May 21, 2012

The Federal Aviation Administration (FAA) is reaching out to aircraft operators in an effort to educate them about the Traffic Collision Avoidance System II (TCAS) and data the agency has collected on resolution advisories (RAs).

FAA figures indicate approximately 41-percent of TCAS RAs involve aircraft operating under FAR Parts 91, 91K, 125 and 135. This is likely due to the fact that these aircraft often operate into Class C and D airspace where IFR traffic is more likely to conflict with VFR traffic.

“We’re still seeing something like 65- to 70-percent of the RAs are issued between TCAS equipped aircraft and smaller aircraft operating under VFR,” said Dan Tillotson, a program manager at ARINC who works with the FAA on the TCAS awareness initiative.

Using a network of 20 Mode-S ground sensors placed throughout the U.S., Tillotson said the FAA has been analyzing the circumstances surrounding RAs in an effort to better understand not only what leads to their issuance, but also the actions flight crews take when faced with an RA.
“Somewhere on the order 65 to 70 percent of the RAs that require a maneuver are complied with,” said Tillotson. FAA wants to know why in more than 30 percent of the RAs studied, pilots did not follow the guidance issued by TCAS to avoid a possible collision.

Part of the reason could be the way TCAS guidelines are written in the U.S. While International Civil Aviation Organization (ICAO) standards state RAs should be complied with except when to do so would endanger the safety of the aircraft, the FAA guidelines are slightly different.

Tillotson explained that FAA regulations direct pilots to follow the TCAS RA unless they have made a definitive visual acquisition of the other aircraft. While FAA has data on whether or not pilots responded to the RA, their data does not specifically identify why a pilot might not have complied with an advisory. For example, if an RA was issued in the airport traffic pattern, it is likely that the pilots had visually acquired the other aircraft and were able to maintain separation.

TCAS RAs are most often issued in areas of high traffic density – the Northeastern U.S., for example. Most occur in Class C or D airspace, according to Tillotson. That, he believes, makes a certain amount of sense given that VFR and IFR traffic are most likely to mingle in those airspaces.

Still, “We were surprised at the concentration of RAs around Class C and Class D airports,” said Tillotson.

While striving to further understand encounters that trigger TCAS alerts, Tillotson said the FAA also wants to establish a dialog with operators about the development of best practices in responding to RAs.

“The best idea is to have a consistent response policy. While the RA doesn’t necessarily trigger an emergency maneuver, the time to discuss whether to respond should not be just after the RA. There should be policy on how RAs are complied with,” he suggested. “If you’re going into an area with closely-spaced approaches or high density approaches where you suspect there may be RAs, that should be part of the approach briefing.