Definition
TCAS I is the first-generation airborne collision avoidance system that monitors nearby transponder-equipped aircraft and issues Traffic Advisories (TAs) to alert the pilot to potential conflicts. TCAS I provides bearing, range, and relative altitude information for surrounding traffic but does not issue resolution instructions; the pilot must visually acquire the traffic and decide on any avoidance action.
Plain English
An onboard system that watches for nearby aircraft and warns the pilot when one is getting too close, but only tells the pilot that traffic is there — it does not tell the pilot what to do about it.
Context Anchor
Seen in instrument flying, cockpit traffic displays, and discussions of traffic alert and collision avoidance equipment.
Derivation
TCAS stands for Traffic Alert and Collision Avoidance System. The Roman numeral 'I' marks it as the earlier, simpler version of the system, before TCAS II added the ability to issue avoidance commands.
Why Pilots Care
Pilots in busy airspace use TCAS I for traffic awareness but must still maintain visual separation since it offers no avoidance instructions.
Intuition Check
Do not assume TCAS I is the same as full TCAS with climb-or-descend commands. TCAS I gives traffic alerts only; it helps you find the threat, but it does not tell you the avoidance maneuver to fly.
Example Sentence 1
The TCAS I display showed traffic two miles ahead and 500 feet below, prompting the pilot to look outside and confirm the other aircraft visually.
Example Sentence 2
During the approach, TCAS I provided a traffic advisory but no guidance on evasive action.