Prompted by industry demand, the FAA is considering approving the installation and use of TCAS II aboard helicopters but is concerned about certification challenges. Major operators have been successfully using TCAS II for several years in regions such as the North Sea.
“Over the past few years, we have received numerous applications for certification of TCAS II installations in rotorcraft,” Stephen Barbini, flight analyst, aircraft certification at the FAA’s rotorcraft directorate, told attendees at the EASA Rotorcraft Symposium in Cologne, Germany. He noted that most, if not all, interest in installing TCAS II in rotorcraft is related to offshore operations, which must contend with reduced visibility and dense traffic. As an example, he noted that there are 2,500 flights per day in the Gulf of Mexico.
The system has proven its worth in areas such Europe, Africa and Trinidad and Tobago, which have several documented reports of collisions avoided thanks to resolution advisories (RAs) provided by TCAS II.
However, despite all the expected and proven benefits, the FAA is wary of dangers and serious limitations stemming from the fact that TCAS II specification requirements are modeled for transport-category airplanes, according to FAA experts. In particular, TCAS II algorithms require vertical speed of 1,500 feet per minute, which rotorcraft cannot always attain. Barbini noted that “the specification requires that the aircraft attain a rate of climb of 1,500 fpm within five seconds of an RA being annunciated.” He sees this as the greatest challenge to installing TCAS II in rotorcraft. For example, an Airbus Helicopters AS365 N3+ Dauphin, at mtow in ISA conditions, has a maximum vertical speed of 1,321 fpm. A helicopter following an RA without the required rate of climb faces the risk of a midair collision.
A research project that evaluates TCAS II in fixed-wing aircraft with performance characteristics similar to helicopters has recently been conducted. In this research, rate of climb is treated as variable, Barbini said. The FAA is currently reviewing the findings of the project.
Another problem is the vertical polarization of the upper directional antenna. It may prevent the system from “seeing” aircraft directly above, Barbini said. Therefore, the helicopter can be commanded, via an RA, to climb vertically into a collision.
The traffic density of the environment in which the helicopters operate also provides a significant limitation. TCAS II provides reliable surveillance up to a traffic density of 0.3 aircraft per square nautical mile, Barbini pointed out. Numerous TCAS II-equipped aircraft operating in near vicinity of each other can saturate systems or even create unnecessary RAs for passing aircraft. In such situations, the solution–albeit less-than-ideal–is to set TCAS II to traffic alert-only. This is what Barbini suggested for news coverage of accidents and sporting events, for example.
He suggested that helicopters might be better suited for TCAS I, which only makes crews aware of nearby traffic. “It uses correct assumptions,” Barbini emphasized. The next generation of TCAS, ACAS-X, will address rotorcraft but is projected to enter service in 2025. o
Finally, Barbini urged the industry to take the lead, form a working group and develop a method to safely install TCAS II in rotorcraft.