Aviation Investigation Report A98H0003

1.18.6  High-Intensity Radiated Fields

  1. - General
  2. - JFK International Airport Environment
  3. - En Route Environment
  4. - Theoretical Worst-Case HIRF Environment
  5. - MD-11 HIRF Certification Environment
  6. - Effect of HIRF on VHF Communications
  7. - Effect of Resonance on HIRF Energy  General (STI1-105)

Modern aircraft transmit and receive RF signals in the atmosphere external to the aircraft. In addition, RF signals are conducted and radiated within the aircraft, through electrical cabling, to control and communicate with various electronic systems. High-intensity radiated fields (HIRF), produced by powerful radar transmitters or lightning, will partially penetrate a commercial aircraft through apertures in the aircraft's hull.[95] HIRF may couple onto cabling within the aircraft structure and distort or corrupt the signals carried on these cables, thereby disrupting the normal functions of the associated aircraft systems. In addition, if the HIRF gradient within the pressurized area of the aircraft exceeds 23 kilovolts per centimetre, an electrical discharge may be induced between narrowly separated conductors.[96] In this latter case, physical damage to electrical components may occur and flammable materials in the surrounding area may ignite. The HIRF environment in the vicinity of the occurrence aircraft was studied to determine whether the ambient field strength was sufficient to produce such an effect.  JFK International Airport Environment

An assessment of the HIRF environment at JFK International Airport was derived from a 1998 study[97] of the peak and average field intensities to which aircraft operating in US civil airspace could be exposed. (STI1-106) During normal approach and departure operations in the airspace on and around airports, a peak field strength of 3 kilovolts per metre can occur in the 2 to 6 gigahertz (GHz) frequency band.  En Route Environment

For the en route portion of the occurrence flight, the most significant known HIRF environment was produced by an AN/FPS-117 air route surveillance radar, located near Barrington, Nova Scotia. (STI1-107) At 0109, the occurrence aircraft passed within 10 nm of this radar site at an elevation angle of approximately 30 degrees from the horizontal. A maximum field strength of 20 volts per minute (V/m) can be produced by the Barrington radar, at a slant range of approximately 10.5 nm. However, because this radar is optimized to achieve optimum gain at relatively low elevation angles, a field strength of approximately 4.3 V/m was produced by the Barrington radar in the external environment surrounding the occurrence aircraft. A maximum combined field strength of approximately 12.1 V/m was produced by the Barrington radar and all other background emitters in the external environment surrounding the occurrence aircraft.  Theoretical Worst-Case HIRF Environment

An estimate of the most severe HIRF environment, (STI1-108) during any phase of flight, was developed for airspace where fixed-wing commercial operations are permitted. Field strengths were calculated for surface emitters and airborne intercept radars, operating at the minimum separation distances permitted under instrument flight rules. Mobile and experimental transmitters, and transmitters located inside restricted, prohibited, and danger areas, were not considered. This methodology produced a worst-case peak field strength of 7 200 V/m, which is assessed to occur in the 4 to 6 GHz frequency band.  MD-11 HIRF Certification Environment

The MD-11 aircraft certification was subject to special HIRF test conditions (STI1-109) imposed by the FAA and the JAA. Test procedures were specified for the MD-11 to demonstrate an acceptable level of aircraft systems protection from the effects of HIRF. The MD-11 HIRF test environment was more stringent than the HIRF certification guidance that currently exists for new aircraft, and exceeded the theoretical worst-case environment. For example, in the 4 to 6 GHz band, where the highest theoretical field strengths are assessed to exist, the MD-11 test condition specified a peak field strength of 14 500 V/m, about double the peak field strength of the theoretical worst case. In the 1 to 2 GHz frequency band, where the Barrington radar operates, the MD-11 test condition specified a peak field strength of 9 000 V/m.  Effect of HIRF on VHF Communications

Aircraft antennas are designed to receive RF signal energy in specific frequency ranges and to conduct this RF energy to the radio or radar receivers in the aircraft. (STI1-110) Aircraft radios are designed for operation at frequencies assigned in accordance with national and international RF spectrum allocations. These RF spectrum allocations are developed to ensure that authorized high-power RF sources will not interfere with aircraft radios and radars. If a HIRF source were to operate within the assigned frequency range for an aircraft radio, the HIRF energy within the frequency range to which the radio receiver was tuned would be demodulated and amplified, adversely affecting VHF communications. However, modern radio receivers are designed to prevent radio signals from being amplified to unsafe power levels. In general, there is no relationship between the degradation or disruption of VHF communications owing to EMI, and the presence of field strengths sufficient to induce an electrical discharge between proximate conductors. The service record for Douglas commercial aeroplanes does not contain any instances of HIRF-induced degradation or disruption of VHF communications, or the presence of field strengths sufficient to induce electrical discharges between proximate conductors.  Effect of Resonance on HIRF Energy (STI1-111)

When a travelling wave is reflected back upon itself, the incident and reflected wave energy may combine to form a spatially stationary, reinforced wave. For an electromagnetic waveform, such as HIRFs, reinforced wave phenomena or resonance can occur in closed cavities, along a length of wire or around the perimeter of an aperture. When resonant conditions exist, the energy density of the reinforced wave may be up to 25 times greater than the energy density of the incident wave. In practice, resonant gain factors rarely exceed a single order of magnitude.

[95]    In transport aircraft, the ratio between external and internal electromagnetic field strength ranges from 2 to 40, depending on the location within the aircraft and the radio frequency. However, resonance conditions at specific points within the aircraft can, in theory, produce localized field gradients that are up to 25 times stronger than the ambient field strength.

[96]    At sea level, sparking between proximate conductors is unlikely to occur until the field gradient around the conductors exceeds about 31 kilovolts per centimetre (kV/cm). At a pressure altitude of 8 000 feet, the maximum altitude for most commercial aircraft cabins, the equivalent field gradient is about 23 kV/cm.

[97]    Heather, F. "High-Intensity Radiated Field External Environments for Civil Aircraft Operating in the United States of America," US Navy Technical Memorandum NAWCADPAX-98-156-TM, December 1998.

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