ATM Modernization, Business & GA, Commercial

Safety in Avionics: Kudos for TCAS Stance

By David Evans | January 1, 2002
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The Federal Aviation Administration (FAA) has taken a significant step toward the oft-proclaimed theology of a "single level of safety" that should greatly help reduce the threat of mid-air collisions. Traffic alert and collision avoidance system (TCAS) technology must be installed in all cargo aircraft, FAA declared in an Oct. 31, 2001, notice of proposed rulemaking (NPRM). That document features a laudable statement of principles about the safety of avionics systems in general (see Docket No. FAA-2001-10910).

Often a punching bag for critics who believe the FAA doesn’t act until jarred by a crash out of its Rip van Winkle-like torpor into a spate of belated regulatory activity, the agency in this case seems genuinely deserving of kudos. FAA is calling not only for fleetwide equipage, but also for fleetwide standardization of collision avoidance technology.

Advocates of alternative collision avoidance systems to TCAS, notably ADS-B (Automatic Dependent Surveillance-Broadcast), must demonstrate that their approach provides an equivalent level of protection. By doing so, FAA has placed the burden of proof on ADS-B enthusiasts. ADS-B, which is designed to fulfill the surveillance function based on signals from the satellite-based GPS system, provides the potential to track aircraft without the need of ground radar. However, ADS-B was not intended to provide collision avoidance (see sidebar, page 46).

Cargo airline pilots, who want TCAS, are heartened. "We’re belatedly happy," said Mike Cronin, speaking for the Coalition of Airline Pilots Associations (CAPA), a group of labor unions representing most of the pilots flying for the major cargo carriers. Cronin, a recently retired American Airlines B777 captain, said of the FAA initiative, "We think they got it right."

For the past few years, cargo pilots have complained bitterly that FAA’s failure to mandate TCAS for their airplanes was an affront to the single-level-of-safety ethic. They claimed they were flying in the same airspace as passenger aircraft, and that any collision between big jets would wreak fearsome destruction in the air and on the ground. The declaration in an earlier FAA document that the cargo pilots’ lives essentially were worthless for cost-benefit purposes was particularly galling.

FAA now explains that, when the original TCAS rules were published in 1989, the intent was to protect passengers from midair collisions. Cargo planes were excluded because they flew mostly at night, in small numbers (about 375 planes), while nearly 4,000 passenger planes were flying, mostly in the day. At that point in history, the NPRM recalled, the cost-benefit analysis simply did not support TCAS for cargo planes. Today, some 1,150 cargo jets are flying at all hours of the day. Changed circumstances, FAA declared, warrant expanding the TCAS requirement to include cargo planes.

No doubt, TCAS has benefited safety. Given gaps in incident reporting, the benefits may even be understated. In the first six years since the installation of TCAS began for passenger airliners, the number of pilot-reported near mid-air collisions (known as NMACs) dropped a whopping 60 percent (imagine such a trend for runway incursions, where the numbers have risen!). On the other hand, there have been some close calls involving cargo aircraft, where the effect on aircrews and ground controllers was akin to a shot of chilled water to the heart. In February 1999, a TCAS-equipped passenger jet took evasive action to avoid a non-TCAS equipped cargo jet. In March 1999 a very near miss occurred involving two cargo jets. Officially, the planes came within a half-mile of each other (with zero feet vertical separation) in the nighttime incident over Salinas, Kan. However, the copilot of one of these jets recalled hearing the sound of the other aircraft whooshing by. (He was in the lavatory at the time, thinking it was a hell of a place to die.)

Not only are there more planes in the sky, they’re being stacked closer. Under reduced vertical separation minimum (RVSM) operations, airplanes will be flying past one another with a scant 1,000 feet difference in altitude. For flights between 29,000 and 41,000 feet beyond ground radar coverage in trans-oceanic airspace, the traditional 2,000-foot vertical separation is being halved. Yet there is no FAA mandate for TCAS on airplanes exercising such RVSM privileges, despite the heightened risk of the euphemistic NMAC. Collision-avoidance technology clearly can help plug this regulatory gap in the safety defenses.

FAA is taking a commendably conservative approach to alternatives to TCAS technologies. One of the primary benefits of TCAS is its ability to coordinate evasive maneuvers between two opposing airplanes, i.e., one climbs, and the other descends. For airplanes not equipped with TCAS, uncoordinated avoidance maneuvers chosen independently by the pilots could actually place them on intersecting paths. The same could occur if one airplane is equipped with TCAS and the other is not. In other words, collision avoidance could turn into collision convergence.

For this reason, FAA is taking a flinty-eyed view of ADS-B as a substitute. "To be considered as an alternative to TCAS, the system must be equivalent to and interoperable with TCAS," the NPRM declared. In other words, ADS-B must be shown capable of supporting coordinated avoidance maneuvers now enabled by TCAS-equipped aircraft, and it must be interoperable with these airplanes to facilitate the same function in the case of two planes, one with ADS-B and the other with TCAS. Some in the industry have advocated ADS-B in lieu of TCAS, which would avoid burdening cargo operators intent on deploying ADS-B with the cost of also having to install TCAS II.

"Those wishing to make the case for ADS-B…must fully resolve these issues" of capability and compatibility, the NPRM declared. The FAA laid down this marker: "To allow the use of ADS-B (or any other future technology) as an alternative to TCAS. … Any equivalent must be shown to provide the same level of safety and coordinated maneuvers presently available with TCAS [emphasis added]."

Indeed, the FAA expounded on a larger concern–the number of functions that prudently should be contained in one box of avionics. Just as the value of real estate is based on the cliché, "location, location, location," air safety is built on the trinity of "redundancy, redundancy, redundancy." If TCAS drops off line, pilots lose only their collision avoidance capability. Other vital functions remain unaffected. Not so in the event of ADS-B failure (see sidebar, page 44). In discussing its reservations about employing ADS-B for collision avoidance, FAA has issued a profound and important statement of philosophy about safety in avionics.

Indeed, the NPRM leads from its concern about one system to systems design in general. First, the document cautions about adapting a system not intended originally for collision avoidance to perform that task. Second, FAA warns against the potential erosion of the principle of redundant, independent systems. In declaring not only an equivalent level of safety, but also equivalent redundancy, FAA has presented the ardent advocates of alternatives to TCAS with a high wall to climb over.

Reservations About TCAS Alternatives

The Federal Aviation Administration recognizes that ADS-B is being evaluated as a potential equivalent collision avoidance system to that of TCAS II. However, the agency believes there are several significant issues that pose challenges. Here are comments from the recent notice of proposed rule making.

On ADS-B for Collision Avoidance

An aircraft equipped with ADS-B would broadcast its…identification, along with position, velocity and other time-sensitive surveillance information to other aircraft and would receive the same information from other aircraft. These capabilities are only fully realized when all aircraft in the system have an operating ADS-B system.

ADS-B has not been developed to provide resolution advisories (RAs) or to perform coordinated maneuvers with the many TCAS- and transponder-equipped aircraft in the National Airspace System (NAS).

TCAS II-equipped airplanes are afforded collision avoidance protection from other TCAS II and all transponder-equipped airplanes…only an airplane with ADS-B will be able to detect another airplane equipped with ADS-B.

A system that is not interoperable with TCAS would require significant costs for the high levels of equipage to realize the safety benefits equivalent to TCAS.

Airplanes that may be equipped with ADS-B and TCAS II would assign priority to TCAS II as the collision avoidance system of last resort, with ADS-B as part of an airborne surveillance system. The FAA is concerned about the possible display of traffic from multiple sources such as TCAS II, ADS-B and traffic information services (TIS).

On the Theology of Independent Systems

The FAA has relied upon independent communication, navigation and surveillance (CNS) capabilities for decades to provide safety in the NAS.

Independence of CNS capabilities allows a pilot to complete a flight safely…even with the loss of any one of the airplane’s CNS components. For example, with the loss of surveillance, whether it is primary radar or secondary surveillance radar, a pilot can still navigate and report the airplane’s position through communications with ATC. This independence is compromised in a system where navigation and surveillance functions are tied to a single system.

(An) ADS-B failure…would result in simultaneous loss of navigational capability and the surveillance function (situational awareness).

TCAS II functions independently from ground-based CNS systems…. ADS-B functioning…creates a scenario whereby a failure in ADS-B could affect the primary and backup means of separation.

Any use of ADS-B as a replacement for TCAS II must be able to address this independence issue and demonstrate other acceptable methods of achieving this redundancy.

Source: FAA-2001-10910

ADS-B, a Primer

ADS-B is intended to support surveillance of aircraft while airborne and on the ground. Surveillance capabilities include primary radar and secondary surveillance radar. Primary radar, a ground-based system, detects actual aircraft location by measuring reflected energy from the target. Secondary surveillance radar, also known as Mode-S, interrogates aircraft transponders and determines aircraft location and other information through a reply.

ADS-B uses the Global Positioning System (GPS) and a radio frequency link to broadcast information between aircraft equipped with ADS-B, as well as between aircraft and ground-based ADS-B receivers.

An aircraft equipped with ADS-B would broadcast its aircraft identification, along with position, velocity and other time-sensitive surveillance information, to other aircraft and would receive the same information from other aircraft. These capabilities are only fully realized when all aircraft in the system have an operating ADS-B system.

Source: FAA-2001-10910

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