What do TCAS, the transponder and IVSI do? Learn how they interact, what TA and RA alerts mean, and why altitude reporting matters.
In real-world aviation, TCAS uses replies from nearby aircraft transponders to detect collision threats and issue alerts. The transponder supplies identity and altitude surveillance data. An IVSI indicates climb or descent rate; when integrated with TCAS, it can also present traffic and the vertical guidance needed to avoid a conflict.
The three systems are connected, but they are not interchangeable: the transponder communicates, TCAS assesses traffic, and the IVSI indicates vertical movement or displays TCAS guidance.
What does each system do?
TCAS detects collision threats, the transponder makes the aircraft electronically visible, and the IVSI displays vertical speed.
| System | Full name | Primary function | What the pilot sees |
|---|---|---|---|
| TCAS | Traffic Alert and Collision Avoidance System | Tracks suitably equipped aircraft and predicts possible conflicts. TCAS I provides traffic advisories; TCAS II can also issue resolution advisories. | Traffic symbols, relative altitude, vertical trend and spoken or visual alerts. |
| Transponder | Not an abbreviation | Replies to secondary surveillance radar and TCAS interrogations. Depending on its mode and equipment, it provides a squawk code, pressure altitude, Mode S identity and other surveillance data. | A control panel showing the selected code, operating mode and IDENT status; it does not itself display nearby traffic. |
| IVSI | Instantaneous Vertical Speed Indicator | Shows the aircraft's rate of climb or descent, usually in feet per minute. A combined TCAS/IVSI also acts as a traffic and resolution-advisory display. | A vertical-speed needle or scale, sometimes surrounded by TCAS traffic symbols and coloured advisory sectors. |
A conventional vertical speed indicator responds to changes in static pressure but has noticeable lag. A traditional IVSI adds acceleration sensing to reduce that delay, although “instantaneous” does not mean perfectly immediate. Our explanation of how a vertical speed indicator measures climb and descent covers the underlying instrument in more detail.
Not every IVSI includes TCAS. A plain IVSI only reports vertical speed. Combined TCAS/IVSI units were common retrofit instruments, while newer flight decks may put traffic on a navigation display and RA guidance on the primary flight display instead.
How do TCAS, the transponder and IVSI work together?
TCAS depends on transponder replies to find cooperative traffic, then presents its assessment on a dedicated display, combined IVSI or modern flight-deck screen.
- The transponder replies. An aircraft with an operating Mode C or Mode S transponder supplies encoded pressure altitude and identification data appropriate to its mode.
- TCAS tracks the target. The observing aircraft's TCAS measures range, bearing, relative altitude and closure over time. Some installations supplement this with ADS-B data, but ADS-B and TCAS remain distinct functions.
- Threat logic evaluates the encounter. TCAS considers factors such as closure rate, altitude separation and the time remaining before the projected closest approach.
- The display identifies the threat. Traffic may appear around an IVSI or on another cockpit screen, accompanied by relative-altitude figures and climb or descent arrows.
- TCAS II issues vertical guidance if required. A resolution advisory identifies vertical speeds that should be achieved or avoided. When both aircraft have compatible TCAS II systems, their advisories are coordinated through Mode S so they do not command conflicting responses.
The altitude transmitted by a transponder is normally pressure altitude referenced to the standard datum, rather than the altitude shown after setting local QNH. This common reference lets TCAS compare two aircraft consistently. Changing the cockpit altimeter setting is therefore not a valid way to correct an erroneous TCAS altitude tag.
What do TA and RA alerts mean?
A traffic advisory warns that another aircraft may become a threat; a resolution advisory tells the crew which vertical response is required or prohibited.
- TA — Traffic Advisory: TCAS may announce Traffic, traffic and highlight the target. The crew searches for the aircraft and prepares for a possible RA. A TA alone is not vertical manoeuvring guidance.
- RA — Resolution Advisory: TCAS II may call for a climb, descent, reduced vertical rate, level-off or another vertical response. Classic TCAS II guidance is vertical, not a command to turn towards or away from the displayed symbol.
On a combined TCAS/IVSI, red sectors normally mark vertical speeds to avoid. Corrective advisories may also provide a green target sector, depending on the instrument and advisory type. The pilot follows the approved RA procedure rather than steering by the apparent position of the traffic symbol, which is not a primary-radar picture.
TCAS I can issue TAs but not RAs. An aircraft carrying only a transponder can be detected by another aircraft's TCAS, but it receives no warning of its own unless it also has suitable traffic-alert equipment.
Why can TCAS traffic or guidance be missing?
Missing traffic or guidance usually comes from the selected mode, absent altitude reporting, incompatible equipment or a display that is only a basic IVSI.
- Transponder in standby: A transponder in STBY normally does not answer routine radar or TCAS interrogations, so other TCAS-equipped aircraft cannot obtain normal cooperative tracking data from it.
- Altitude reporting disabled or faulty: Without valid Mode C or Mode S altitude, TCAS cannot provide full vertical threat protection. Incorrect encoded altitude can also produce misleading relative-altitude information and requires an avionics check.
- Intruder has no suitable transponder: TCAS cannot reliably detect non-cooperative traffic such as an aircraft with its transponder switched off. It is not a replacement for an outside visual scan.
- TA ONLY selected: This mode displays traffic advisories but deliberately inhibits resolution advisories. Full TCAS II operation generally requires TA/RA mode when procedures permit it.
- Plain IVSI installed: A basic IVSI has no traffic-display capability, even if it looks similar to a combined TCAS/IVSI gauge.
Panel labels vary. ALT may describe transponder altitude reporting without confirming that TCAS RAs are armed, while TA ONLY and TA/RA refer to the collision-avoidance mode. Crews use the aircraft checklist because air/ground switching and integrated controls differ between installations.
In a flight simulator, a blank TCAS display can also mean that AI or multiplayer traffic is disabled, the aircraft is not supplying electrical power to the unit, or the add-on cannot read the simulator's traffic source. A practical legacy example is the FS2004 panel combining TCAS II, an IVSI and a transponder, which illustrates why all three controls and displays may appear together while performing separate jobs.