FM Immune

[Maxflyer]

What exactly is meant by the term FM Immune?

[Circuit Basher]

Empty bank balance!! ;) :D

Talking from memory (so am not going to be quoting verbatim rules here - that is your longer term challenge!), it's now a requirement for any aircraft flying IFR in UK Airspace to be fitted with radio equipment that does not interfere with the performance of the VOR / DME, commercial radio, etc. FM Immune equipment is certified not to have any spurious artefacts in its transmission spectrum (or radiating from the box itself) outside the permissible levels. FM immune equipment is also now at 8.33kHz channel spacing.

Due to my lack of ability to provide a detailed explanation, I just found the attached overview:
FM IMMUNITY (CAA AIRWORTHINESS NOTICE 84)

WHAT DOES IT MEAN?

* Aircraft navigation and communication receivers in most current aircraft were designed to eliminate commercial FM radio interference. The design specification is now 40 years old and things have moved on. The restrictions under which commercial radio stations have operated for the last 30 or 40 years will be lifted on 1st January 2001. This means that they will be able to broadcast within an increased bandwidth and with increased power causing breakthrough (FM interference) in most navigation receivers in use today. This FM interference may cause loss of sensitivity and / or erroneous navigation information.

HOW WILL IT AFFECT MY AIRCRAFT?

* If your aircraft is used for IFR or IMC flying then you must take action before 1st January 2001. This means making your navigation receivers FM immune. Old analogue avionics such as Bendix/King KX170 & 175 series and Narco Nav211 series etc. cannot be modified and must simply be replaced. More modern sets such as Bendix/King KX155/165 series and Narco MK12D may be modified or service exchanged. Please note however that not all modern sets can be modified and often the cost of a modification is greater than a new replacement.

WHAT IF I NEVER FLY IFR OR IMC?

* If you only fly under VFR conditions you may placard your equipment to say that it does not comply with FM immunity and down grade it to class 3.

* Please read carefully the terms and conditions of class 3 equipment. If you fly with non FM immune equipment after 1st January 2001 in class A, B, or C airspace or under IFR/IMC conditions you will be breaking the law.

[Mark 1]

This should explain it (courtesy of those folks at Leicester).

http://www.aerocoms.com/fm.htm

[bookworm]

For the avoidance of doubt, the text that Circuit Basher quotes is not AWN 84. It is Aerocoms's interpretation of AWN 84. In particular:

If you fly with non FM immune equipment after 1st January 2001 in class A, B, or C airspace or under IFR/IMC conditions you will be breaking the law.

is flawed.

There is no requirement for the carriage of FM immune equipment (or any particular radionavigation equipment) outside controlled airspace under IFR.

[Flyin'Dutch']

FM immunity only applies to VOR Navigation sets.

Bookworm is correct but for completeness probably a good idea to add that you can even fly within class D with non immune stuff in VFR and even IFR but you will need prior permission for that from ATC.

The 8.33kHz spacing has nowt to do with FM immunity.

The filters which were intially heralded as the saviours of old sets are under scrutiny. They were/are very expensive so most people who needed FM immunity did choose to go the whole hog and got a Garmin 430 which addressed the issue nicely and also allows 8.33KHz spacing and throws in a GPS navigator at the same time.

FD

[IO540]

Is there any evidence that lack of FM Immunity has ever caused a problem with any GA avionics?

[Flyin'Dutch']

Not as far as I am aware.

However I think that if you deng into something on an approach using your ILS/VOR and were not compliant at the time, your insurance will have an easy day denying your claim.

After all in today's society the only people to be afraid of are your insurers and the other party's sollicitors.

FD

[Keef]

FM immunity relates (in reality) solely to the bottom end of the VHF NAV band, and solely to receivers.

It all happened when the Government sold off the "guard band" - the gap in the frequency allocation between FM broadcasting and aircraft navigation. So the FM broadcasters now come right up to the edge by 108MHz. If you're flying an ILS on 108.notalot, your receiver will have a hard job coping with Radio Free Toytown as you fly your approach.

So we all have to buy new radios to fly IFR in most controlled airspace. I blame the Government (but then. I often do - they're supposed to be in charge).

There was an early theory (now mostly disproved) that the problem of FM interference could be solved with filters rather than new radios. Those of us who know a bit about radio were sceptical - how do you make a filter with a very sharp cutoff between 107.95 and 108.00, and yet broad enough to go all the way to 118 or even 136 MHz...

[bluskis]

Just to clarify, FM immunity in ground bound equipment is as described by Circuit Basher, that is the equipment must not interfere with other equipment.

Thats not what it means in avionics shorthand, here it means that the equipment itself is immune from others interfering emissions.

[Timothy]

I don't know if this accords with forum etiquette (I am sure L will tell me if not :( ) but I will have two NAVs and two COMs, all FM Immune, to sell in about October.

W

[gasax]

Are n't you glad that you willingly (?) pay your radio licensing fee to the Radio Communications Agency and they respond to protecting your interests by making your equipment redundant?

What would we do without these wonderful cost recovery agencies?

[IO540]

Keef,

You are right about filters; however an ACTIVE filter (rather than just a box with some chunks of quartz in it) can be made as good as you want.

I did see some passive filters for this job for about £1000, about 2yrs ago. The material cost in them was about £50!

There is no reason why a suitable filter cannot be made.

[Keef]

IO540

I've seen some cavity filters that were very impressive, but hardly suitable to aviation use.

Active filter - in my way of looking at it, you're talking a "front end" rather than a filter. Yes, that could be done - might even cost £50, plus a few hundreds of £k to get it certificated. A bit like the DSP on my HF kit here - brilliantly effective.

There was a better answer ... but the Government wouldn't have made much money out of that. And we wouldn't have had to pay for the solution.