by Vincent J. Mellone

The upsurge in TCAS incident reporting to the ASRS points to a growing controversy between flight crews and air traffic controllers. While flight crews appear to wholeheartedly endorse its use, others, specifically air traffic controllers, feel that the difficulties and teething problems of this new technology may mean that the genie is out of the bottle -- and out of control.

Background for TCAS II

Since 1955, the aviation community has struggled to conceive and implement an automated airborne collision avoidance system as a backup to the air traffic control system. In 1981, Administrator J. Lynn Helms committed the resources of the Federal Aviation Administration (FAA) to the development of an airborne system by 1985. Based on 1987 congressional legislation, the FAA mandated the installation of Traffic Alert and Collision Avoidance System (TCAS II) equipment on all airliners by the end of 1993. To date, approximately 70 percent of the air carrier fleet and over 400 business aircraft are TCAS II equipped. Two thousand TCAS II aircraft have flown over 2.5 million revenue miles protected by a technology that is generating discussion and even controversy among pilots and controllers.

ASRS Takes a Look

On July 29, 1992, at the request of the FAA's Office of Aviation Safety and the National Transportation Safety Board (NTSB), the ASRS provided an analysis of TCAS II incident reports. A random sampling of 170 TCAS II incident reports were coded and analyzed by a team of ASRS pilot and controller analysts in a Quick Response (QR) effort. An ASRS QR is an intensive, time-limited analysis of ASRS incident data. Although lacking some of the features of a more traditional research effort, QRs are capable of providing a useful and quantifiable "snapshot" of topical issues. This analysis made the overall assessment that TCAS II has definitely enhanced safety, but still has a number of technical and human-machine interface problems.

Note: As all TCAS units in current use are TCAS II systems, TCAS and TCAS II are used synonymously in this review.

TCAS II Saves

There are many incident reports where flight crews assert that TCAS "saved the day." An air carrier captain writes:

• "On base leg...we were cleared by Center for [the] visual...Immediately after accepting the visual, Center reported pop-up traffic at 11-12 o'clock, level. I noticed an RA on the TCAS II with visual commands to pull up. After climbing 200 to 300 feet, I noticed a [light aircraft]...cross under us about 200 feet below. The alert Controller at Jacksonville Center, reinforced by the [TCAS II] RA command, ...prevented a possible mid-air. TCAS works." (ACN 213749)

And in another air carrier report...

• "Hazy holiday weekend in Southern California (LA basin). Many, many VFR aircraft in [the] area. My crew alert for traffic. TCAS scope cluttered with traffic. On departure...climbing... [a] traffic conflict [at] 12:30, 3 miles, 500 to 1,000 feet above [was noted] on TCAS. I hoped to climb (zoom) above it as soon as it was acquired visually. However, it was not acquired visually until after evasive action was taken based on TCAS II RA and ATC traffic advisory. TCAS and ATC saved the day." (ACN 179784)

This is the good news about TCAS, but these benefits have not been gained without some side-effects.

TCAS II: Issues and Incidents

Most TCAS II issues reported to the ASRS encompass anomalous or erroneous operation of TCAS II equipment, TCAS-induced distraction, airborne conflicts provoked by TCAS, and non-standard use of TCAS.

TCAS II Equipment Issues

Early versions of TCAS II equipment displayed some hardware and software anomalies. The reliability of TCAS equipment has improved considerably, but there continue to be some problems -- as this recent report illustrates. An air carrier flight crew departed an airport at night in mixed meteorological conditions. With high terrain very near, they experienced a very frightening situation:

• "Climbing through 1,200 feet [on departure] we had a TCAS II Resolution Advisory (RA) and a command to descend at maximum rate (1,500 to 2,000 feet per minute). [The flight crew followed the RA and began a descent.] At 500 feet AGL we leveled off, the TCAS II still saying to descend at maximum rate. With high terrain approaching, we started a maximum rate climb. TCAS II showed a Traffic Advisory (TA) without an altitude ahead of us, and an RA [at] plus 200 feet behind us...Had we followed the TCAS directions we would definitely have crashed. If the weather had been low IFR, I feel we would have crashed following the TCAS II directions. At one point we had TCAS II saying 'Descend Maximum Rate,' and the GPWS (Ground Proximity Warning System) saying 'Pull Up, Pull Up.' [The] ATC [Controller] said he showed no traffic conflict at any time." (ACN 201637)

Erroneous Mode C

Controllers obtain altitude information about traffic displayed on their radar from the Mode C function of the aircraft's radar transponder. If Mode C provides erroneous altitude information, the controller will see an incorrect altitude displayed on the scope. Should a controller note a disparity between displayed and assigned altitude, he/she will ask the flight to confirm its altitude, and may request that the flight crew reset their transponder, use a different transponder, or disable Mode C.

Like a controller, TCAS II uses Mode C information to determine vertical separation on other traffic. Should Mode C even temporarily provide erroneous altitude information, an erroneous Resolution Advisory command to climb or descend may result. Unlike a controller, TCAS II cannot query the flight crew to determine if the problem lies with malfunctioning equipment. Note the following pilot report:

• "Our flight [air carrier X]...was at flight level 260...We observed a TCAS II advisory [TA] of traffic at 12 o'clock, 1,000 feet above at about 15 miles on an opposing heading. Shortly after, we observed traffic on [the] TCAS II display descend from 1,000 feet above to 500 feet above. TCAS II commanded a descent of at least 2,000 feet per minute to avoid traffic.
  
  "We queried...[ATC about the] traffic. They told us we had an air carrier jet (Y) 1,000 feet below us on a converging heading.... At about the same time we visually acquired air carrier (Y) about 500 to 1,000 feet below our altitude. [The] Controller confirmed he was assigned flight level 250. We observed no traffic above, nor did the Controller have any traffic above us. Our TCAS II continued to command a descent and continued to show...[a] traffic conflict 500 feet above us. [The] Controller advised that air carrier (Y)'s Mode C did momentarily show 26,500 feet and then returned to flight level 250 on their scope. We had altered course slightly to the right to offset [the] conflict, but did not follow the TCAS II RA. If we had followed the TCAS II RA we, in my opinion, would have impacted the opposing aircraft." (ACN 210599)

The First Officer of the other aircraft (air carrier Y) writes:

• "...[Center] called traffic at 12 o'clock about 12 miles at 26,000 feet. I...spotted traffic about 7 to 8 miles out. Then we heard the air carrier jet (X) report a TCAS advisory saying the guy was showing 500 feet above them. Our aircraft was not TCAS II equipped. I continued to observe our traffic as he passed over us and in a slight right-hand bank. He appeared closer than the standard 1,000 feet separation. ATC then told us our Mode C showed 26,500 feet. The Captain and I both verified 29.92 [QNE] and 25,000 feet on our altimeters...we were at 25,000 with a faulty Mode C (which pilots have no way of monitoring). This could have been a potentially fatal situation had it not been visual [and daylight] conditions." (ACN 210600)

TCAS II Distraction

Pilots frequently cite TCAS II related auditory and workload interference with normal cockpit duties, as is noted in the following report from an air carrier First Officer.

• "...we received two TCAS II-advisories, corresponding to departures. The departures are cleared to 10,000 feet, [and] arrivals...[at] 11,000 feet. The TCAS II reacted to the closure rate of the departing aircraft and our inbound flight. [The] RA was ignored as traffic was in sight. The real problem is that the TCAS II alert caused such a distraction in the cockpit that two or more radio calls from Approach Control were missed." (ACN 203411)

Non-Standard Use of TCAS II

Some pilots are using TCAS displays to maintain "visual" separation. Consider the following controller report:

• "I was training a developmental [controller] on Arrival Control. We had an air taxi (X) for sequence to visual approach Runway 15. The developmental pointed out aircraft (Y) [to air taxi (X)] and the pilot responded, 'Is he following someone out there at 800 feet? ' The developmental was going to clear him for the visual approach when I stopped him and asked [the pilot of air taxi (X)]...if he had aircraft (Y) in sight. He said not visually, but had him on TCAS II. This seems to be happening more and more...It appears [that pilots]...are using TCAS II instead of looking out the window. As an air traffic controller I cannot have pilots using TCAS for visual separation to maintain spacing (as on one occurrence a crew offered to do). There is no TCAS II separation." (ACN 202301)

TCAS II-Induced Conflicts

Many TCAS incident reports received at the ASRS allege that pilot response to erroneous TCAS commands has promoted a conflict where, initially, none existed. Consider the following near mid-air collision (NMAC) where the TCAS II RA may well have been triggered by the high climb rate of air carrier (Y).

A Controller's Dilemma

• "Air carrier (X) was inbound on the...STAR level at 10,000 feet. Under my control, air carrier (Y) departed...on the...SID, climbing to [an] assigned altitude of 9,000 feet. Approximately 14 miles SW...I issued traffic to air carrier (X) that air carrier (Y) was leveling at 9,000. Air carrier (X) responded after a few seconds that they were descending. I again told air carrier (X) to maintain 10,000 feet. Air carrier (X) responded 'OK, we've got an alert saying go down.'
  
  "Simultaneously, air carrier (Y) was getting an alert to climb. They both followed the TCAS II [RAs] and almost collided. Later, [the pilot of air carrier (X)] ...indicated [that] his TCAS II was showing zero separation. They passed in the clouds without seeing each other. When pilots start taking evasive action, our equipment cannot update quickly enough for the controller to help. Both aircraft were issued traffic as prescribed by our handbook (merging target procedures). [Air Carrier] Company directives, I'm told, dictate that pilots must respond/follow the TCAS II alert advisories." (ACN 224796)

The Captain of Air Carrier (X)

• "[We] air carrier (X) received a Traffic Advisory [TA] annunciation followed by [an] immediate RA annunciation...'Descend Crossing Descend.' We immediately pushed over following the RA to 2,000 to [a] 2,500 foot per minute descent. Intruder was showing 12 to 12:30, 800 feet below [and] climbing. I instructed the First Officer to advise ATC of [the] descent. ATC advised [the] intruder was [an] air carrier who was on a different frequency...and that he was leveling off. [The] Controller advised us to level and then climb. Both aircraft passed to each others' right on the same altitude. During the event things got confusing. [The] RA called for descent and then increased descent. ATC is calling for a level-off followed by an immediate climb." (ACN 224912)

The Controller of Air Carrier(Y) Writes...

• "[A] pilot can now disregard ATC instructions based on TCAS II alerts, even if the controller has issued traffic to that aircraft and has applied the correct separation." (ACN 224982)

Pilot/Controller Conflict

In a recent congressional subcommittee hearing on TCAS II, Air Line Pilots Association (ALPA) President, J. Randolph Babbitt, testified, "Line pilots have strongly endorsed TCAS II and would emphatically resist any efforts to reduce its operational effectiveness." The National Air Traffic Controllers Association (NATCA) President, Barry Krasner, countered at the same hearing that "...TCAS [II] is highly disruptive to the air traffic control environment." The U.S. Government's General Accounting Office, in a summary of TCAS II surveys, detected that controllers have strong "negative" feelings while there is widespread pilot acceptance of TCAS II.

The pilot community, particularly ALPA, sees TCAS II as a "...last ditch, they-may-have-hit-if-something-is-not-done, piece of equipment..." that gives the pilot a precious way out if "...the ATC system has somehow unaccountably failed." Both the FAA and the airlines exhort TCAS II equipped flight crews to "...follow the RA..." when it is contrary to ATC instructions. In contrast, NATCA's Krasner ominously warns that TCAS II "...is an airborne system that works improperly and actually erodes an already precarious margin of safety in the skies." NATCA favors a moratorium on further TCAS II installations and calls for restricting alerts to TAs only. ALPA recognizes that TCAS II is not without its technical faults, but considers its enhanced safety value well worth the "growing pains." The issue appears to be settling down to two strongly different viewpoints: 1) NATCA wants the decision to separate aircraft to remain in the hands of the air traffic controller "...who has verified information to provide required separation;" 2) The pilot community wants an independent airborne collision avoidance system that leaves the decision for last-minute collision separation action in the cockpit.

The majority of TCAS II reports being received at the ASRS are favorable to the technology. According to on-the-scene reporters, TCAS II has prevented numerous near-midair collision situations and averted potential air disasters. However, there is growing friction between flight crews in compliance with TCAS II RAs and wary air traffic controllers. TCAS II is a developing challenge for both constituencies.

A sampling of excerpts from contrasting ASRS reports underscores the frictional issues:

• "We were level at FL230...RA sounded with command to descend...alert ended at 22,400 feet...Center later admonished us for descending... traffic level at FL240...we listened to our TCAS RA." (ACN 205812)
  
  
• "ATC took offense to TCAS and its use in the air traffic system.... ATC is not too fond of TCAS because it takes away their authority." (ACN 206966)
  
  
• "Pilots should question TCAS II traffic if they're concerned, rather than 'doing their own thing.' " (ACN 195990)

The optimal blending of TCAS II within the National Airspace System is expressed in the following ASRS excerpt:

• "Both the controller and [we] were pleased with how well TCAS aided in the situation. No passengers or crew were injured in the climb." (ACN 195211)

In the interest of serving the aviation public, it behooves pilots and controllers to find an operational accommodation that masters the safety benefits of TCAS II.

TCAS II Incident Reporting

The information provided thus far in this article has been qualitative in nature; we will now provide a quantitative analysis of the 1,997 TCAS II related incident reports that have been received at the ASRS. Approximately 7 percent of these reports are from controllers, and the remaining 93 percent from pilots. As the air carrier and business aircraft fleets moved to comply with FAR 121.356, the reporting of TCAS II incidents increased significantly between 1990 and 1992 -- as can be seen in Table 1.

Table 2 provides the airspace involvement for all ASRS database TCAS incident records -- for the aircraft of the principal reporter of an incident (primary aircraft). (Note that airspace categories are not necessarily mutually exclusive.) The single airspace category in which the majority of TCAS incidents occurred was in "Other" Controlled Airspace, meaning within controlled airspace but outside of TCAs, ARSAs, etc. The next most commonly involved airspace was in Terminal Control Areas (TCAs), while the third most common airspace category was in the Positive Control Area (above 18,000 feet).

Table 3 indicates the flight phase of ASRS database TCAS incidents. (As with airspace categories, ASRS flight phase categories are not necessarily mutually exclusive.) The flight phase most commonly cited for the primary aircraft was Cruise. Note, however, that if the climb and descent phases of flight are combined, they total 1,151 citations -- 30 percent more than the cruise phase.

Quick Response Data Set

Now lets look at the 170 randomly selected records that comprised the QR data set. There were 159 incident reports by pilots, and 11 controller reports. In 153 of 170 incidents, flight crews received both a TCAS II TA and RA (84%). For those 153 records, Figure 1 shows what pilot avoidance action was based on. Note that in the majority of incidents, avoidance action was based on the RA.

In 111 of 170 incidents, the reporter provided information about visual acquisition of their TCAS II targets. Figure 2 shows when and if traffic was acquired visually by the flight crew. In 68 percent of incidents, the flight crew never sighted their traffic.

Figure 3 shows whether or not ATC observed the TCAS II traffic for the entire 170-report QR data set. In the majority of instances, ATC did "see" the traffic.

Figure 4 indicates that ATC provided traffic advisories in 136 of 170 TCAS II incidents. There were 34 instances in which ATC response to a potential conflict was unknown. According to reporters, ATC did not provide a traffic advisory in 48 of 136 TCAS II incidents (35 percent).

Table 4 shows how reporters felt about TCAS II. Reporters could express more than a single opinion; thus there are 301 individual responses (citations) from 170 reports. In 94 instances, the reporter stated or implied that TCAS enhanced safety. This table provides the key findings of the Quick Response analysis effort, indicating that reporters in the 170-record QR data set strongly endorse the safety value of TCAS II. However, this finding is undoubtedly influenced by the relatively large percentage of these reports that come from pilots rather than controllers.

Recommendations

The ASRS analyst staff are certainly no wiser than those in government and industry who are grappling with TCAS II issues, nor would we make suggestions that conflict with TCAS II standard operating procedures. Nonetheless, review of TCAS II incident reports does suggest that there may be ways to deal with, or even avoid a TCAS incident.

Controllers

1. High rates of climb and descent, particularly in auto-flight aircraft, are often implicated in erroneous TCAS II activation. Controllers should be alert to the possibility of a TCAS II RA for an aircraft while it is in its final 1,000 feet of climb or descent and in proximity of traffic.
   
   
2. Providing traffic advisories in a timely manner to TCAS-equipped aircraft in climb and descent in the terminal area, even though separation has been previously established, may help reduce the number or severity of TCAS events.
   
   
3. Controllers must remember that both FAA and company TCAS II instructions stress pilot compliance with RA commands. Venting anger or annoyance at pilots can heighten tensions and promote distraction.

Pilots

1. As with any technology, TCAS II is not foolproof. Be alert for anomalous operation of the unit, particularly in proximity to terrain.
   
   
2. TCAS incidents are most likely to occur in the climb and descent phases of flight. It is recommended that flight crews, where possible, reduce vertical rates when closing on target altitudes to those recommended in the Airmen's Information Manual (AIM). Auto-flight aircraft with programmed high rates of vertical speed may be particularly prone to erroneous TCAS "triggering."
   
   
3. Alerting ATC prior to or concurrent with executing your RA response, whenever cockpit workload and frequency congestion permit, could help controllers reduce the impact of the avoidance maneuver on other traffic in the area.
   
   
4. TCAS II works well at providing separation in conflict situations. In a number of instances where pilot response to TCAS RAs has been delayed or tentative, close air-misses have resulted. This suggests that, once the flight crew has decided to respond to a TCAS II alert, their response should be immediate and complete.
   
   
5. Excessive responses to TCAS II alerts have also caused many problems. It is suggested that pilots limit their response to RAs to that commanded by their TCAS II equipment. Exceeding TCAS II vertical commands into the next cardinal altitude could provoke an unintended near miss or loss of separation with other traffic.

Summary

TCAS II, in the opinion of much of the pilot community, has been instrumental in reducing risks associated with airborne conflicts. Many in the ATC community, however, feel that TCAS II has increased workload and derogated the primary responsibility of the air traffic controller -- that of providing traffic separation. These sharp divisions between the pilot and controller constituencies, with resulting friction, may, if not adequately managed, result in an overall negative impact on flight safety. For some controllers, it is inconceivable that ATC should not be "in-the-loop" prior to or during TCAS activation. As industry and government grapple with TCAS problems and issues, it is important for pilots and controllers to jointly understand and manage the TCAS environment -- so that the genie remains under control.