FM Immunity

Frequency modulation immunity support

Very high frequency omnidirectional range (VOR) and instrument landing system (ILS) require additional protection against interference by modifying current equipment to allow for frequency modulation. Despite this being a known issue, it is not yet common practice.

To assist our military stakeholders, we promote the safety of State aircraft operation within a very high frequencies (VHF) interfering environment by providing guidance and helping them through the specific nature of their needs.


Since 1 January 2001, VHF (Very High Frequency /30 - 300 MHz) broadcasting stations in Europe have been allowed to operate with reduced restrictions and increased transmitter power levels. This has significant implications for aircraft with VHF navigational receivers, especially VHF Omnidirectional Range (VOR) and Instrument Landing System (ILS).

Consequently, for safety reasons, VOR and ILS receivers in aircraft are now required to be protected against potential interference from VHF broadcast transmissions. This entails the use of FM (Frequency Modulation) immune VHF equipment through the modification of existing equipment or re-equipage.

Some States have mandated the carriage of FM Immune VHF avionics for en-route and at airports. However, exemptions for State aircraft may still be negotiated on a bilateral basis.

Aircraft operators and aircrew are to refer to national aeronautical publications (Aeronautical Information Publication/AIP, Aeronautical Information Circular/AIC) for current official policy and procedures.

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For more information about this support service, please contact us.