New HIRF testing rules a step closer

 - September 15, 2006, 11:37 AM

The FAA last month closed a three-month comment period on a notice of proposed rulemaking (NPRM) that would amend long-standing HIRF (high-intensity radiated fields) testing requirements for newly certified airplanes and helicopters.

Writing that the amendments are necessary to protect aircraft from HIRF broadcast by high-power radio-frequency transmitters, the FAA has proposed stricter lab testing requirements. Concern about HIRF has “increased substantially in recent years,” according to the FAA document, because of increased use of computer processors, databuses and other electronics aboard aircraft and ever more powerful ground transmitters. Also a concern, composite materials used in aircraft production may not always provide adequate electromagnetic shielding, the FAA wrote.

Certification policies that address the effects of HIRF on aircraft electrical and electronic systems have been in effect for the last 15 years. During this time, the FAA and OEMs have worked to define the HIRF environments, the requirements for aircraft HIRF protection and the methods for verifying the HIRF protection effectiveness. But while plenty of statistical data on the problem exists, the actual HIRF environment constantly changes with new RF transmitter development and activation. The biggest concern is that a critical system such as the engine’s FADEC could be knocked offline by HIRF, leading to an in-flight engine shutdown.

Rotorcraft operating VFR would be required to meet even more stringent HIRF certification standards than airplanes because they operate closer to transmitters and do not have to comply with the same minimum altitude restrictions. In its background assessment of the dangers related to HIRF, the FAA recounted the details of a March 1999 incident in which a Robinson R44 passed over a high-frequency, high-energy antenna in Portugal and subsequently had to make a forced landing.

Flying past the antenna, the pilot recalled strong interference in the navcom and intercom followed by illumination of the low rotor RPM and clutch lights. He performed an autorotation and landed successfully, although the main rotor suffered damage in the process. Accident investigators blamed the forced landing on electromagnetic and RF interference.

Special conditions addressing HIRF have been a part of the TC and STC process for many years, but the FAA said the prevalence of higher-power transmitters and increased use of electronics in modern aircraft require the changes to HIRF laboratory testing procedures.