@Richard Pearse: That proved to be a more thought-provoking (and research provoking) question than I expected.
I’m spoilering the answer since it’s a major pilot geek-out that’s a total hijack to the actual thread the others are having.[spoiler]My carrier services many small town fields across the US that don’t have 24/7 control towers or radar approach control. Having said that, between my being now on bigger airplanes and preferring to be snug in bed well before the wee hours, I don’t visit those places and situations much any more. They’re still in my iPad though.
Anyhoo …
Many ILS, and certainly the vast vast majority at major airports*, are equipped with DME. Absent a marker beacon, the next common legal alternative to identify the final approach fix (FAF) is a cross radial from a nearby (10-15 miles max) VOR.
That seems to be their pro-forma choice when the reality is the airport is served by 24/7 radar approach control. Where approach radar is available you can consider the FAF radar identified, but as a practical matter controller workload isn’t going to permit him/her to call it out for you unless you’re the only jet in the sky.
I vaguely recall one middle-sized city we used to serve in the 727 that needed the cross radial to be identified onboard to find the FAF late at night when local ATC had gone home. Absent FMS I’m betting that still goes on in the wee hours all over our fair land.
There are a scant few that still use an LOM co-located with a marker beacon with no nearby VOR assistance. I was surprised to still find one where the LOM was a designated IAF. So in a steam gauge airplane one could transition from the enroute structure by homing to the NDB then maneuvering via a holding pattern entry or even complete holding pattern to align with the localizer and descend to FAF altitude before fixing the FAF via marker or LOM passage and starting descent to the runway all with no radar hand-holding. Shades of my Dad in the 1950s; hard to believe it’s expected even as a tertiary backup plan today.
I found one fun situation where there’s a VOR situated almost but not exactly along the final approach course about halfway between the FAF and the runway. http://flightaware.com/resources/airport/CID/IAP/ILS+OR+LOC+RWY+09/pdf . The ILS has no LOM or markers; you fly laterally and vertically off the ILS while using the VOR’s DME for identifying the FAF and a later LOC-only stepdown fix. And, unless you like using a stopwatch, the LOC-only MAP if the GS is unusable.
Talk about a setup to screw up if somebody ends up flying the VOR course guidance by mistake. Besides being 1/4 mile off course laterally you’d be at about 1000 AGL descending on what you thought was the localizer when suddenly you get cone of confusion and station passage. Oops! To make it really easy to goof, the ILS has the same base identifier as the VOR: ICID vs CID. Even if you’re doing the right thing it’s kinda weird to have the DME count down to zero then start counting back up while you’re still correctly on your way towards the runway.
The real bottom line nowadays is this:
There’s a general overlay rule that says a properly functioning FMS system using IRS with DME/DME updating or a properly functioning GPS with or without IRS can be used to identify any fix anywhere any time.
So as a practical matter for most jets most of the time nowadays the answer is we’re there when the FMS-driven moving map shows we’re there. Since everybody needs to have ADS-B installed by IIRC Jan 2019 for commercial and Jan 2020 for GA, it won’t be long before FAA, ATC, and the rest of the alphabet soup can assume 100% GPS & FMS or FMS-lite equipage. At which point damn near all ground-based infrastructure becomes optional.
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- At my home base we have 8 runway ends. Probably 80% of my arrivals are on 2 of them and 95% on just 3 of them. One end that I’ve very rarely used is a ILS without DME whose final approach fix is over the water. So no marker beacon or LOM. I had never noticed until I was looking just now that (absent FMS) the final fix is radar only.[/spoiler]