The International Civil Aviation Organisation (ICAO) defines all RAs as involving either “a manoeuvre intended to provide separation from all threats” or “a manoeuvre restriction intended to maintain existing separation.” ICAO further breaks down this term as either a corrective RA, which “advises the pilot to deviate from the current flight path” or a preventive RA, which “advises the pilot to avoid certain deviations from the current flight path but does not require any change in the current flight path.”
Traffic-alert and collision avoidance system (TCAS II) logic, currently at Version 7.1, automatically identifies a risk of midair collision and issues alerts for affected pilots. The system is known in some world regions as the airborne collision avoidance system (ACAS). The risk identification is based upon in-flight analysis of the relative trajectories of two or more closing aircraft and second-by-second projection of their future relative positions. In Europe, installation of Version 7.1 software was mandated with effect on 1 December 2015.
Based on secondary surveillance radar transponder signals, this mature technology enables — within defined distances — automatic exchange of position, time, velocity and other parameters between equipped aircraft.
Most importantly, TCAS II provides an independent, last-resort safety net'' irrespective of air traffic control (ATC) separation standards or the pilots’ situational awareness, according to EUROCONTROL. The agency also notes, “While assessing threats, it does not take into account the ATC clearance, pilot’s intentions or autopilot inputs.”
Despite TCAS II’s accuracy and reliability, accident and incident investigations reveal that pilots at times take actions contrary to the resolution advisory (RA), the system’s most urgent collision-avoidance alert. In this circumstance, the logic generates a TCAS RA reversal — e.g., a “Climb, climb NOW” RA seconds after a “Descend, descend” RA. TCAS RA reversals also occur because the pilot of a TCAS-unequipped aircraft changes the flight path from a climb to a descent (with unknown intentions) or while relying solely on visual traffic avoidance.
The TCAS RA reversal also could be prompted, for example, by a pilot’s misguided decision to comply with an opposite ATC instruction during the RA. Overriding the TCAS II logic this way likely will increase the risk of collision. Therefore, pilots and controllers must understand these functions well and comply with the responses mandated by flight operations standards, according to EUROCONTROL.
Hazards & Effects
Upon identification of the collision risk, TCAS II detection algorithms trigger calculation of coordinated avoidance manoeuvres for the affected aircraft. The device in each aircraft initially alerts the pilot by a simultaneous and direct flight instrument display and by aural annunciation of the respective RA. (The system generates RAs within one second of the detected conflict because one second is the “situation refresh” interval of TCAS II.)
Coordination of the RAs essentially means requiring the pilot of Aircraft A to climb while requiring the pilot of Aircraft B, also at risk, to descend — or TCAS II could require the pilot of either aircraft to maintain an altitude.
EUROCONTROL frequently reminds the aviation community that safe separation depends on the pilot of each aircraft promptly and accurately following (i.e., complying with) the initial RA. Therefore, whenever TCAS II generates an RA, the flight crew must comply — even if complying is contrary to the instructions received from the controller.
The SKYbrary article "ATC Avoiding Instructions Opposite to TCAS RA" describes controversial history surrounding risk scenarios that require TCAS RA reversals. Notably, some controllers intervened improperly based on their awareness of a TCAS RA (i.e., from an RA downlink).
Contributory factors in two influential accidents — the 2001 Yaizu accident and the 2002 Überlingen midair collision — and several serious incidents led to several technological and procedural improvements related to TCAS RA reversals, according to EUROCONTROL.
The SKYbrary article "ATC Avoiding Instructions Opposite to TCAS RA" explains the ATC involvement this way: “When a loss of separation is likely to occur or has occurred, the controller will issue instruction(s) to one or both aircraft. These instructions may conflict with a TCAS RA, creating the potential for confusion and the possibility of an inappropriate response."
“Such a conflict often arises because of the much more rapid update of relative position available to the flight crew via TCAS [as noted, an update every second] than to ATC with even the best radar-refresh rates [e.g, an update every 4.8 seconds). Controllers must also:
- potentially react to more than one conflict detection source (e.g., radar display and short term conflict alert (STCA);
- assess the situation;
- develop a solution in a very short period of time; and
- communicate this solution to the aircrew as quickly and clearly as possible."
“Controller detection of a potential conflict may be delayed due to tasks with other aircraft under control and communications with conflicting aircraft may then also be delayed because of [radio frequency] congestion or misunderstandings between the controller and the pilots.”
The SKYbrary article "RA Downlink" explains the potential effect of an TCAS RA reversal this way: “The information presented to the controller might be outdated at the moment it appears on the situational display (due to surveillance latency), which would make it outdated and therefore potentially unsafe. Even if the controller is only using the RA [downlink] “for information” regarding the direction [i.e., sense] of the level change, this information may be not up to date (e.g., due to an RA reversal) at the time it is presented to the controller. Even a delay of a few seconds could be significant.”
Technologically, the improved reversal logic of TCAS Version 7.1 already has been a major step forward. This upgraded software monitors RA compliance in coordinated encounters (i.e., as noted, when both aircraft are TCAS II equipped).
When the system detects that an aircraft is not responding correctly to an RA, TCAS II issues a reversal RA to the aircraft pilot, requiring manoeuvres in accordance with the RA. EUROCONTROL guidance explains: “In single-equipage encounters (i.e., when only one aircraft is TCAS II equipped), Version 7.1 will recognise the situation and will issue a reversal if the unequipped threat aircraft moves in the same vertical direction as the TCAS II–equipped aircraft. Although the reversal logic change is transparent to flight crews, it will, nevertheless, bring significant safety improvements.”
Procedurally, TCAS RA reversals have long been an element of international technical standards and practices. These include ICAO standards and recommended practices (SARPs) and European Commission Implementing RuIes–Operations (IR-OPS) and European Commission EU OPS 1, which requires that,
“An operator shall establish procedures to ensure that:
... When undue proximity to another aeroplane (RA) is detected by [ACAS/TCAS II], the commander or the pilot to whom conduct of the flight has been delegated must ensure that any corrective action indicated by the RA is initiated immediately, unless doing so would jeopardize the safety of the aeroplane;
The corrective action must:
- Never be in a sense opposite to that indicated by the RA; and,
- Be in the correct sense [e.g., climb or descend] indicated by the RA, even if this is in conflict with the vertical element of an ATC instruction.”
Accidents & Incidents
- T154 / B752, en-route, Uberlingen Germany, 2002 (On 1st July 2002, a Russian-operated Tu154 on a passenger flight collided at night with a cargo Boeing 757-200 over Überlingen, Germany with the consequent loss of control of both aircraft and the death of all occupants. The collision occurred after an ATC control lapse had led to a conflict which generated coordinated TCAS RAs which the B757 followed but the TU-154, in the presence of a conflicting ATC instruction, did not.)
- A319/A319, en-route, South west of Basle-Mulhouse France, 2010 (On 29 June 2010, an Easyjet Switzerland Airbus A319 inbound to Basle-Mulhouse and an Air France Airbus A319 outbound from Basle-Mulhouse lost separation after an error made by a trainee APP controller under OJTI supervision during procedural service. The outcome was made worse by the excessive rate of climb of the Air France aircraft approaching its cleared level and both an inappropriate response to an initial preventive TCAS RA and a change of track during the ensuing short sequence of RAs by the Training Captain in command of and flying the Easyjet aircraft attributed by him to his situational ‘anxiety’.)
- A320 / B738, en-route, near Córdoba Spain, 2014 (On 30 October 2014, a descending Airbus A320 came close to a Boeing 737-800 at around FL 220 after the A320 crew significantly exceeded a previously-instructed 2,000 fpm maximum rate of descent assuming it no longer applied when not reiterated during re-clearance to a lower altitude. Their response to a TCAS RA requiring descent at not above 1,000 fpm was to further increase it from 3,200 fpm. Lack of notification delayed the start of an independent Investigation but it eventually found that although the A320 TCAS equipment had been serviceable, its crew denied failing to correctly follow their initial RA.)
- A320/B738, vicinity Delhi India, 2013 (On 2 September 2013, a B737 crew were not instructed to go around from their approach by ATC as it became increasingly obvious that an A320 departing the same runway would not be airborne in time for a landing clearance to be issued. They initiated a go around over the threshold and then twice came into conflict with the A320 as both climbed on similar tracks without ATC de-confliction, initially below the height where TCAS RAs are functional. Investigation attributed the conflict to ATC but the failure to effectively deal with the consequences jointly to ATC and both aircraft crews.)
- E55P, Blackbushe UK, 2015 (On 31 July 2015 a Saudi-operated Embraer Phenom on a private flight continued an unstabilised day visual approach to Blackbushe in benign weather conditions. The aircraft touched down with excess speed with almost 70% of the available landing distance behind the aircraft. It overran and was destroyed by impact damage and fire and all occupants died. The Investigation concluded that the combination of factors which created a very high workload for the pilot "may have saturated his mental capacity, impeding his ability to handle new information and adapt his mental model" leading to his continuation of a highly unstable approach.)
- RJ1H/UNKN, vicinity Malmo Sweden, 2009 (On 13 October 2009, an Avro RJ100 being operated by Malmo Aviation on a scheduled passenger flight from Stockholm Bromma to Malmo in day VMC came into proximity with a unseen light aircraft crossing below which activated a TCAS RA which was followed. The flight crew were unaware that they were outside controlled airspace at the time. No abrupt manoeuvring occurred and none of the 85 occupants were injured.)
- A310 / B736, en-route, Southern Norway, 2001 (On 21 February 2001, a level bust 10 nm north of Oslo Airport by a climbing PIA A310 led to loss of separation with an SAS B736 in which response to a TCAS RA by the A310 not being in accordance with its likely activation (descend). The B736 received and correctly actioned a Climb RA.)
- A319 / PRM1, en-route, near Fribourg Switzerland, 2011 (On 10 June 2011 an ATC error put a German Wings A319 and a Hahn Air Raytheon 390 on conflicting tracks over Switzerland and a coordinated TCAS RA followed. The aircraft subsequently passed in very close proximity without either sighting the other after the Hahn Air crew, contrary to Company procedures, followed an ATC descent clearance issued during their TCAS ‘Climb’ RA rather than continuing to fly the RA. The Investigation could find no explanation for this action by the experienced crew - both Hahn Air management pilots. The recorded CPA was 0.6 nm horizontally at 50 feet vertically.)
- B752 / B752, en-route, north of Tenerife Spain 2011 (On 20 November 2011, a problem in reading the altitude labels on the ATC radar control display led to a Finnair Boeing 757 being cleared to make a descent which brought it into proximity with a Thomas Cook Boeing 757 in day VMC. Co-ordinated TCAS RAs were generated onboard both aircraft but when the Finnair aircraft failed to respond to its Climb RA and continued descent, the other aircraft, which had responded correctly to its initial RA, received a further RA to reverse their descent to a climb. The Finnair aircraft reported retaining visual contact with the other aircraft throughout.)
- C525 / B773, vicinity London City UK, 2009 (On 27 July 2009, a Cessna 525 departing from London City failed to comply with the initial 3000 ft QNH SID Stop altitude and at 4000 ft QNH in day VMC came into close proximity on an almost reciprocal heading with a Boeing 777-300ER. Actual minimum separation was approximately 0.5nm laterally and estimated at between 100 ft and 200 ft vertically.)
- DH8A/DH8C, en-route, northern Canada, 2011 (On 7 February 2011 two Air Inuit DHC8s came into head-to-head conflict en route over the eastern shoreline of Hudson Bay in non radar Class ‘A airspace when one of them deviated from its cleared level towards the other which had been assigned the level 1000 feet below. The subsequent investigation found that an inappropriate FD mode had been used to maintain the assigned level of the deviating aircraft and noted deficiencies at the Operator in both TCAS pilot training and aircraft defect reporting as well as a variation in altitude alerting systems fitted to aircraft in the DHC8 fleet.)
- DH8D / DH8D, vicinity Sudbury ON Canada, 2016 (On 14 October 2016, two Bombardier DHC8-400s received coordinated TCAS RAs as they came into opposite direction conflict near Sudbury, an uncontrolled airport, as one was descending inbound and emerging from an overcast layer and the other was level just below that layer after departing. Both aircraft crews ignored their RAs and their respective visual manoeuvring brought them to within 0.4nm at the same altitude. The Investigation noted that the conflict had occurred in Class ‘E’ airspace after the departing aircraft had cancelled IFR to avoid a departure delay attributable to the inbound IFR aircraft.)
- F100 / EC45, vicinity Bern Switzerland, 2012 (On 24 May 2012, a Fokker 100 descending visual downwind to land at Berne and an EC145 helicopter transiting the Bern CTR (Class 'D' airspace) VFR came within 0.7 nm horizontally and 75 ft vertically despite early traffic advice having been given to both aircraft. The Investigation attributed the conflict to the failure of the F100 crew to follow either their initial TCAS RA or a subsequent revised one and noted that although STCA was installed at Berne it had been disabled "many years before".)
- F900 / B772, en-route, near Kihnu Island Estonia, 2013 (On 17 October 2013, a Falcon 900 climbing as cleared to FL 340 and being operated as a State Aircraft equipped with TCAS II v7.0 initially responded to a TCAS RA against crossing traffic at FL 350 in day VMC in the opposite direction to the one directed and prescribed separation was lost as a result. The Investigation concluded that the F900 crew had commenced a climb on receipt of a TCAS RA 'ADJUST VERTICAL SPEED' when a reduction in the 800 fpm rate of climb was required. Safety Recommendations were made in respect of TCAS RA requirements for State Aircraft.)
- GLF5 / A319, south-eastern France, 2004 (On 16 September 2004, a loss of separation occurred over Geneva between Air France A319 and a Gulfstream V which commenced descent without clearance by ATC and with coordinated TCAS RAs not followed by either aircraft.)
- ICAO Doc 4444: PANS-ATM Chapter 15.
- ICAO Doc 8168: PANS-OPS Volume 1, Chapter 3.