In the previous article, we discussed some of the sources of and the historical basis for HIRF testing. With the increased use of “Fly by wire” controls of aircraft, the high intensity of radio frequency transmitters caused interference with these controls, at times with very serious effects. Test levels had to be increased, and methodology changed for components and subsystems, as well as complete systems and the full aircraft. RTCA’s DO-160B, which was in effect until 1989, had as its highest radiated susceptibility level ‘Category Z,’ which was 1 V/m from 35 MHz to 1215 MHz, with a bump to 2 V/m from 118-136 MHz. The lowest level in DO-160C (December 1989) would be 5 V/m, with a new maximum of 200 V/m up to 18 GHz.[1]
In 1988, the SAE “was requested to develop guidance for designers’ aircraft, aircraft engine, and electronics components on how to maximize protection of airborne avionics and electronic systems from the adverse effects of high energy RF fields through which aircraft may fly.”[2] The SAE would create three groups, or Panels, to address the subject. Panel 1 would validate the HIRF environment. Panel 2 would support the FAA by writing the high-level advisory material for their rule making effort. Panel 3 would write the recommended practices to meet the environments they would identify. This work was focused on environments found in the Continental U.S. and its territories. A similar effort was underway in Europe through EUROCAE with Working Group 33, members of which participated in SAE’s AE4R HIRF Subcommittee. To assure uniformity and completeness, the Electromagnetic Effects Harmonization Working Group, or EEHWG, would assemble the data and information generated by the groups involved.
It was found that the environment for rotorcraft (helicopters) was different than that of commercial or private fixed wing aircraft. Helicopters are cleared for lower flight and closer approach to transmitters than fixed wing aircraft. Helicopters also have a more open flight deck than most aluminum skin aircraft. Thus, the resulting fields experienced had the potential to be higher. Other considerations were addressed for fixed wing aircraft as well. The result was the generation of four HIRF environments:
- Fixed Wing Aircraft Severe HIRF (not used in FAA HIRF guidance AC 20-158B)
- HIRF Environment I – Aircraft Certification
- HIRF Environment II – Aircraft Normal
- HIRF Environment III – Rotorcraft Severe HIRF
The predicted levels of HIRF had to be validated. To support this, the FAA conducted some flights in their S-76 helicopter. This 1993 work resulted in a three-volume report entitled “FAA Technical Center Final Report DOT/FAA/CT-93/5-I, S-76 High Intensity Radiated Fields”. This helped to establish and validate the rotorcraft environment. NASA would fly a 757 near several transmitters and measured fields in the flight deck and electrical equipment bays. The NASA data was compared to computer models. Earlier studies were performed by Ohio University Avionics Engineering Center under contract with the FAA. In 1988, a DC-3 with RF field measuring equipment flew near four sites:[3]
- 100 kHz: Loran-C, Carolina Beach, North Carolina.
- 195 and 11.939 MHz: Voice of America, HF Broadcast, Greenville, North Carolina.
- 5 MHz: Over-the-Horizon Radar (OTH-B), Moscow, Maine.
- 3 GHz: TPS-75 Radar, Baltimore, Maryland.
Many other sources were considered, especially any significant transmitter within a five-mile radius of the airport or landing craft. These included airport-based transmitters (VOR, Glide Slope, several radars, communications and telemetry, marker beacons, and the like). Also included were all other transmitters, such as television, radio, public and private licensed transmitters, radars, communication systems – both terrestrial and space, and any other emitters of various types. The largest U.S. airports with the most significant emitters were studied first, focusing on emitters over 100 Watts.
Some transmitters were continuous wave (CW) or amplitude or frequency modulated (AM or FM). Others, such as radars and some communication systems, were pulse modulated with different amplitude signatures. As a result, peak field levels and average field levels were defined. These levels may not have been from the same transmitter since peak fields from a pulse modulated source with a very low duty cycle may have a very low average field strength compared to other sources.
In the next article, the field levels.
Endnotes
[1] It should be noted that the power requirements from 2 V/m to 200 V/m increased by 10,000 times, power increases as the square of the voltage. Almost overnight, test labs required radical changes in the equipment needed to perform these tests.
[2] Fred Heather, High intensity radiated field external environments for civil aircraft operating in the United States of America, Report No.: NAWCADPAX–98-156-TM, (Naval Air Warfare Center, Aircraft Division, 1988), 3.
[3] Ibid, 5.
