Which substitution is appropriate during an ILS approach when the glide slope becomes inoperative?

When the glide slope becomes inoperative during an Instrument Landing System (ILS) approach, it is necessary to switch to using the localizer (LOC) component of the ILS for guidance. The localizer provides lateral guidance along the runway centerline, which allows for a non-precision approach when the glide slope is not functional.

By utilizing the localizer minimums instead of the ILS minimums, the pilot can safely execute an approach using the available guidance. It is crucial to note that localizer minimums are typically higher than ILS minimums, reflecting the absence of the vertical guidance that the glide slope provides.

This choice maintains operational safety, adhering to regulatory requirements, and ensures that pilots apply an appropriate set of approach standards when the glide slope is not available. The other options, which involve using VOR, GPS, or standard minimums, do not provide the necessary lateral guidance that is essential for safely landing the aircraft in this scenario.