GASIL 1/2001 - "as a result of the 1979 world radio conference the decision was taken to allow FM broadcasters to use the 105 to 108 Mhz .. .."
http://www.wws.princeton.edu/ota/disk3/1982/8221/822104.PDF#search='world%20radio%20conference%201979'
I wonder why the CAA granted exemption certificates until 1999 - 20 years after FM broadcasters were permitted to use these bands? Were these certificates the work of idiots or did the CAA have reasonable grounds for their grant? What has changed since 1999? Why did the CAA not prohibit the use of non FM immune VORs? They were clearly capable of prohibiting the use of non FM immune loc.
I suspect the real reason is the CAA and other national organisations within Europe realised there was a very small but never the less real risk FM broadcasts could make the integrity of loc unsafe which in itself presented a serious safety hazard whether in controlled or uncontrolled air space for obvious reasons. Never the less they felt the risk sufficiently small to grant the exemptions that were given (or maybe they gave in to the political pressure) or maybe they realised that the USAF would not be relying solely on loc approaches. They also realised that outside controlled airspace given the widespread use of other means of cross checking position it remained "safe" to use non FM immune VORs.
I would be interested to know the effects of FM interference. What would be the indicative signs. Would a half competent pilot have ample warning that all was not well. Would he in any event be cross checking his VOR with GPS, NDBs, DME, radar, dead reckoning. I would have thought so. If tracking two VORs would both be effected at the same time - I would have thought not. I find it hard to imagine that a pilot would not see some warnigns signs that all was not well and have plenty of time to resolve the issue.
I am not advocating flying with non FM immune equipment. The fact remains that there are many aircraft without, including those used for IMC training. Are they operating illegally or irresponsibly? I am far from convinced they are.