In the previous articles, we have discussed the need for testing HIRF, the assumptions that were made to determine the field strengths, and the environments they would apply to. There were four HIRF environments created:
- 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
These environments are based on the type of aircraft and the potential for high-level exposure. For Fixed Wing Severe, these were the worst-case estimated field strengths that could be exposed to civil aircraft under Visual Flight Rules. With the potential of an exceptionally close approach to a transmitter, these levels exceed certification requirements.
For certification of Part 23 and Part 25 aircraft, HIRF I was established as suitable test levels. HIRF II was a subset of these levels, less severe, which may be encountered during normal flight operation of civil aircraft. The need for very high levels was found for Rotorcraft and established as HIRF III.
| Frequency | Fixed Wing Severe | HIRF I (Certification HIRF) |
HIRF II (Normal HIRF) |
HIRF III (Rotorcraft Severe HIRF) | |||||
| Start | Stop | Peak | Average | Peak | Average | Peak | Average | Peak | Average |
| 10 kHz | 100 kHz | 50 | 50 | 50 | 50 | 20 | 20 | 150 | 150 |
| 100 kHz | 500 kHz | 60 | 60 | 50 | 50 | 20 | 20 | 200 | 200 |
| 500 kHz | 2 MHz | 70 | 70 | 50 | 50 | 30 | 30 | 200 | 200 |
| 2 MHz | 30 MHz | 200 | 200 | 100 | 100 | 100 | 100 | 200 | 200 |
| 30 MHz | 70 MHz | 30 | 30 | 50 | 50 | 10 | 10 | 200 | 200 |
| 70 MHz | 100 MHz | 30 | 30 | 50 | 50 | 10 | 10 | 200 | 200 |
| 100 MHz | 200 MHz | 90 | 30 | 100 | 100 | 30 | 10 | 200 | 200 |
| 200 MHz | 400 MHz | 70 | 70 | 100 | 100 | 10 | 10 | 200 | 200 |
| 400 MHz | 700 MHz | 730 | 80 | 700 | 50 | 700 | 40 | 730 | 200 |
| 700 MHz | 1 GHz | 1400 | 240 | 700 | 100 | 700 | 40 | 1400 | 240 |
| 1 GHz | 2 GHz | 3300 | 160 | 2000 | 200 | 1300 | 160 | 5000 | 250 |
| 2 GHz | 4 GHz | 4500 | 490 | 3000 | 200 | 3000 | 120 | 6000 | 490 |
| 4 GHz | 6 GHz | 7200 | 300 | 3000 | 200 | 3000 | 160 | 7200 | 400 |
| 6 GHz | 8 GHz | 1100 | 170 | 1000 | 200 | 400 | 170 | 1100 | 170 |
| 8 GHz | 12 GHz | 2600 | 330 | 3000 | 300 | 1230 | 230 | 5000 | 330 |
| 12 GHz | 18 GHz | 2000 | 330 | 2000 | 200 | 730 | 190 | 2000 | 330 |
| 18 GHz | 40 GHz | 1000 | 420 | 600 | 200 | 600 | 150 | 1000 | 420 |
Table 1: HIRF Field Strengths Found in FAA Advisory Circular AC 20-158B
The FAA is aware of missing data in Table 3 of the circular. The above table is accurate.
The levels shown are for field strengths exposed to the exterior of the aircraft in question.
To determine the proper category for a given system or function, the HIRF certification level (HCL) classification is assigned. FAA AC 20-158B, paragraph 7.3.1, states:
The HCL classification assigned to the system and functions may be different from the design assurance level assigned for equipment redundancy, software, and complex electronic hardware. This is because HIRF environments can cause common cause effects. The term “design assurance level” should not be used to describe the HCL because of the potential differences in assigned classifications for software, complex electronic hardware, and equipment redundancy. The HIRF safety assessment should include all electrical and electronic equipment, components, and electrical interconnections, and should assume that they are potentially affected by HIRF.
The system HIRF certification level is divided into three categories based on the severity of failure conditions.
- System Level A – Catastrophic – is for electrical and electronic system that performs a function whose failure would prevent the continued safe flight and landing of the rotorcraft/airplane.
- System Level B – Hazardous – is for electrical and electronic system that performs a function whose failure would significantly reduce the capability of the rotorcraft/airplane or the ability of the flight crew to respond to an adverse operating condition. This also applies for airplanes approved for instrument flight rules (IFR) operations.
- System Level C – Major – is for electrical and electronic system that performs a function whose failure would reduce the capability of the rotorcraft/airplane or the ability of the flight crew to respond to an adverse operating condition.
Since the levels given in Table 1 are for the outside of the aircraft, it is necessary to know the expected attenuation the aircraft may provide, also known as the transfer function of the aircraft. Generic values are provided in Appendix A.3 of the FAA circular. However, there may be benefits to the airframe manufacturer to know what the transfer function is for a particular aircraft. With improved attenuation, coupled fields internal to the airframe will reduce the fields exposed to avionics. Reduced fields can imply less need for high grade filters and shields, reducing weight and cost.
Next – Some methods used to evaluate an aircraft for transfer function levels and how this relates to common standards used for avionics testing.
