Why can a localiser be used without a glideslope, but not vice versa?

Question

An ILS consists, at its most basic, of two components: a localiser (to provide lateral guidance) and a glideslope (to provide vertical guidance). The two are typically used together, for an ILS approach, but the localiser can also be used on its own, for a localiser-only, or simply localiser (LOC for short), approach (for instance, if the glideslope is inoperative or nonexistent, the aircraft lacks a functioning glideslope receiver, or the pilot is shooting a back-course approach). For a localiser-only approach, the pilot uses a specified series of navigational fixes to mark off each segment of the approach; each of these segments has a specified minimum safe altitude (which is shown on the approach plate), and the pilot uses this information, and their onboard baroaltimeter, to maintain terrain clearance during the approach.

While localiser-sans-glideslope approaches are quite common, there are, to the best of my knowledge (feel free to correct me if I'm mistaken here), no examples of the reverse: a glideslope-sans-localiser approach, where the pilot uses the glideslope for vertical guidance but relies on ground- or satellite-based navigation fixes for lateral guidance.

What allows the use for an approach of a localiser without a glideslope, but not a glideslope without a localiser?

Answer 1

Adding to @J.Hougaard's practical reasons, there is one technical reason in the existing standards I can think of:

The identification signal (morse code) is provided by the localizer, not the G/S, so G/S-only approaches would be unidentifiable. (ICAO Annex 10 Vol 1 § 3.1.3.9)

Side note: I'm not sure if there can be a workaround for it using existing airborne equipment, since a G/S works on UHF (which is transparent to the pilot, i.e., the pilot only selects the LOC VHF frequency, and the G/S UHF frequency is simply paired).

The identification is likely one of the reasons behind the following:

Inoperative localizer. When the localizer fails, an ILS approach is not authorized. (FAA AIM)

There is no mention of an operative G/S remaining. Whereas a failed G/S does not prohibit using the LOC for a LOC-only approach (after adjusting the minima).

Answer 2

Because there would be no benefit to doing so.

where the pilot uses the glideslope for vertical guidance but relies on ground- or satellite-based navigation fixes for lateral guidance.

As you correctly point out, if using only a glideslope, the pilot would have to rely on other navigation facilities for the horizontal guidance, for example a locator/NDB or RNAV. But if a locator/NDB or RNAV approach is available, the pilot could just use that approach for the vertical guidance as well.

For an NDB approach, there is no actual vertical guidance, but the pilot will descend manually, typically while cross checking distance to threshold using a DME. This is relatively inaccurate, so an NDB approach typically has a relatively high minimum descend altitude (the altitude at which the pilot must visually see the runway in order to continue).

The reason an ILS approach (LOC+GP) generally has lower minima is because a LOC and GP combined is very accurate. If you were to combine an NDB with a GP instead, you would still have accurate vertical guidance, but not very accurate lateral guidance. Because of this, it would probably not be possible to define lower minima compared to a NDB/DME approach. What good would a lower minima do you, if, by the time you reached, say, 200ft, you find out that you are not actually aligned with the runway, because the lateral guidance is so inaccurate? So even with accurate vertical guidance, you would need time to align with the runway once you get below the ceiling, which means our theoretical NDB+GP approach would need minima similar to that of a pure NDB approach. Hence, there isn't really anything to be gained.

Answer 3

Let's consider the ILS on its own, with no supplement navigation aids like RNAV / GPS. Assume IFR conditions.

• A LOC+GS approach can get you onto the runway.
• A LOC-only approach simply won't work, you need at least one more piece of information, e.g. DME.
• A LOC+DME approach would be similar to a VOR+DME approach. You'd step down your altitude as your DME reading decreases. At the end you either see the runway well enough to switch to visual, or execute a missed approach.
• A GS-only approach provides no information whether you are on the extended runway centerline. If you follow the GS all the way down, you would crash into something, e.g. the hangar or the planes on a nearby taxiway.

For the same reason, when flying an ILS approach, you should capture the localizer first, then the glideslope; never the other way around.

So we must pair GS with some other navigation aid, if LOC is unavailable. But what?

• You cannot pair GS+DME. GS+altimeter already gives you a DME of sort.
• If you have RNAV, might as well go completely RNAV anyway.
• VOR+GS works, provided that the aircraft have two nav radios. Tuning directly to the GS channel is possible with the existing hardware, but the cockpit controls and displays are not designed to do that at the moment.
• NDB+GS would also work, but with a lower accuracy.

However, as pointed out above, the danger of misplacing the aircraft horizontally is greater than vertically. A VOR+GS would have relatively inaccurate lateral guidance, especially at long distances, making the accurate signal of the GS useless in improving the minimum altitude of the approach.