Definition
The higher landing minimums published on an Instrument Landing System (ILS) approach chart that apply when the glideslope component is unusable or out of service. With no glideslope guidance available, the approach reverts to a non-precision localizer-only approach, which uses a higher minimum descent altitude (MDA) instead of a decision altitude (DA) and typically requires greater visibility.
Plain English
When the part of the ILS that gives you up-and-down guidance to the runway isn't working, you can still fly the approach using only the side-to-side guidance, but you have to stay higher and need better visibility to land.
Context Anchor
Seen on instrument approach charts in the minimums section, especially when the glideslope is reported out of service or unreliable.
Derivation
Glideslope combines glide, meaning a smooth descent, with slope, meaning an angle or slant. Inoperative means not working. Together, the phrase points to the minimums used when the normal ILS descent path is not working.
Why Pilots Care
These higher limits maintain safety margins without vertical guidance, preventing descent below a safe altitude.
Intuition Check
Do not read this as meaning the whole ILS approach is automatically unusable. It means the glideslope part is not usable, so the pilot must use the separate published minimums for that condition.
Example Sentence 1
ATIS reported the ILS glideslope out of service, so we briefed the ILS glideslope inoperative minimums and planned for a localizer-only approach.
Example Sentence 2
With the glideslope out, we flew the approach using the published ILS glideslope inoperative minimums of 480 feet and one mile visibility.