HIRF Explained

By Gordon Walker — December 04, 2012

To the many young people beginning their careers in the field of aviation, I say “Welcome”. To those who have specifically chosen the avionics side of the business, I would add “Congratulations”. You’ve made an awesome career choice. My own career has spanned five decades, from the 1970s to the present day, and I’ve never had cause to
regret my decision. When I abandoned my first career choice – “Rock Star” – by selling my 1962 Fender Mustang guitar for $200 on a Toronto bus, I committed to a life in aviation that has been fulfilling and rewarding. Having always been fascinated by aircraft, and all things mechanical and electronic, the world of avionics has allowed me to immerse myself in an industry full of exciting and innovative technology. The advent of the microchip in particular, has led to a veritable revolution in the aviation electronics world.

The cockpit of today bears little resemblance to those of the past: the myriad of complex instruments replaced by a few simple video screens; the heavy control yoke, once connected to ailerons and elevators by means of cables, pulleys and bellcranks, replaced by a lightweight plastic computer joystick; and of course, most significantly, the highly skilled pilot, once responsible for physically flying and navigating the aircraft, replaced by a data entry clerk, whose primary job is to monitor the operation of the autoflight system, and look good for the public in a snazzy uniform.

These improvements and innovations are all made possible through the use of digitalized electronic hardware, and brilliantly designed software. Computers using digital data busses communicate commands and feedback signals between navigation systems (GPS/IRS/ILS etc.), flight control surfaces, cockpit video instrumentation, control inputs (joysticks/switches/selectors), engines, and system controls (bleed air valves, fuel pumps, fire bottle squibs etc.). These digital communications are, essentially, voltage pulses travelling along wires. When voltage is present, a digital condition of “1” exists; when voltage is removed, a digital condition of “0” exists.

By designing protocols in which timed sequences of pulses represent information or “intelligence”, digital systems are able to communicate information and send command/feedback signals throughout the network, or in our case, the aircraft. For example, imagine a simplified engine control system concept. To control the speed of the
engine, the engine control computer could use a protocol with as few as two pulses. If the computer sends two pulses of voltage to the fuel control module on the engine, the module will deliver more fuel and cause an increase in RPM. If the computer sends only a single pulse, the module will interpret this as a “decrease speed” signal, and reduce fuel to the engine. Should the computer send no pulses, the fuel module on the engine will not change the amount of fuel it is delivering, thus keeping engine RPM constant. These three conditions would be represented digitally as 11 for increase speed/fuel or 01 for decrease speed/fuel, and 00 for maintain current speed/fuel. The actual protocols are, of course, far more complex than this but the concept is the same. One can easily imagine how this concept can be used for all of the various digital systems aboard an aircraft, such as 11 meaning turn left, 01 meaning turn right, and 00 meaning maintain heading.

The digital concept is fantastic in its simple elegance. There is no maybe condition, only 1 being ON and 0 being OFF. Wouldn’t it be grand if life was so clear and simple? Not so fast. Imagine the consequences of erroneous transmissions of those 1s and 0s. Surely that’s not possible, is it? The answer, unfortunately, is YES, and that brings us to this month’s subject: High Intensity Radio Frequency (HIRF) Explained.

We’ve looked at advanced digitized computer concepts. Now let’s take a step back and revisit some very old electrical principles in order to understand the concept of HIRF problems.

The old fashioned step-up/step down transformer works on the concept of electromagnetic induction. Whenever current passes through a wire, a magnetic field is created around the wire. Similarly, whenever a wire moves through a magnetic field, or has a magnetic field move over it, a voltage is created and a current flow is induced into that wire. When current passes through the windings of a transformer’s primary coil, a magnetic field is set up around that coil. This magnetic field passes over the transformer’s secondary winding, and induces a current flow in that winding. (Whether the voltage is stepped up or stepped down depends on the ratio of turns in the primary, to turns in the secondary.)

Digital busses and circuitry can act in the same way as the secondary winding of a transformer. When an electromagnetic field passes over them, a voltage can be induced, which could be interpreted as a digital pulse. This can occur when an aircraft flies through an area of strong electromagnetic radiation. Such conditions exist in the vicinity of high-powered radio transmitter antennas. Consider the huge increase in transmitter sites as a result of the recent increases in wireless communications, satellite communications and navigation systems, as well as high powered radio and television antennas, all of which are transmitting the type of HIRF which can lead to interference with digital avionics systems.

To exacerbate this problem, aircraft were once made of aluminum, the purpose of  which was to reflect, absorb and discharge radiated RF energy. Composite airframes offer none of this protection. Composite materials are invisible to electromagnetic energy, and these electromagnetic fields are therefore able to penetrate the airframe and induce unwanted voltages into digital databus wiring and electronic circuitry. With recent booms in the personal electronic device market, passengers carrying all manner of cell phones, smart phones, electronic tablets, handheld games, navigation systems, and so on, can also pose a potential threat to an aircraft’s digital systems. Many of these devices, such as cell phones are actually radio transmitters, and even those that are not transmitters still contain electronic clock/oscillators which could potentially generate a frequency that is similar to that being used by an avionics system.

How then, do we protect our carbon fibre, fully digitized, fly-by-wire, FADEC-powered aircraft from the perils of HIRF and onboard electromagnetic interference (EMI)? A big part of the answer lies in a piece of information we’ve already touched on. The fact that a metal airframe is able to absorb and reflect electromagnetic energy means that by putting a metal “shield” around wires, we can prevent electromagnetic interference. When electromagnetic energy passes over a shielded wire or wires, most of the energy is absorbed or reflected by the shield, rather than inducing unwanted voltages into the wires themselves. Shields can also be used on wiring which may emit strong electromagnetic fields, such as ignition or generator circuits.

Shielded wiring goes a long way towards reducing problems associated with electromagnetic interference, but like most things, it only works when the appropriate level of shielding is employed, properly installed, and maintained. Failure to do so could result in engine failures, loss of flight controls, navigation errors and catastrophic loss of an aircraft. Whether you’re new to the field of aircraft maintenance, or an old, long-in-the-tooth character like me, it’s important to always be thorough, diligent, aware, and conscious of technological changes that affect our industry.

Q: What is the most effective way of reducing HIRF interference?

Answer to previous question:

Q: Why is inertial navigation preferred over GPS for unmanned aircraft?

A: Inertial navigation does not require any external inputs from radio stations, satellites or magnetic compasses

About The Author

Gordon Walker

GORDON WALKER entered the avionics industry after graduation from Centennial College in 1980. His career with Nordair, Air Canada, CP Air, PWA, and ultimately Canadian Airlines took him to many remote corners of Canada. Since leaving the flight line to pursue a career as a college professor, Walker has continued to involve himself in the aviation/avionics industry, by serving on several CARAC Committees concerning the training and licensing of AMEs, being nominated to the CAMC Board of Directors, and being elected President of the National Training Association. (NTA).

View all articles by Gordon Walker.

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