TCAS II, the fully capable version, issues two kinds of alerts: Traffic Advisories (TAs) to help pilots visually acquire intruders, and Resolution Advisories (RAs) to command vertical escape maneuvers. When two TCAS-equipped aircraft encounter each other, their systems coordinate through the Mode S data link to ensure opposite vertical commands — one climbs, the other descends. Following the 1986 Cerritos mid-air collision, Congress passed Public Law 100-223 mandating TCAS II on all passenger aircraft with more than 30 seats. By 1990, commercial TCAS systems were flying, and by 2003 ICAO mandated ACAS (the international version) worldwide. Today, more than 25,000 aircraft carry this silent sentinel, a machine that negotiates with other machines to keep humans alive.