using a radio-modem, and 2 others were hoppin' and boppin' to a CD of Jackie Chan songs! I suppose even a SG gives up when it hears Jackie Chan singing!
I have been approached to help with a survey of electrical anomalies that might be influenced by external natural events, in particular the Aurora Borealis. Other factors may be solar flux, and meteorite ionisation related.
To compile as much data as possible, we are asking for reports to include the following:
1- Date and Time
2- A/C Type
3- Rough Geographic Area (EG - North Germany)
4- Short account of event (EG - FMS Lockup for 5 mins, FMS independent operation, map shift about 5km east, etc, etc)
All the received reports will go confidentially, and will be compared to flux data supplied by NASA. The results will be posted on this page later, with a resume of the findings.
Can you please send your reports to me (airnig), or directly to:
NUMBER / ORIGIN / DATE : 003 / E-mail / 280498
Do Portable Electronics Endanger Flight?
By Tekla S. Perry and Linda Geppert
Report on new studies into the effects of PED's or Portable Electronic Devices when used on board aircraft.
The business travellers on the airplane continue to tap on their laptop computers, oblivious to the loudspeaker. A teenager, plugged into his Gameboy with earphones, shoots down another space alien. A couple sharing a portable CD player daydream to their favourite opera. Eventually, prompted by flight attendants making a final check down the aisles, they all grudgingly turn off what are being called portable electronic devices (PEDs).
Except you. You're sitting by a window, across the aisle from the teenager, and somehow the flight attendant fails to notice that your computer is still running. "Good," you think, "I am right in the middle of drafting this presentation. At least I can finish one more slide. It's a silly rule, anyway. My computer is shielded, I'm sure any avionics are shielded, and the aluminium skin of the aircraft is a shield as well. My computer won't do any harm."
Maybe not. Maybe not your computer, in this seat, on this particular airliner, on this flight. But can a computer or other PED, in the right place, on the right airplane, seriously affect the airplane's instrumentation or communications? You bet.
"If it were up to me, I would shut PEDs down, period," Finbarr O'Connor, electromagnetic compatibility manager of R&B Enterprises, told IEEE Spectrum, the magazine of the Institute of Electrical and Electronics Engineers. R&B Enterprises is a testing, training, and consulting company that specializes in electromagnetic compatibility.
"I would feel better," O'Connor says, "if they were not allowed in the passenger compartment at all. They should be stowed. The potential for them to be turned on accidentally is high. I have no doubt that PEDs have the potential to affect avionics." O'Connor's opinion counts. He is a member of Special Committee 177 (SC-177) of RTCA Inc., a non-profit organisation that has been charged with investigating the effect of PEDs on avionics systems, and he chaired that body's Test Procedure Subcommittee.
Interference under scrutiny
RTCA, formerly called the Radio Technical Commission for Aeronautics, recommends standards and offers guidance to the aviation industry. Currently, most airlines in the United States and elsewhere voluntarily follow an RTCA recommendation issued on Sept. 16, 1988, that prohibits the use of PEDs during takeoff and landing. That recommendation was issued mostly to lessen any possibility of interference with aircraft avionics, but also to reduce the chance of passengers being injured by PEDs that might bounce around on a flight and to prevent passengers from being distracted from safety announcements.
A new study that RTCA has been working on marks the organisation's third visit to the issue of interference from portable electronics. (The first time was in the 1960s.) In a draft report now wending its way through a lengthy approval process, SC-177 has made recommendations concerning the use of PEDs. Spectrum obtained a copy of this draft, which is being circulated to members of the aviation industry.
Until recently, evidence of the danger of PEDs has been almost exclusively anecdotal. But, says O'Connor, the RTCA committee has now developed a reporting system. Feedback has been gathered from pilots who were given forms that they could use to report directly to SC-177 incidents of suspected interference.
The committee asked flight crews to verify the cause-and-effect relationship between a PED and the apparent interference if they could do so without compromising safety. This they did by watching the affected instruments while the device was turned on and off and then repeating the experiment later, either by moving the device to another location in the aircraft's cabin or by operating it in another mode.
In addition, the US National Aeronautics and Space Administration maintains a database of safety incidents of all types in its Aviation Safety Reporting System. From January 1986 through June 1996, ASRS collected voluntarily submitted reports on almost 69,000 incidents of all types. Flight crewmembers submitted most of the reports. In two searches of the ASRS database (from January 1986 to June 1994 and from June 1994 to June 1996), NASA researchers turned up 62 reports in which such words as "passenger electronics," "laptop," or "tape player" occurred. In 52 of these (slightly less than 0.08 percent of the total), the flight crew suspected that a portable electronic device operated by a passenger was interfering with aircraft systems.
The reports vary widely in the amount of correlation between the PED and the interference. The weakest correlations are those in which the flight crews who observed an anomalous instrument reading were aware of PEDs on board and suspected them of causing interference.
Roughly half of the reports show a slightly stronger connection, in that abnormal instrument readings returned to normal when the PEDs were turned off. In the strongest reports, the problem reappeared when the device was turned back on. According to the RTCA draft, out of the 33 reports that SC- 177 received, six were of the on-off-on variety.
A report selected from the ASRS database illustrates this type of incident. In March 1993, a large passenger aircraft was at cruise altitude near Dallas/Fort Worth International Airport when the No. I compass suddenly precessed 10 degrees to the right. The first flight attendant was asked to check whether any of the passengers were operating electronic devices. She reported that the passenger in seat X had just turned on his laptop computer. The report continues: "I asked that the passenger turn off his laptop computer for a period of 10 minutes, which he did. I then slaved the No. I compass, and it returned to normal operation for the 10-minute period. I then asked that the passenger turn on his computer once again. The No. 1 compass immediately precessed 8 degrees to the right. The computer was then turned off for a 30-minute period, during which the No. I compass operation was verified as normal."
The report states that it was evident to all on the flight deck that the operation of the laptop computer was adversely affecting the operation of the No. 1 compass. It concludes: "I believe that the operation of all passenger-operated electronic devices should be prohibited on airlines until the safe operation of all of these devices can be verified."
Guilt by radiation
What is going on here? The culprit is electromagnetic emissions from the PEDs, which interfere with avionics systems, most commonly radio navigation and communications. Co-conspirators are the aluminium air-frame, which can act as a shield, a resonant cavity, or a phased array, and the sensitivities of the avionics. The radiation from the devices can couple to the avionics through the antennas, the wiring, or directly into the receiver. According to some experts, avionics should bear some of the responsibility for eliminating the interference problem. Systems should be designed to better withstand interference from PEDs.
In general, manufacturers of affected devices have a responsibility to design immunity into their products, according to Bennett Kobb, editor of Spectrum Guide: Radio Frequency Allocations in the United States, 80 MHz-300 GHz, and contributing editor to Telecommunications Reports. "There can be substantial differences in the level of interference immunity between what is technically possible, what is cost-effective, and what is reasonable for policy makers to expect from manufacturers," he told Spectrum. Avionics manufacturers did not respond to Spectrum's requests for comments.
The PEDs operate at frequencies from a few tens of kilohertz for AM radios to 133 MHz for laptop computers. When the harmonics of these signals are taken into account, the emitted frequencies cover almost the entire range of navigation and communication frequencies used on the aircraft. The frequency and intensity of the radiation also depend on what mode the device is being operated in.
To complicate matters further, different types of avionics have different sensitivities, according to Al Helfrick, associate professor in the engineering technology department at Embry-Riddle Aeronautical University. Daytona Beach, Fla. A radiation source may cause total destruction of a navigation signal on one channel while nearby channels are completely unaffected. Another type of receiver may be sensitive to the modulation of the signal or to the number of individual radiators.
The frequency bands that avionics systems use dot the electromagnetic spectrum from a few kilohertz to several gigahertz. At the low end, omega navigation, which is used to determine aircraft position through groundbased transmitters, operates in the frequency range of 10-14 kHz. From 108-118 MHz is the VHF omnidirectional range finder (VOR), a radio beacon that is used to navigate from point to point.
At 328-335 MHz is the glideslope system used during landings. And above I GHz is the distance-measuring equipment (DME), which gauges the space between the aircraft and a ground-based transponder and which is used throughout flight, from take-off to landing. Also in the spectrum above I GHz are collision avoidance, global positioning, and cockpit weather radar systems.
Most at risk among these systems are those that have antennas located at various points outside the skin of the aircraft to pick up the navigation and communication signals. "Those are the instruments that we cannot harden because they are built to receive very small signals," says Dave Walen, manager of electromagnetic effects for Boeing Commercial Airplane Group. "We rely on those sensitive receivers to pick up small signals in space, and that is the primary concern we have with carry-on electronic devices."
Once the antennas have picked up the signals, they run through coaxial cables to communications or navigation receivers generally located below the floor of the cockpit, explains Todd Degner, manager of avionics engineering at American Airlines. The output of those receiver boxes then goes to cockpit indicators or to other computers in the plane, or both.
Most navigation signals, for example, go to a cockpit indicator and also to the autopilot computers. The wires that connect the receivers to the indicators or computers are twisted, shielded pairs, or twisted, shielded triples, depending on whether the signal is digital or analogue.
Often the wires from the antennas to the receivers run along the fuselage inside the aircraft skin, passing less than a metre from a PEDwielding passenger. The thin sheet non-conducting material that forms the inside of the passenger compartment, typically fibreglass, offers no shielding whatsoever between the PED and the wiring.
Boeing's Walen confirmed to Spectrum that wires critical to the functioning of the aircraft are generally shielded. At American Airlines' Degner believes that because the cables are so well shielded most of the interference from PEDs is due to radiation that the antennas pick up, and then transmit to the cockpit instruments or the navigation computers.
Shielding could be damaged during servicing or could degrade over time. The effectiveness of shielding also depends upon good grounding. This is difficult to maintain over time because of the nature of aluminium's surface chemistry: aluminium oxidizes rapidly in air, thereby increasing the resistance of the electrical connection to ground. In that case, the wires could pick up interfering signals directly.
Even with shielding in mint condition, electromagnetic interference can still couple to the aircraft's navigation or communication systems. Although the aluminium skin of the aircraft forms an excellent electromagnetic shield, it has holes through which the radiation can escape. In airliners, the biggest holes are the windows.
To further compound the problem, the aircraft's aluminium skin is essentially a resonant cavity. In the presentation by Embry-Riddle's Helfrick on aircraft attenuation at the 13th Digital Avionics Systems Conference (held 30 Oct - 3 Nov,1994, in Phoenix, Ariz.), he showed that depending on the frequency that the PED emits, in certain locations or hot spots, in the aircraft, the device can create standing waves. The result is a signal intensity that is even larger than it would be in free space. To get to the bottom of the issue, Helfrick had measurements performed on 20 aircraft, from light general aviation aircraft to large transport airplanes. A spectrum analyser and a horizontally polarised biconical antenna were placed on the ground 30 to 50 metres from the centre-line of the aircraft. Inside the airplane, a signal generator was turned on and moved around until the point of least attenuation was found. Horizontal and vertical polarisations for more than 10 frequencies were measured.
These measurements were then repeated, but without the aircraft present. (The aircraft was towed away and the signal generator was placed on a pole about 3 metres above the ground, where the centreline of the airplane had been.) Helfrick defined the attenuation as the ratio of the received signal with the radiator inside the aircraft to the level received with the aircraft removed.
"Some measurements, particularly frequencies below the VHF navigation band, actually produced gain." Helfrick reported. From these lower frequencies, "the windows become a phased array; you just have to put your radio or whatever into the right spot." Because of these hot spots, signal attenuation can vary greatly from seat to seat.
For frequencies in the VHF navigation band and higher, Helfrick found that the smallest attentuation occurred with the radiator at the window, "and for all practical purposes, the attenuation was 0 db."
SC-177, however, rejected Helfrick's measurement technique and chose instead to measure the attenuation (or path loss) at the receiver in its new investigation. Thus, the committee lumped the attenuation of the antenna and its wiring together with that of the fuselage.
Emissions from Portable Radios
To protect avionics systems from interference, intentional radiofrequency (RF) emitters like CB radios, remote-controlled toys, and walkie-talkies are banned outright on commercial airline flights.
Most, but not all, airlines extend the ban to portable radios and television receivers. Although those devices are not intentional emitters, the local oscillators of AM radio receivers produce signals in the 1 MHz range, and of TV sets, up to 800 MHz. FM receivers typically generate signals from 98.7 to 118.7 MHz, covering the entire VHF navigation band.
One incident reported to ASRS involved portable radios. In January 1993, on a flight from Denver, Colorado, to Newark, NJ, the aircraft lost all directional gyros (electromechanical devices that indicate orientation) at cruise altitude. The captain instructed the flight attendant to go through the cabin and tell all passengers to turn off their electronic devices. She reported back that about 25 passengers with portable radios had been listening to a football game and one passenger was using a lap top computer.
After five minutes, the gyros still failed to operate properly. On rechecking, the flight attendant found that the radios were still on. Using the loudspeaker system, the captain immediately told all passengers using such devices to turn them off because they were affecting the navigation equipment. After 90 seconds, the gyros corrected themselves to the proper heading. But 20 minutes later, they "began moving off the correct heading by as much as 20-30 degrees."
The captain then picked up the mike and told the passengers that if they did not turn the radios off, he could have the flight attendant go through the cabin and confiscate all radios until they reached Newark. Within two minutes, the gyros began to swing back towards the correct heading. No further incidents occurred.
(Originally published in 'Air Line Pilot')
Tekla S. Perry is senior Editor and Linda Geppert is Senior Associate Editor of IEEE Spectrum, the magazine of the Institute of Electrical and Electronic Engineers.
NUMBER / ORIGIN / DATE : 004 / E-mail / 030500
In the automotive industry it is not unusual for component and system
EMI immunity test levels to exceed 100 V/m, with a variety of modulation
requirements. These high immunity levels are achieved with relatively modest
increase in cost, due to effective design practices. It is clearly absurd
for avionics manufacturers to insist that EMI immunity is too costly, when
low profit margin auto parts makers routinely achieve high levels of
The only EMI susceptibility that cannot be effectively removed at the level of the victim circuit is corruption of low level signals at the receiving antenna. To remove this threat, it will be necessary to reduce the emissions levels allowed by consumer electronic devices. This won't happen, as the political pressure brought to bear on regulatory agencies will be relentless. If anything, the trend seems to be to allow more aggressive emitters, like digital cell phones (TDMA, GSM).
It seems prudent to require that all consumer electronics devices be stowed for the entire flight.
NUMBER / ORIGIN / DATE : 005 / E-mail / 150900
Several instances have been reported to IFALPA headquarters recently about the continued use of mobile phones on board aircraft.
To examine the potential dangers of use of mobile phones on board aircraft, the UK Civil Aviation Authority has carried out tests on two parked aircraft at London Gatwick Airport. It found evidence that calls produced interference levels that could disrupt aircraft systems. Faults that could be attributed to operation of mobile phones include
o false cockpit warnings
o malfunctioning of aircraft systems
o interference in pilots' headsets
o distraction of cabin crews from their normal duties.
The UK CAA bans the use of mobile phones from its flights once an aircraft's engines start running. The tests were carried out on aircraft equipment certificated before December 1989 which had not previously been tested for immunity from interference from mobile phones. The Gatwick tests revealed that an aircraft's internal doors do not block mobile phone signals but that passengers in the path of the transmission did reduce the signal.
Dan Hawkes, head of the Avionics Section of the UK CAA said: "the tests showed that a mobile phone used near an aircraft's flight deck or avionics equipment bay will produce interference." The CAA is now recommending that notices reminding passengers of the mobile phone ban are placed in airport departure lounges. It also suggests that airlines should be encouraged to look into installing mobile phone detection equipment. IFALPA's President, Captain Ted Murphy, when asked to comment on this topic said "We need to remind all operational personnel that the result of this CAA investigation means that mobile phones must be switched off at all times on the aircraft, including before take-oft and after landing". [see Safety Bulletin O1SABOO6 -IFALPA lntranet]