What do TCAS, a transponder and an IVSI do?

TCAS detects transponder-equipped traffic, predicts collision risks and warns the crew; TCAS II can also command a vertical avoidance response. A transponder replies with the aircraft’s code, identity and pressure altitude as equipped. An IVSI shows climb or descent rate and, in combined installations, can display TCAS traffic and guidance.

In our Aviation & Real-World Flying coverage, we use the real cockpit definitions. Simulator panels sometimes put all three functions on one gauge or control panel, but they are not interchangeable: the transponder communicates, TCAS assesses traffic, and the IVSI indicates vertical movement.

What does each system do?

TCAS predicts traffic conflicts, the transponder makes an aircraft electronically visible to compatible surveillance systems, and the IVSI shows vertical speed.

TCAS is not an onboard primary radar and does not receive the controller’s radar picture. It relies mainly on cooperative replies from other aircraft, so ATC may see traffic that TCAS cannot.

What is an IVSI instrument in aviation?

An IVSI is a vertical speed instrument designed to respond faster than a conventional pneumatic VSI when a climb or descent begins.

A conventional VSI measures the pressure difference created as static pressure changes, but its calibrated leak introduces noticeable lag. A traditional IVSI adds an acceleration-sensitive mechanism that produces an earlier indication. The word instantaneous means reduced lag, not perfect or delay-free measurement; turbulence and abrupt acceleration can still cause brief needle movement.

Not every IVSI contains TCAS. A plain IVSI has only a vertical-speed indication, while a combined unit adds traffic symbols, relative-altitude tags and RA sectors around the same dial. Modern glass flight decks may instead place traffic on a navigation display and RA guidance on the primary flight display.

Our detailed explanation of how VSI and IVSI instruments sense climb and descent covers the pressure system and instrument lag. In a simulator, an IVSI-labelled gauge may read vertical-speed data directly without modelling the pneumatic behaviour of the real instrument.

How do TCAS and a transponder work together?

The transponder supplies surveillance replies; TCAS uses those replies to track traffic, calculate risk and generate an advisory when required.

1. The intruder’s transponder replies. Mode C and Mode S equipment can report encoded pressure altitude. Mode S also supports selective addressing and communication used to coordinate compatible TCAS II resolution advisories.
2. TCAS builds a track. The observing aircraft estimates range, bearing, closure and, when altitude data is available, relative altitude and vertical trend.
3. Threat logic predicts the encounter. TCAS considers time to the projected closest approach as well as separation. A nearby aircraft flying away may be less significant than a more distant aircraft closing rapidly.
4. The display classifies the traffic. Symbols distinguish ordinary or proximate traffic from a traffic advisory or resolution advisory. The exact shapes and colours depend on the display.
5. TCAS II coordinates compatible RAs. If both aircraft have suitable TCAS II and Mode S equipment, the systems exchange information so that their vertical commands complement rather than oppose each other.

The altitude sent by the transponder is normally pressure altitude referenced to 1013.25 hPa or 29.92 inHg, not the indicated altitude obtained from the local QNH setting. Using a common datum lets two TCAS installations compare altitude consistently. Turning the altimeter-setting knob will not correct a bad transmitted altitude; the encoder or avionics data source must be checked.

Some installations use ADS-B information to support traffic surveillance and reduce interrogations. ADS-B, a traffic display and TCAS are still separate functions, and the presence of ADS-B traffic does not by itself prove that resolution advisories are available.

What do TA and RA mean in aviation?

A traffic advisory draws attention to a possible threat, while a resolution advisory provides vertical guidance intended to avoid the conflict.

• TA — Traffic Advisory: TCAS highlights the aircraft and may announce Traffic, traffic. The crew looks for the traffic and prepares for a possible RA. A TA alone is not an instruction to climb, descend or turn.
• Preventive RA: TCAS identifies vertical speeds that must not be entered. If the aircraft already complies, a major flight-path change may not be required.
• Corrective RA: TCAS commands a change such as climbing, descending, levelling off or reducing vertical speed. Spoken wording varies with the TCAS standard and installation.

TCAS I provides TAs but cannot issue RAs. TCAS II provides both when the aircraft, operating mode, altitude data and encounter permit them.

What is the classic TCAS vertical response?

Classic TCAS II resolution advisories command or prohibit vertical speed; they do not supply a left or right escape turn.

On a combined TCAS/IVSI, red sectors show vertical speeds to avoid, while a corrective RA may also provide a green target sector. Newer displays may put equivalent guidance on a vertical-speed tape. The pilot follows the RA indication and approved procedure rather than turning towards or away from the apparent bearing of the traffic symbol.

Under standard operating procedures, an RA is followed unless doing so would endanger the aircraft, even when it conflicts with an ATC vertical instruction. ATC is advised as workload permits, and the crew returns to the clearance after the conflict is resolved. Simulator users can compare how different cockpits represent these stages in our guide to TCAS traffic advisories and resolution advisories in flight simulators.

What does TA/RA mean on a transponder panel?

TA/RA is a TCAS operating selection, not a special transponder type or squawk code. It often appears on a shared transponder and TCAS control panel, which causes the common name “TA/RA transponder”.

• STBY: The transponder normally does not provide routine surveillance replies, and the collision-avoidance system is not in normal operating mode.
• XPDR, ON or ALT: These labels concern transponder replies or altitude reporting. They do not necessarily mean that TCAS resolution advisories are armed.
• TA ONLY: Traffic surveillance and TAs are available, but RAs are deliberately inhibited.
• TA/RA: TCAS II may issue both types of advisory when all other conditions are met.

Panel labels and automatic air/ground switching differ between aircraft, so the aircraft checklist takes precedence over a generic switch sequence. Changing the squawk code or pressing IDENT will not cure a blank TCAS display. Our guide to squawk codes and transponder operating modes explains what those controls actually change.

Does TCAS work in airports and near the ground?

A transponder remains useful at airports, but TCAS is an airborne collision-avoidance system rather than a runway-incursion or ground-traffic warning system.

Airport procedures may require an operating transponder on the ground so that surface surveillance can identify the aircraft. Do not assume that every aircraft should be placed in standby after landing; use the published procedure and aircraft checklist.

TCAS installations use air/ground information, radio altitude and threat logic to filter aircraft known to be on the surface and to inhibit inappropriate advisories near the ground. Some traffic may still appear on the display, but a blank or limited display while parked can be normal. TCAS does not confirm that a runway or taxiway is clear.

Can TCAS see every aircraft?

TCAS cannot reliably detect an aircraft that lacks a suitable operating transponder.

An aircraft with no transponder, a failed unit or a unit in standby may be invisible to TCAS even if primary radar or a pilot can see it. A target without valid altitude reporting may appear with limited information, but TCAS cannot perform the same vertical threat assessment or issue a normal coordinated RA against it.

An aircraft carrying only a transponder can be detected by another aircraft’s TCAS, but it receives no warning of its own. TCAS therefore supplements rather than replaces visual scanning, ATC separation and normal procedures.

Why is TCAS blank or not giving guidance in a flight simulator?

A blank simulated TCAS usually results from missing traffic, an incorrect operating mode, an unpowered display, restrictive filters or an aircraft add-on that does not read the selected traffic source.

1. Confirm what is installed. A plain IVSI or generic vertical-speed tape has no traffic function. Some simulated aircraft provide a traffic display but do not model full TCAS II resolution advisories.
2. Check electrical power and brightness. The avionics bus, relevant display and TCAS or transponder control must be powered. A dimmed display can look completely dead.
3. Enable a supported traffic source. AI traffic, live traffic and multiplayer aircraft are handled differently by each simulator and add-on. Traffic must also contain usable position and transponder data.
4. Select the correct modes. Set the transponder and TCAS controls as the simulated aircraft’s checklist requires. TA ONLY intentionally prevents RAs; standby can prevent all useful traffic surveillance.
5. Check range and altitude filters. A short display range or an above/below altitude filter can hide valid targets without indicating a fault.
6. Use a genuine conflict scenario. TCAS does not issue an RA merely because another aircraft is visible. Closure, predicted separation, altitude, phase of flight and advisory inhibits all affect the result.
7. Consider add-on compatibility. A custom aircraft may use its own traffic logic and support only particular simulator traffic sources. Incomplete AI transponder data can also produce traffic without relative altitude or prevent an RA.

A mistake we see often is treating the traffic symbol as a conventional radar contact and expecting guidance whenever two aircraft look close on the map. TCAS responds to a predicted collision threat, not simple map distance, and many simulators simplify its coordination, ground filtering and altitude-reporting behaviour.