Several times, I have talked with people in the general public about the use of electronics on aircraft, often with the same response: “There is no good reason they have us turn off our electronics.” However, those of us in the EMC industry, and especially those in the aerospace aspect of the industry, know how true the issues can be. June 8, 2011, ABC News addressed 75 possible incidents of EMI on aircraft[i]. Keith Armstrong, in his Banana Skins series, has nearly 100 documented issues relating to aircraft. On December 12, 2020, a flight computer on Virgin Galactic SpaceShipTwo rebooted due to an EMI issue, causing an abort of the test flight. Thankfully, pilots C.J. Sturckow and Dave Mackay were not hurt and were able to glide to a safe landing.
Concerns about interference in commercial avionics date back to the late 1950s. A special committee of the RTCA, SC-88, with support of the FAA, was formed to study the use of electronics by passengers. On April 12, 1963, DO-119 was published but without regulatory limits placed on the radiation emissions of portable electronic devices. However, responsibility for assuring compliance with FAR 91.21 (was FAR 91.19 at the time) remained with the operator of the aircraft.
By the early 1980s, portable computers were making their way onboard, and some airlines subsequently banned their use. One computer trade magazine suggested their readers avoid any airlines that did not permit them to use their computers. A new subcommittee, SC-156, studied the issue. Their first meeting on December 1-2, 1983, was attended by representatives of aviation, computer industry, and the press. This work resulted in DO-199 – Potential Interference to Aircraft Electronic Equipment from Devices Carried Aboard, a two-volume study and recommendations, published on September 16, 1988. In the middle of this period, DO-160C was released, which greatly increased susceptibility test levels and developed new test methodologies.
In 1992, a new special committee, SC-177, was formed to look at how personal electronic devices (hereafter PEDs) would or could interfere with aircraft electronics and systems. International agencies such as the ICAO were contacted along with the FAA. One hundred thirty-seven incidents were reviewed, with laptop computers being the greatest number of incidents. They determined that the difference between test methods was part of the issue. Commercial electronics are measured at 3 meter or 10 meter distances, while aerospace standards require 1 meter test distances. To validate these issues, testing was performed on commercial electronics that had met FCC and CISPR regulations. The tests used DO-160C test methods with some modifications. Instead of having a conductive ground plane, the table was non-conductive, as used in commercial EMI testing. It was also 80 cm above the ground plane. No conducted emissions testing was needed since all the electronics are battery-operated and not connected to the aircraft.
Also of interest was the coupling path of the interference. The committee analyzed how the interfering signals were being coupled from the PEDs to the electronics. This involved first looking at which systems were most vulnerable, what frequencies they were operating in, and determining if the coupling was into cabling or antenna inputs. The most often affected system was VOR, with other navigation systems, autoflight/instrument landing systems, and VHF communications. One test found VHF susceptibility (break squelch) at 2 µV/m from the first row of seats.
The committee recommended that the use of PEDs should be prohibited “during any critical phase of flight” – which we know as the 10,000’ rule and that PEDs with transmitters should be prohibited unless determined safe for use. They found that the potential for interference from PEDs was real and that passengers willfully use transmitting devices during landing and takeoff, and although they should not cause interference due to frequency coordination, signals were coupling into aircraft systems.
DO-233 called for further testing to be performed, that the FAA and the FCC should work together and work with industry to determine the emission characteristics from PEDs. It also called for educating the general public about these issues, and to continue work with avionics and the FAA concerning regulation and further the efforts to harden aircraft systems to these signals.
In part 2 of this blog, we will look at the next RTCA documents that address some of these issues.
[i] See https://abcnews.go.com/Blotter/safe-cellphone-plane/story?id=13791569
