In a previous post, we highlighted some of the procedural safeguards used to provide adequate separation between airborne aircraft. With this post, we’ll examine other methods to ensure aircraft remain safely separated from each other. All these procedures, methods, and equipment work together to maximize the safety of the national airspace system.
Transponder: The most basic form of electronic collision avoidance equipment is the transponder. This device emits an electronic signal that allows air traffic control (ATC) to locate an aircraft’s position with radar. For the past several years, transponders have been capable of providing aircraft altitude as well (known as Mode C, or altitude encoding). If aircraft get too close to one another, ATC receives an audiovisual warning. Controllers can then relay a traffic alert to the aircraft involved. Over time, transponders have continued to evolve and have paved the way for newer forms of electronic traffic avoidance.
TCAS: Additionally, all large aircraft are required to possess a traffic collision avoidance system (TCAS). TCAS is a form of “portable radar,” which works independently of ATC’s ground-based radar. TCAS detects the transponder signals of other aircraft and, when traffic is nearby, issues alerts. These alerts may include traffic advisories (TAs) and/or resolution advisories (RAs). TAs are a kind of “heads up” to advise aircraft of a possible conflict. RAs announce when a conflict is imminent and evasive action is required. RAs even tell aircraft what type of maneuver to execute and, when received, supersede all ATC directives. Though established ATC procedures are usually sufficient to maintain separation, TCAS is great for “belt and suspenders” reinforcement.
ADS-B: Automatic dependent surveillance-broadcast (ADS-B) is the latest technological marvel for air traffic separation. ADS-B, through its use of both a highly accurate GPS receiver and a datalink, allows an equipped aircraft’s position, speed, and altitude to be broadcast to other ADS-B equipped aircraft, as well as to ATC, in real time. Think of ADS-B as a highly accurate version of TCAS and radar combined. In fact, ADS-B is slated to replace traditional ATC radar. Though the technology is available now, future mandates will further enhance the safety potential of this system.
The national airspace system is itself designed to promote aircraft separation. All controlled airspace requires all IFR (instrument flight rules, which includes ALL airline traffic) flights to maintain radio contact with ATC. This enables controllers to notify aircraft of potential traffic conflicts. Additionally, airports with operating control towers require EVERY aircraft to establish radio communication in order to operate in their terminal airspace. The larger/busier the airport, the more traffic separation rules are in place.
Class D airports, the smallest tower-controlled fields, require all aircraft in their airspace to establish radio communications. This requirement permits ATC to supervise all types of flight operations in the airspace, as well as to issue traffic advisories/alerts.
Medium size airports, located in Class C airspace, require both radio communications and an operable Mode C (position + altitude reporting) transponder in order to enter the airspace. As these airports generally have more traffic than Class D, the transponder requirement adds another layer of traffic separation safety. Additionally, the Class C airspace is larger than Class D, which keeps unqualified/non-participating aircraft farther from the airport.
The nation’s busiest airports are surrounded by Class B airspace. In addition to the requirements for radio communications and a transponder, all aircraft must have a specific clearance to operate within Class B. This keeps many small, private aircraft from transiting the area near the major airport. Class B has even larger dimensions than Class C, which ensures more maneuvering space for the greater amount of traffic.
While we’ve just glossed over the basics of the airspace & equipment characteristics, additional details further enhance the safety procedures for air traffic separation. In addition, ATC can and does address issues that the equipment, airspace, and procedures we’ve discussed cannot. Though we’ve spent two posts covering numerous traffic avoidance issues, the national airspace system still contains additional safeguards. If you’ve ever heard that it’s safer to fly than to ride in a c