By Andy Hoag, Aireon Vice President of Product Development, and Dr. Giuseppe Sirigu, Ph.D., Aireon Manager Modeling & Analysis
On Saturday, March 1, 2025, multiple aircraft approaching Reagan National Airport (DCA) experienced alarming Traffic Collision Avoidance System (TCAS) Resolution Advisories (RA), indicating a potential risk of mid-air collision. Typically, these TCAS RA alerts are a critical safety barrier, enabling pilots to execute maneuvers to maintain safe separation. However, on March 1, the number of alerts was significantly higher than expected, when several events were recorded in a single day compared to less than three on a normal month in the same area.
According to the Aireon analysis of the day’s events, all of the recorded TCAS alerts involved an unidentified “intruder” aircraft at an altitude exactly 2,300 feet — regardless of the altitude of the approaching aircraft. This clearly ruled out common errors such as TCAS ghosting or filtering issues with ownship traffic.
Additionally, Aireon compared each RA and detailed information about the intruder aircraft’s position and discovered the intruder was not broadcasting identity via ADS-B or Mode S, its signals indicate it operated only with a basic Mode 3/A transponder and altitude-reporting Mode C. The data cannot say for certain, but it is possible the intruder was airborne or related to a ground-based transmitter used for testing or spoofing.
Globally, such TCAS advisories are not uncommon — in fact, according to Aireon’s Safety Dashboard, more than 5,000 TCAS RAs occurred worldwide in February 2025 alone, with 67% demanding corrective maneuvers to avoid collisions. Typically, Resolution Advisory hotspots correlate with dense air traffic areas and specific airspace management practices.
