Traffic Alert and Collision Avoidance System (TCAS), poronounced “tee-cass,” was developed to reduce the risk of midair collisions by providing electronic awareness as well as the pilot’s eyes. Civilian aircraft do not have radar that can spot another airplane such as military airplanes have. Most airline and corporate airplanes do have weather radar, but this does not display aircraft. The TCAS system uses three systems to plot the position of aircraft. A directional antenna receives a Mode S transponder signal from nearby aircraft to provide a bearing or direction. The Mode C function of the transponder displays the altitude of the nearby aircraft. Lastly, the timing of the Mode S interrogation/response protocol is measured to determine the distance of the aircraft.
There are two basic types of TCAS. TCAS I is required for turbine-powered passenger-carrying aircraft that have 10 or more seats but less than 31 seats. However, many smaller airplanes may install TCAS I. The GARMIN G1000 avionics can have TIS, TAS or TCAS. This system’s display and audio provide Traffic Advisories (or TAs) to assist the pilots to see and avoid the traffic. Recently, while flying a Cirrus with a G1000 avionics package, I received a TA that changed the color of the target airplane from white to yellow, displayed a yellow “Traffic” display on the Primary Flight Display (PFD), and provided an audible alert of, “Traffic, two o’clock, one mile.” I then spotted the traffic and maneuvered the airplane to avoid the nearby traffic. TCAS I does not provide guidance on how to maneuver to avoid traffic but only alerts you of the traffic and its relative location to you. The pilot’s response to a TA should be to establish visual contact with the aircraft, although pilots may not deviate from an assigned altitude based only on a TA. FAR 91.113 (b) states, “When weather conditions permit, regardless of whether operation is conducted under instrument flight rules or visual flight rules, vigilance shall be maintained by each person operating an aircraft so as to see and avoid other aircraft.”
(See Figures 1, 2 and 3)
TCAS II provides Traffic Advisories, but also provides Resolution Advisories (or RAs) that are guidance to maneuver the airplane on a vertical plane to avoid the target aircraft. RAs are directions to climb, increase climb, descend, increase descent, or maintain and monitor the current altitude by announcing “monitor vertical speed.” It does not provide left/right or turning guidance to avoid traffic. TCAS II is required in the US for commercial aircraft with 31 or more passenger seats and for aircraft with a max takeoff weight of 33,000 lbs. or more.
TACS II displays four levels of traffic:
- Other traffic is displayed as a hollow white or cyan (blue) diamond (non-threat traffic outside the protected area).
- Proximity traffic is displayed as a solid cyan diamond (within 6 NM and plus or minus 1,200 feet vertical).
- Traffic Advisory (TA) is displayed as an amber/yellow circle (considered a threat within 900 feet vertically).
- Resolution Advisory (RA) is displayed as a red square (considered an immediate threat and provides a vertical escape maneuver).
(See Figure 4)
An arrow on the right side of the traffic display pointed up or down indicates that the traffic is climbing or descending at least 500 feet per minute.
The + or – sign and number indicate the relative altitude in hundreds of feet above the traffic symbol when the intruder traffic is above your aircraft and below the traffic symbol if the intruder is below your aircraft. i.e., -10 indicates that the traffic is 1,000 feet lower than your aircraft altitude. Some aircraft can change this to absolute altitude, but few pilots operate in this configuration.
A TCAS II Resolution Advisory (RA) changes the color of the intruder aircraft to red on the PFD or other display and provides an audible warning such as “Traffic, Climb.” The pilot should start a climb or descent as directed within 5 seconds and climb at a rate of 1,500 to 2,000 feet per minute. This is shown as a green target on the traffic display. If a greater rate is required, another audible warning will be given. This aural warning will state “increase climb/descent” and requires a 2,500 minimum vertical speed and a response within 2.5 seconds. The 2.5 seconds to respond also applies to a TCAS “Reversal,” where the direction to respond vertically is changed by the TCAS. At about 300 to 500 feet of altitude change, the TCAS will announce “level off” and move the green target to zero on the Vertical Speed Indicator (VSI). After the target airplane is clear, the TCAS will announce “clear of conflict,” at which point the pilot will return to the previously assigned altitude. Pilots are required to notify Air Traffic Control (ATC) if they are deviating/changing altitude for a TCAS Resolution Advisory and report to ATC again when they are clear of the conflict from the intruder aircraft.
(See Figures 5, 6 and 7)
Older versions of TCAS would have had the pilots climb or descend to just over 1,000 feet before directing them to “Adjust Vertical Speed,” which meant reducing vertical speed or leveling off. That had the potential of causing a secondary collision conflict. The latest version is more succinct, with a simple direction to “level off.” The latest version, TCAS 7.1, also directs pilots to level off between 300 to 500 feet from their original altitude. I still see most pilots in simulator training grossly exceed the new requirement of climbing/descending at the recommended 1,500 feet-per-minute and to level off when directed, not to exceed 500 feet. The required pitch adjustment is smaller than most pilots perceive.
ATC regularly separates aircraft traffic by time, speed, distance, and altitude. They also frequently point out traffic to each aircraft to make them aware of nearby aircraft and to help the pilot pick up sight of the aircraft to see and avoid it visually. This is stated something like this: “Challenger 605, Boeing 737 at one o’clock, two miles, 1,000 feet above your altitude.” But when a pilot reports to ATC that they’re deviating for an RA by stating “Challenger 605, TCAS RA,” ATC may not assign an altitude to the aircraft but must let the pilots follow the TCAS electronic corrections and guidance. If ATC had provided the pilots’ aircraft an assigned heading to avoid the target traffic, and it subsequently received an RA, the pilots should follow the TCAS for up or down vertical guidance and follow ATC direction for lateral guidance. After the pilots advise ATC that they are “clear of conflict,” ATC may resume control of the aircraft.
This rule of relinquishing ATC control to TCAS goes back to an accident on July 1, 2002, in Southern Germany, where ATC directed the airplane to descend, and the TCAS directed the airplane to climb. The crew descended, which was contrary to the TCAS instructions that advised the crew to climb. This resulted in a midair collision and fatal accident. The other airplane’s TCAS also directed it to descend. Had both airplanes followed the TCAS, there would not have been an accident, according to the investigating authority. This accident changed or clarified the rule to follow the TCAS guidance if conflicting instructions are provided by ATC. The advantage here is that the TCAS for two aircraft communicate with each other, coordinating the response, so one will be directed to climb and the other to descend.
Lately, there have been multiple near-midair collisions and one midair accident. Reference a go-around a few days ago, on February 2nd, the FAA made the following statement: “An air traffic controller instructed Air France Airlines Flight 378 to perform a go-around at Detroit Metropolitan Wayne County Airport around 7:05 p.m. local time because a previous arrival was slow to exit the runway.” Earlier that same day at the Goodyear, Arizona Airport, I heard an excited Air Traffic Tower Controller direct two light airplanes in the traffic pattern who were getting too close to each other to immediately change altitudes and headings. On January 14th at Phoenix Sky Harbor Airport, Delta Airlines and United Airlines had to respond to their respective TCAS Resolution Advisories after getting too close. The airline’s TCAS II provides an alert or caution notice when aircraft are about 40 seconds apart and provides an increased warning and Resolution Advisory when about 25 seconds apart.
On January 29th, an Army Blackhawk UH-60 helicopter (callsign PAT-25) collided midair with American Airlines Flight 5342 operated by PSA, a Bombardier CRJ-700, over the Potomac River at Ronald Reagan Washington National Airport (KDCA). The CRJ-700 was about 330 feet AGL, performing a circling approach to runway 33 at night when the collision occurred. The CRJ-700 was flying North up the Potomac River on the Mt Vernon Visual Approach to Runway 1 when the control tower asked if they could take Runway 33, to which they replied yes. The tower advised Blue Streak 5342 (the CRJ-700) to circle to runway 33 after passing the Wilson Bridge and cleared them to land. The Army Blackhawk helicopter was flying South down the Potomac River. The tower controller reported the CRJ traffic to the helicopter, stating, “Traffic just South of the Willson Bridge is a CRJ at 1,300 feet for runway 33. Do you have the traffic in sight?” Tower then directed the helicopter to pass behind the CRJ. TCAS RAs and aural annunciations would have been inhibited at this altitude. This was the first fatal FAR Part 121 Airline Accident in the US since the Colgan Air Flight 3407, a Bombardier Q400, crashed because of an aerodynamic stall in Buffalo, New York, on February 12, 2009.
Limitations and Inhibiting Functions (AC 20-151C)
- To prevent nuisance or unwanted RAs, TCAS alerts, warnings, and aural annunciations are inhibited below certain altitudes.
- TCAS “Increase Descent” RAs are inhibited below 1,450 feet while descending (1,650 feet while climbing) Above Ground Level (AGL).
- TCAS “Descend” RAs are inhibited below 1,000 feet while descending (1,200 feet while climbing) AGL.
- All RAs are inhibited below 900 feet while descending (1,100 feet while climbing) AGL.
- All TCAS aural traffic advisories (voice messages) are inhibited below 400 feet while descending (600 feet while climbing) AGL.
- TCAS will not track or display non-transponder equipped aircraft.
- GPWS alerts, stall warnings, and wind shear take precedence over a TCAS RA.
While the threat of a midair collision is reduced with the implementation of TCAS, as we have seen, it is not eliminated. Remain vigilant.
