Interference is any unwanted signal or noise that disrupts communication. It can reduce clarity, cause partial loss of signal, or in severe cases, completely block communication on a frequency.

Common sources of interference in VFR operations include:

Thunderstorms and Atmospheric Disturbances. Electrical discharge in thunderstorms generates broadband electromagnetic noise across a wide frequency spectrum, including VHF. This noise appears as crackling, static, or "salt and pepper" sounds in the receiver. While modern aviation radios have some filtering and noise blanking, they cannot eliminate all weather-generated interference. The closer you are to an active thunderstorm, the worse the interference. This is one reason why VFR pilots should avoid flying near thunderstorms—in addition to the turbulence and severe weather hazards, radio communication becomes unreliable.

Aircraft Electrical and Engine Systems. The engine ignition system generates electromagnetic noise, particularly on magnetos. Other sources include navigation lights, landing lights, and avionics systems. If a radio is not properly shielded or if antenna cables are not well-grounded, these sources can leak noise into the receiver. During aircraft maintenance, a proper electrical grounding check is performed to minimize this type of interference.

Adjacent Frequency Interference. Because channels are spaced only 8.33 kHz apart, adjacent channels are relatively close in frequency. If a nearby aircraft is transmitting on an adjacent channel with high power, its signal can "bleed over" into your frequency, especially if your radio's receiver filters are not sharp or if the transmitting aircraft has poor frequency stability. Modern radios have good selectivity (ability to reject adjacent channels), but this remains a potential source of interference at busy airfields.

ADF Oscillation. If you have an ADF receiver tuned to an NDB station, improper tuning or worn components can cause the ADF to oscillate internally, generating an audio tone that may be heard on the communication radio frequency if antenna connections are shared or located close together. This is typically an aircraft maintenance issue and is corrected through proper ADF tuning procedures and antenna isolation.

Radio Equipment Malfunction. A faulty transmitter or receiver can produce spurious signals or interference. For example, a transmitter with poor frequency stability may drift away from its assigned frequency, causing interference to adjacent channels. Periodic aircraft maintenance and radio certification ensure that equipment remains within specification.

As a pilot, you can minimize the risk of interference by: