The definition of radio interference can be described as a range of situations where the reception of radio signals is disrupted by transmissions from unauthorized users, devices emitting spurious transmissions whether or not by design or disturbances in atmospheric conditions. This could lead to pilots not receiving or understanding ATC directions and missing important information regarding their flight.
Aircraft communication radio's should be equipped with functions or technical features which help the pilot suppressing these unwanted interferences. Power line filtering and digital signal processing (DSP) in the form of noise blankers and noise suppressing should be standard issue in every radio these days.
The aircraft, its equipment and the engines should also be designed with these requirements in mind so they do not emit any unwanted RF energy causing interference (think high voltage strobe lights) and possible loss of communications.
The cause of the interference can be either from inside the aircraft or it has its origin or influence from outside the aircraft. For the pilot it can sometimes be difficult to know/hear the difference. Problem source detection can be relatively as easy if a certain interference occurs when switching on a device or aircraft light or changes with engine RPM.
Atmospheric conditions can cause layers of air with different temperatures to exist above each other and these are able to guide or funnel VHF communications over greater distances than normal line of sight would provide. This would have the effect of hearing communications of airports which are well 'over the horizon' and, of course, you may be heard by them too.
On HF, long range communications depends on the ionosphere and its density which is caused by solar activity. The ionization layers are located between altitudes of 50 to 400 km, which is a lot higher than the 10 to 12 km these airliners usually fly at. As such aircraft altitude does not have such a great influence on range as is has on VHF.
Unauthorized or Malicious User
Unauthorized transmissions occur when spurious emissions from transmitters on lower frequencies are not adequately filtered. Either the 3rd, 5ft or 7th harmonic may manifest themselves in the aviation band between 108 and 136 MHz and have their base frequency somewhere in the HF and/or low VHF band. Good transmitter, antenna, low pass filter design and proper shielding should prevent this from happening.
Transmissions by malicious users misguiding pilots by issuing false messages have occurred in the past. These are easily to detect by experienced pilots as these messages are not well timed and are non well formed and have occurred during phases of flight (take off and landing) where such messages are not normally issued. Should you hear these, notify ATC asap so these users are taken care off by the proper law enforcement agencies.
The wiring for the radio's, transponders, GPS and other EFIS panels should be connected to a separate avionics bus and each device must have a separate ground wire to a common ground bus on a central fuselage ground so that any ground loops are avoided. It is advisable to keep high power or current carrying wiring away from antenna cabling and use accepted EMI suppression techniques to make sure that RF radiation remains outside of the aircraft radio's.
These devices generate high voltages to ignite or flash the light at regular intervals, easy to detect should you 'hear' them. They need proper grounding of the wiring from the high voltage (HV) power supply to the light unit, ground them at one end only. Make sure the 12/24 volt wires to the power supply are thick enough to handle the expected maximum peak current, usually between 4 and 10 amps. Run the wiring on a separate bus with their own fuse.
Modern aircraft now use LED based strobe lights, the advantages are low weight and longevity of the LEDs. Although they do not use high voltages to generate the flash, these device may use a microcontroller to create the flash patterns. These should be sufficiently shielded too.
Spark ignited engines are inherent sources of radio interference. Sparks are wide band noise emitters, much like thunderstorm lighting is. Modern engines have good noise suppression resistors built in the spark plugs and shielded HV cables so this should not be an issue in todays aircraft anymore.
You will not find these problems with diesel engines, but as these may use a FADEC system containing a microcontroller shielding is important here too.
Alternators & generators
The brushes in a generator and rectifier diodes alternators can fail for any reason or become really bad with age if not taken care of properly on time (lack of maintenance). Interference is easily recognized as this will vary with engine RPM. Read more in our article on power generation and discover how they operate.
Radio settings, DSP
The proper use of the squelch knob (some are automatic), which blocks the white noise you hear in the audio when no station is transmitting, preventing from this type of interference.
Modern digital signal processing (DSP) functions as audio and IF bandwidth filtering, noise blanking and reduction for suppressing engine ignition pulses and other interference, should be switched on when needed.
Not every modern aircraft radio is equipped with DSP at this time. Today, modern HF/VHF communication radio's do have these digital signal processing functions built-in and their advantages are widely known and accepted. The next logical step is to implement DSP technology in our aircraft radio's by the manufacturers.
Active noise reducing or canceling headsets are becoming more and more common with pilots these days. These actively listen for noise (within a certain frequency band response so you will continue to hear the engine) in the cockpit with separate microphones in the headset and cancel out or reduce this, thereby improving cockpit communications and radio reception. This is of course a specialized application of DSP.