Definition
A line on an instrument approach chart showing the published minimums for a straight-in Instrument Landing System (ILS) approach to a specific runway. The 'S-ILS' line lists the Decision Altitude (DA) and required visibility for an aircraft flying the full ILS approach (using both localizer and glideslope) and landing straight ahead on the runway aligned with the approach.
Plain English
It's the row on the approach chart that tells you how low you can descend, and how much visibility you need, when flying a full ILS approach straight in to the runway.
Context Anchor
Seen in the minimums section of an instrument approach chart, including approach charts used when planning timed approaches from a holding fix.
Derivation
The 'S' stands for 'Straight-in' and 'ILS' for 'Instrument Landing System.' The label distinguishes a straight-in ILS approach from a circling approach or a localizer-only approach to the same runway, each of which has its own minimums line.
Why Pilots Care
The S-ILS line gives the lowest legal minimums available on the approach because it uses the full ILS (vertical guidance from the glideslope plus lateral guidance from the localizer). Reading the wrong line — for example, using S-LOC minimums while flying a full ILS — would either cost you usable altitude or, worse, lead you to descend below safe limits for the equipment you're actually using.
Intuition Check
S-ILS does not mean the whole procedure has no turns. It means the minimums listed are for the straight-in ILS landing to the runway named on the chart.
Example Sentence 1
Briefing the approach, the captain noted the S-ILS minimums of 200 feet DA and 1,800 RVR.
Example Sentence 2
After departing the holding fix on schedule, the pilot flew the S-ILS without deviation to meet the published arrival time.