Tcas

TCAS I (Traffic Alert and Collision Avoidance System I)

Function: Provides traffic awareness only — alerts pilots to nearby aircraft but does not issue avoidance commands.
Operation:
- Uses Mode C transponder signals from other aircraft to determine relative altitude and range.
- Requires both aircraft to have working transponders.
Alert type:
- TA (Traffic Advisory) only – warns the crew to “look out” for traffic.
- No Resolution Advisory (RA) – it doesn’t tell pilots to climb or descend.
Display: Traffic shown as symbols (e.g., amber circle for TA, white diamond for other traffic).
Typical users: Smaller general aviation or regional aircraft.

👉 In short: TCAS I = Traffic awareness only (TA). TCAS II = Traffic awareness + Collision avoidance guidance (TA + RA).

TCAS II (Traffic Collision Avoidance System)

Function: Detects nearby aircraft using their transponder signals and warns pilots of potential collisions.
Operation:
- Compares Mode C altitude data from other aircraft.
- Determines bearing using two antennas.
- Requires both aircraft to have working transponders (no protection otherwise).
Alerts:
- TA (Traffic Advisory): Warns of nearby traffic – amber circle.
- RA (Resolution Advisory): Suggests climb/descend action – red square.
- Proximate traffic: Nearby but not a threat – white/cyan diamond.
Types of RA:
- Preventive: Maintain current vertical speed.
- Corrective: Change vertical speed immediately.
Inputs used:
- Aircraft configuration (gear/flaps),
- Pressure altitude (Mode C),
- Radio altimeter (to limit RAs near the ground).
Display: On the electronic VSI or EFIS navigation display.

👉 In short: TCAS II detects other transponder-equipped aircraft, warns pilots of possible collisions, and gives climb/descend instructions if needed—only in the vertical plane.

Here’s a clear comparison between TCAS I and TCAS II 👇

Feature	TCAS I	TCAS II
Main Function	Alerts pilot of nearby traffic (situational awareness)	Alerts pilot and provides collision avoidance instructions
Type of Alerts	TA only (Traffic Advisory)	TA + RA (Traffic & Resolution Advisories)
Avoidance Guidance	None	Provides vertical maneuver (climb/descend) guidance
Collision Prediction	Basic proximity detection	Calculates time to closest approach and relative altitude
Transponder Requirement	Requires Mode C (altitude-reporting) transponders on both aircraft	Same (Mode C or Mode S required)
Display Symbols	White/cyan diamond (proximate), amber circle (TA)	Same + red square for RA
Computational Complexity	Simple	More advanced (predictive algorithms)
Used by	General aviation, small/regional aircraft	Commercial airliners, large transport aircraft
Vertical Guidance	None	Yes (vertical only)
Lateral Guidance	None	None (TCAS II operates only in the vertical plane)

✅ Summary:

TCAS I → Warns pilots of nearby aircraft (awareness only)
TCAS II → Warns pilots and tells them what to do (avoidance maneuver)

TCAS II does not physically control the aircraft — it only provides vertical guidance commands to the pilots.

Here’s how it works:

RA (Resolution Advisory) is a visual and aural instruction such as:
- “CLIMB” / “DESCEND”
- “INCREASE CLIMB” / “ADJUST VERTICAL SPEED”
- “MONITOR VERTICAL SPEED” (preventive RA)
The pilot must manually fly the maneuver by adjusting pitch and power.
TCAS II continuously monitors both aircraft; if the situation changes, it may issue a reversal RA (e.g., from “CLIMB” to “DESCEND”).
Once the threat passes, it announces “CLEAR OF CONFLICT,” and the pilot returns to normal flight.

✅ In short: TCAS II provides advisory guidance only (RA) — it tells the pilot what to do vertically, but does not automatically move the controls or autopilot.

(Automatic avoidance actions are a feature being explored in future systems like ACAS Xa/Xo, not in TCAS II.)