Although the maximum range for ILS in p3d is fixed at 25NM I have seen planes intercepting as far back as 35-40nm from the airport in flightradar24, which made me curious how much DME distance is generally considered fine for ILS interception in real life? Does it vary from airport to airport or between old and new transmitters or perhaps aircraft even?

An ILS localiser is typically calibrated and flight-checked to 10NM from the runway threshold, and protected from interference to 25NM. Reception (and use of) of the signal may well be possible beyond that, but, to put it broadly, YMMV and there is absolutely no guarantee that the signal will be usable, accurate or safe so until you are within the promulgated range it is wise to treat any indications with a great degree of caution.

In real life, ILS frequencies have a very long range (close to 100 NM), much longer of what is represented in the flightsim world.

Cheers, Ed

High altitude VORs do have a very far range radius given their power and omni-directional antennas; ILS localizers being much less powerful and with highly directional antennas (yagi arrays) do not not have so much range.

It may be that a carrier signal could be available for a RMI without the modulated 90 &150 hz for directional cues but more likely the aircraft was on RNAV prior to the IAF / actually receiving the signal. The pilot could've also used VOR nav prior to IAF if a VOR was located on field. Lastly ATC directions and/or visual ground cues could have helped especially if it is a commonly flown flight by the pilot.  

31 minutes ago, spokes2112 said:

It may be that a carrier signal could be available for a RMI without the modulated 90 &150 hz for directional cues but more likely the aircraft was on RNAV prior to the IAF / actually receiving the signal. The pilot could've also used VOR nav prior to IAF if a VOR was located on field. Lastly ATC directions and/or visual ground cues could have helped especially if it is a commonly flown flight by the pilot.  

Got that?

13 minutes ago, Ron Attwood said:

Got that?

It was my first thought too 

AIM 1-1-9 LOC reception is 35 degrees of centerline to 10nm and 10 degrees from centerline to 18nm. Per the AIM "Unreliable signals may be received outside these areas." 

I know for our commercial sims (Level D FFS) the manufacture hard codes localizer range to 18nm. 

As been previously stated if the aircraft is equipped with an RNAV (area navigation) capable system (such as an FMS) the system is capable of extending a track. i.e. If the course from the IF (Initial Fix) to the FAF (Final Approach Fix) is 124 degrees then the FMS can extend the course outside of the IF and the aircraft suitably equipped can track the extended track. On more advanced FMS (such as the FMS3000 on the Pro Line 21) they have a NAV to NAV capability in which the FMS will automatically hand over guidance to the localizer prior the FAF.

More likely the approach is one of the newer RNAV to ILS type which contains RNAV waypoints that lead to a Capture Fix (CF) or a Computer Navigation Fix (CNF) at which point you start navigating via the Localizer. These approaches can even be tied into a STAR providing for seamless navigation and descent. 

In the US, the FAA  defines a standard service range of 18 nm and this is the distance to which flight standards inspects and certifies the ILS.  In some instances an extended service range is defined and certified.

In Europe the standard range is 25 nm.

The actual signals can be received at longer range depending on terrain, but aren't certified.

 

scott s.

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Some of the major airports like LAX have them certified further out. 

KORD likes to have you intercept about 25-30 miles from the runway.  When doing that in both the EMB145 and MD80, the plane will start small "S" turns since the localizer isn't very sensitive that far away.  It is better to use the RNAV to intercept the course and then flip to conventional about 15-20 out.