Listening to APP for several major (US class B) airports, it seems that every flight is vectored to intercept the localizer. Is it really possible that 100% of flights filed an ILS approach? If so, why? It seems like at least some (perhaps small) fraction would have filed an RNAV approach. Can someone with only GPS still "intercept the localizer" by flying the given vector until the runway is in sight and then get cleared for visual approach, so actual ILS doesn't matter? Or does that case just never come up because ATC knows (via the equipment code in the flight plan) who has ILS equipment available and makes them use it even if that's not what they filed?
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5$\begingroup$ This might just be a terminology thing, but flights (pilots) don't file approaches, they can optionally request them. ATC otherwise chooses the approach that suits conditions and that allows the most aircraft to land per hour. $\endgroup$– egidCommented Nov 13, 2018 at 15:43
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$\begingroup$ If you are instructed to "intercept the localizer" and you are GPS only then you do NOT have the equipment and means to comply. Actual ILS does matter in this case. Flying an assigned vector in IMC hoping to get VMC and runway in sight is not an acceptable means of getting around the issue. Simply reply "unable, request vectors to visual..." (or RNAV) $\endgroup$– Michael HallCommented Nov 13, 2018 at 23:45
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As egid pointed out, pilots typically do not file a specific approach as part of their flight plan. ATC will decide what runways and approaches are in use based on the weather. As the pilots are on their descent to the destination, they will be told which approach to expect. Pilots can always request something different but ATC is trying to get everyone to their destinations as safely and quickly as possible.
It's easier to space and sequence aircraft if they are all on the same approach, rather than trying to get aircraft on multiple approaches to mesh together just right. It's also easier to sequence them if they are all on the same approach course along the localizer, where an RNAV approach may have multiple initial fixes that follow various paths to get to the same final approach fix.
An ILS approach gives ATC more control over the spacing and sequencing of aircraft onto the final approach course. Aircraft can be vectored in anywhere along the localizer. RNAV approaches have more locations where aircraft can join them, which may be inconvenient depending on the direction from which an aircraft is arriving. Not all aircraft may be able to join an RNAV course by vectors, but pretty much any airliner can intercept an ILS localizer.
The same benefits can apply even in visual conditions. The localizer provides a common reference that all pilots can join and follow while they can maintain visual separation from each other, allowing closer spacing of aircraft. It's easier to say "join the localizer" than "join the RNAV RWY 17 final approach course."
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$\begingroup$ Looking at KDFW, the RNAV and ILS approaches appear to have the same fixes, descent profiles, etc. for each runway. They do have different minima, of course, but I assume that doesn't matter in VMC. However, the ILS approaches say SPECIAL AIRCREW & AIRCRAFT CERTIFICATION REQUIRED, so why wouldn't RNAV be preferred as more generally applicable? Or is it assumed that anyone landing there would have the appropriate certifications? I do notice the missed approach is different, but in the normal case, how would ATC even know which you did? $\endgroup$– StephenSCommented Nov 13, 2018 at 19:18
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$\begingroup$ @StephenSprunk the basic ILS doesn't have the extra requirements. I'm going to try to update my answer as well with more accurate info that might address the rest of your comment. $\endgroup$– foootCommented Nov 13, 2018 at 20:53