Radio frequencies (RFs) are among the biggest concerns for pilots in the sky and control teams on the ground. A minor event sends everyone into a panic. High-Intensity RFs (HIRFs) are particularly concerning. They penetrate the aircraft’s skin even at low frequencies and pose a significant threat at medium and high frequencies.

The worst part is that aircraft operate in environments rich with HIRF devices, including radar controls and guidance and communication systems running on advanced transmitters. These systems pose a significant threat to safe aircraft operation.

To this end, industry regulators have developed HIRF testing guidelines. This guide explains the certification requirements and testing standards to help manufacturers evaluate and mitigate risk better.

What is HIRF Testing?

High-Intensity Radio Frequency (HIRF) testing is a form of Electromagnetic Compatibility (EMC) test applicable in high-interference environments. It’s also practical in critical missions with human safety concerns.

HIRF testing is increasingly important within the aviation industry due to reduced electromagnetic shielding. Moreover, modern aircraft comprise more electrical and electronic components. Also, ever-increasing bus and processor power increases HIRF sensitivity. Fortunately, a visit to a reputable DO-160 test lab is often sufficient.

HIRF Testing and Certification Guidelines

Unfortunately, no regulation explicitly specifies the requirements for HIRF testing and certification.

FAA 14 CFR Parts 23, 25, 27, and 29 come close as they define the legal requirements for certification. For instance, the document defines peak and average standards and provides test levels for different operating environments. Sadly, it doesn’t define the requirements for aircraft tests.

Therefore, aviation manufacturers often fall back on the Advisory Circular (AC) 20-158, RTCA DO-160F, and SAE ARP 5583 REV-A.

The Advisory Circular 20-158 is optional and not legally binding. But it specifies one way to achieve HIRF certification. It identifies three risk categories – catastrophic, hazardous, and major. Then it provides the path to attaining HIRF certification for each category.

DO-160F is just as brief. It focuses on conducted and radiated susceptibility tests for aircraft systems and subsystems. Additionally, it defines the maximum allowable conducted susceptibility for various systems. Unfortunately, there’s not much else to gather from the document.

That leaves SAE ARP 5583 REV-A as the most detailed HIRF test and certification guideline. It expands on AC 20-158, providing aircraft-level HIRF test procedures. Moreover, it defines high-level and low-level tests and further explores their various levels.

High-Level HIRF Tests

You need high-level tests for critical missions and high-interference environments. The tests are directive or radiated.

1. Directive High-Level Tests: The technician directly applies a radio frequency current to the aircraft’s skin. Then they measure the current running through the cables. Check Table-9 in the SAE document for the recommended field values and modulation schemes. The document also explains how to comply with the average and peak levels defined in the FAA 14 CFR.
2. Radiated High-Level Tests: These tests utilize antennas and microwave horns to measure the aircraft interior’s conducted susceptibility and field values. Conducted susceptibility tests are within 10 kHz to 400 MHz, while the field values are calculated within 100 kHz to 18+ GHz. Check section 6.3 of the document for further directions.

Low-Level HIRF Tests

For less critical missions and low-interference environments, you need low-level tests. They are almost similar to high-level tests, except for lower field values. Normalize the values and multiply them with appropriate certification environments described in the FAA 14 CFR documents. The modulation schemes are similar to high-level ARP tests.

Three types of low-level (LL) tests are available, Directive Drive (LLDD), Swept Current (LLSC), and Swept Field (LLSF). Subsequent sections of the ARP 5583 REV-A document provide the test details, including data normalization strategies.

Summary

HIRF tests are the best way to evaluate an aircraft’s resilience to radio frequency interference. Unfortunately, no regulation explicitly specifies how to perform HIRF tests. So, you’ll have to be innovative. SAE ARP 5583 REV-A is one of the best resources for certification. But ensure all your tests are within FAA guidelines.