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 blanker and noise suppressing should be standard issue in every airplane 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.
This page discusses some of the external sources which can cause radio interference in our aircraft.
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 without experience. Problem source detection can be relatively as easy if a certain interference occurs only when switching on a device or aircraft light or certain 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 High Frequencies (HF, 3 MHz - 30 MHz), 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 it has on VHF (30 MHz - 300 MHz).
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/must 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 non well formed and have occurred during phases of flight (takeoff 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 wires will act as an antenna picking up interference from external sources outside of the aircraft.