AINsight: Avoiding TCAS Conflicts in the Covid Era

 - November 24, 2020, 5:56 PM

Covid-19 has created several new challenges for the aviation industry. The ramifications have been far-reaching and severe.

From a flight safety perspective, the unprecedented drop in air traffic during the early months of the pandemic, among other factors, has created several unique, and undesired, threats. One unexpected outcome was an increase in the rate of air traffic conflicts even though there was a decrease in the number of flights.

An increase in the rate (events per flight, not count) of conflicts, as measured by TCAS (traffic alert and collision avoidance system) alerts, was surprising. While most of these events were classified as operationally undesirable or nuisance alerts, it must be recognized that TCAS is the last line of defense to prevent a midair collision.

To better frame the issue, aviation has long subscribed to the “big sky theory” for traffic separation. This theory is a mathematical model that uses the gas law approach—the notion that two randomly flying objects are less likely to collide since the three-dimensional space around them is very large relative to the size of each object.

Critical parameters of the big sky theory include the number of flying objects per unit volume, its speed, and its size. In theory, if the number of objects per unit volume, its speed, or its size decrease, the likelihood of collision should also decrease.

The problem identified during the early months of the pandemic was the number of objects flying decreased, but the rate of TCAS alerts increased. With fewer aircraft flying, if everything was proportional, the rate of TCAS alerts should have decreased, but it did not.

Covid by no means “blew up” the big sky theory, but it exposed a vulnerability caused by poor procedural compliance and a general lack of knowledge of TCAS limitations by pilots. In the past, these shortcomings were masked by poor aircraft performance.

In a pre-pandemic world, large passenger loads equated to heavy aircraft weights and poor climb performance at altitude. Post-pandemic, light passenger loads would equate to light aircraft weights and much stronger climb performance; often this was the case when a nuisance TCAS alert was triggered.

Nuisance TCAS Resolution Advisory (RA) events are not new and there are procedures in place to prevent them. The most common nuisance TCAS RA events occur during a level-off maneuver where aircraft are separated by only 1,000 feet. These nuisance events are triggered when two aircraft are in the proximity of each other, and the sum of the vertical speeds of both aircraft or the vertical speed of one aircraft exceeds 1,500 fpm.

These undesired events typically involve one of the following level-off scenarios. In the first scenario, an aircraft is either climbing or descending towards an altitude that is within 1,000 feet of another aircraft that is in level flight. The other scenario involves an aircraft climbing and the other aircraft descending within the same proximity of each other, separated by only 1,000 feet. In each case, there may not be a collision risk, but the TCAS sounds an RA based on a closure rate of 1,500 fpm or greater.

Procedurally the flight crew of each aircraft must respond to commands given by the TCAS to deconflict the situation. Responding to a TCAS RA can also be problematic, in a recent IATA study, “Performance Assessment of Pilot with TCAS Advisories using FDM,” pilots followed the TCAS RA command in only 37 percent of the cases.

To reduce nuisance TCAS RA events, it is recommended that pilots reduce the rate of climb or descent when approaching their assigned altitude or flight level. This action limits the rate of convergence between the two aircraft, lessening the likelihood that an alert will be triggered.

FAA guidance (AC 20-151A) suggests a reduction in the vertical speed to between 500 and 1,500 fpm when between 1,000 and 2,000 feet above or below the assigned altitude. ICAO PAN-OPS Doc. 8168 recommends a vertical speed at or below 1,500 fpm throughout the last 1,000 feet of a climb or descent to the assigned altitude.

Other regulators have adopted easier-to-remember recommendations such as within 2,000 feet of an assigned altitude, the vertical speed should be limited to 2,000 fpm and within 1,000 feet of an assigned altitude the vertical speed should be limited to 1,000 fpm or less.

Some manufacturers publish recommendations in an aircraft’s flight crew operating manual. As an example, Airbus recommends limiting the vertical speed to no more than 1,500 fpm during the last 2,000 feet of a climb or descent.

Contributing to the problem, operators often will not publish these recommendations in company flight operations manuals, and when they do, these recommendations are rarely followed.

Likewise, in addition to the above recommendations, pilots can study the SIDs and STARs for each airport to identify areas where there is the potential for only 1,000 feet vertical separation during a level-off. As an example, if the SID has a “top altitude” of 10,000 feet, there is a good chance that you can find a STAR that has a corresponding altitude constraint of 11,000 feet.

In this case, during the preflight departure briefing, consider a discussion to limit the vertical speed (and the desired vertical mode) 1,000 to 2,000 feet below the “top altitude” unless a higher altitude clearance is received. In general, pilots tend to over brief instrument approaches and under brief SIDs and STARs. 

As mentioned, the challenges related to flight operations during a global pandemic are both unique and plentiful. An increased rate of TCAS RA events largely caused by lightweight aircraft is preventable. This one is easy: adherence to procedures that pre-date Covid-19 will help mitigate these undesirable events.

Pilot, safety expert, consultant, and aviation journalist Stuart “Kipp” Lau writes about flight safety and airmanship for AIN. He can be reached at stuart.lau3@gmail.com.