What minimums should be substituted during an ILS approach if the glide slope becomes inoperative?

In the scenario where the glide slope becomes inoperative during an Instrument Landing System (ILS) approach, pilots are required to substitute the ILS minimums for Localizer (LOC) minimums. This is because the key function of the ILS is to provide both lateral and vertical guidance for an approach using radio signals. When the glide slope, which provides the vertical component, is not operational, the approach can still be executed using the LOC signal, which provides lateral guidance only.

It's important to understand that the minimums for the ILS approach are predicated on having both components functional. When only the LOC is available, the pilot must revert to the LOC approach minimums, which typically indicate a higher minimum altitude for decision making compared to ILS minimums. Following this protocol ensures that the pilot maintains a significant margin of safety due to the loss of vertical guidance.