TCAS (the traffic alert and collision avoidance system) watches the transponders of nearby aircraft and, if one is on a collision course, tells the pilots to climb, descend or level off. Ground proximity and terrain awareness warnings do the same job for the ground, calling out “terrain, terrain, pull up”. Both are now required on airliners in the US, and each was made mandatory after crashes.
Two alarms that changed flying
The two systems solve the same problem: a crew that does not know something is in its way. For TCAS, that is another aircraft. For the terrain warnings, it is a hill, a mountain or the sea. Both are a last line of defence. Air traffic control, charts and the crew’s own checks come first. They exist because those earlier defences have sometimes failed together.
TCAS: what a traffic advisory and a resolution advisory mean
TCAS II, the version used on airliners, works out how quickly each nearby aircraft is closing in range and height. According to the FAA’s guide to the system, it can give two kinds of alert:
- Traffic advisory (TA): “Traffic, traffic.” It helps the pilots look for the other aircraft and prepares them for a possible resolution advisory.
- Resolution advisory (RA): an instruction such as “Climb, climb”, “Descend, descend” or “Level off”, which increases or keeps the vertical separation from the other aircraft. When the danger has passed, TCAS announces “Clear of conflict”.
When both aircraft carry TCAS II, the two systems coordinate their RAs over the Mode S transponder data link, so one aircraft is told to climb and the other to descend. That only works if both crews follow their RA.
Under 14 CFR 121.356, US airline aircraft of more than 33,000 pounds must carry TCAS II with a Mode S transponder. Passenger aircraft with 10 to 30 seats must carry at least TCAS I, which gives traffic advisories.
The collisions that led to it
These events do not have pages on this site yet; Mid-Air Collisions covers them in more detail.
- Grand Canyon, 30 June 1956. A TWA Lockheed Super Constellation and a United Air Lines DC-7 collided over the Grand Canyon, and all 128 people on board were killed. Both were at 21,000 feet. The TWA flight was flying “1,000 on top”, a visual clearance under which air traffic control was not responsible for separating it from other aircraft. The FAA says the collision was a major factor in the creation of the Federal Aviation Agency in 1958 and in major changes to air traffic control.
- Cerritos, California, 31 August 1986. Aeroméxico Flight 498, a DC-9 on approach to Los Angeles under radar control, collided with a Piper PA-28 that had strayed into controlled airspace without clearance. Sixty-four people on the DC-9, three on the Piper and 15 on the ground were killed. The NTSB found that the probable cause was the limits of the air traffic control system to provide collision protection. Among the contributing factors were the limits of the “see and avoid” concept. The FAA’s TCAS guide says Cerritos led to a Congressional mandate (Public Law 100-223). In 1989 the FAA required passenger aircraft with more than 30 seats flying in US airspace to carry TCAS II. The deadline was later extended to the end of 1993.
Überlingen 2002: when TCAS and the controller disagreed
On the night of 1 July 2002, at 21:35:32 UTC, Bashkirian Airlines Flight 2937, a Tupolev Tu-154M flying from Moscow to Barcelona, collided with DHL Flight 611, a Boeing 757 freighter, over Überlingen in southern Germany. All 69 people on the Tupolev and the two pilots of the 757 were killed. Both aircraft had TCAS II.
Both crews received a TCAS traffic advisory at 21:34:42. Seven seconds later, the single controller on duty at Zurich told the Tupolev to descend and expedite. At 21:34:56, TCAS told the Tupolev crew to climb, and the 757 crew to descend. The Tupolev’s copilot pointed out that TCAS said climb. The Tupolev crew followed the controller and kept descending, against their RA. The 757 crew followed theirs. Both aircraft were now descending.
Germany’s accident investigation bureau, the BFU, found the immediate causes. The controller did not notice the developing conflict in time. The descent instruction was given when separation from the 757 could no longer be assured. The Tupolev crew kept descending after TCAS told them to climb. The BFU’s systemic causes included ICAO, manufacturer and airline rules on TCAS that were not standardised, were incomplete and partly contradicted each other. It also cited the practice of leaving one controller to work alone at night.
The rule that followed. On 1 October 2002 the BFU recommended that ICAO give TCAS resolution advisories priority over air traffic control instructions. ICAO’s changes to its procedures and its standards for operations manuals were adopted on 13 March 2003 and took effect on 27 November 2003. The message is now simple: if TCAS gives an RA, follow it, even if a controller says otherwise. TCAS II version 7.1, published in June 2008, also added logic to reverse an RA when one aircraft is doing the opposite of what it was told.
GPWS and EGPWS: “terrain, pull up”
GPWS, 1974. After studies found many accidents in which a working aircraft was flown into the ground, the FAA required ground proximity warning systems on US airliners from 1974. These accidents are explained in Controlled Flight Into Terrain.
EGPWS and TAWS, 2000 rule. The early systems could warn too late. On American Airlines Flight 965 near Cali in 1995, the GPWS worked but did not give the crew enough time to clear the mountains. The FAA’s 2000 rule required a terrain awareness and warning system (TAWS, also called enhanced GPWS). The FAA says it gives much earlier warnings, looks ahead of the aircraft, and keeps working when the aircraft is configured to land. Under 14 CFR 121.354, turbine airliners built after 29 March 2002 must have Class A TAWS with a terrain display, and all turbine airliners had to have it by 29 March 2005.
The callouts are short and blunt. The NTSB’s report on US Airways Flight 1549 records the warnings the Airbus gave as it glided towards the Hudson River with both engines out: “too low, terrain”, “too low, gear”, “terrain, terrain, pull up” and “caution terrain”. In that case the crew knew exactly where they were going.
What these systems cannot do
- TCAS cannot see aircraft without a working transponder. The FAA’s guide says this plainly: since TCAS does not detect aircraft that are not transponder-equipped, or whose transponder has failed, TCAS alone does not ensure safe separation in every case.
- TCAS stops giving RAs close to the ground. In its report on the January 2025 mid-air collision over the Potomac River near Washington, the NTSB explains that on the CRJ700 involved, resolution advisories are automatically inhibited below about 1,000 feet above the ground. Only traffic advisories are given.
- A warning has to be obeyed. On Air Niugini Flight 73 in 2018, the crew of a Boeing 737-800 continued an approach into heavy rain below the minimum altitude. Papua New Guinea’s Accident Investigation Commission found they disregarded 13 aural EGPWS alerts before the aircraft struck the lagoon short of the runway at Chuuk. One passenger died.
- Too many false alarms train crews to ignore them. In its 2000 rule the FAA noted that if an alarm sounds too often as a false alarm, crews either begin to ignore it or are tempted to switch it off. Alert timing is a balance between early warning and nuisance alerts.
Each system is one layer among several, which is the idea behind the Swiss cheese model. For collisions on the ground, the same thinking produced runway status lights and surface radar; see Runway Incursions.
The alarms compared
| System | What it warns about | Required in the US (airlines) | Accidents behind it |
|---|---|---|---|
| GPWS | Terrain close to the aircraft (the original system) | 1974 (Part 121 airlines) | CFIT accidents studied in the early 1970s |
| TCAS II | Aircraft on a collision course, with climb or descend instructions | 1989 rule; full fleet by the end of 1993 | Grand Canyon (1956), Cerritos (1986) |
| Airborne wind shear warning | Wind shear on approach and take-off | Aircraft built after 2 January 1991 (14 CFR 121.358) | Microburst accidents including Delta 191 (1985) |
| TAWS (enhanced GPWS) | Terrain ahead, with a terrain display | New aircraft from 29 March 2002; all turbine airliners by 29 March 2005 | Cali (1995) and CFIT accidents to corporate jets |
| ICAO rule: follow the RA | Gives TCAS RAs priority over ATC instructions | Applicable worldwide from 27 November 2003 | Überlingen (2002) |
Sources
- Federal Aviation Administration: Introduction to TCAS II Version 7.1 (booklet)
- Federal Aviation Administration: Lessons Learned: Bashkirian Airlines Flight 2937 and DHL Flight 611, Überlingen, Germany, July 1, 2002 (summary of the BFU report and the changes after it)
- Federal Aviation Administration: Lessons Learned: TWA Flight 2 and United Air Lines Flight 718, Grand Canyon, Arizona, June 30, 1956
- National Transportation Safety Board: Aircraft Accident Report NTSB/AAR-87/07, Collision of Aeronaves de Mexico DC-9-32, XA-JED, and Piper PA-28-181, N4891F, Cerritos, California, August 31, 1986
- US Code of Federal Regulations (eCFR): 14 CFR 121.356, Collision avoidance system
- Federal Aviation Administration: Terrain Awareness and Warning System, final rule, 65 FR 16736, 29 March 2000
- US Code of Federal Regulations (eCFR): 14 CFR 121.354, Terrain awareness and warning system
- Accident Investigation Commission, Papua New Guinea: Final Report AIC 18-1004, Air Niugini P2-PXE, Boeing 737-8BK, Chuuk Lagoon, 28 September 2018
- National Transportation Safety Board: Aircraft Accident Report NTSB/AAR-10/03, US Airways Flight 1549 (GPWS and EGPWS alerts recorded during the ditching approach)
- National Transportation Safety Board: Aviation Investigation Report AIR-26-02, Midair Collision over the Potomac River, PSA Airlines Flight 5342 and US Army PAT25, January 29, 2025 (TCAS RA inhibit altitude)
Last checked 31 October 2026.