The probability of anyone reading our column and this article not being aware of the recent mid-air tragedy in Washington, D.C., between a PSA airliner and a US Army U-60 Blackhawk is highly unlikely. However, this article is not (directly anyway) about this terrible accident. We do not want to preempt the official investigation regardless of what we think we know, have watched or read, and will wait for the results of what was almost certainly a preventable accident—as most of them usually are.
What we would like to discuss, in a peripheral way though, is the Traffic Alert and Collision Avoidance System (TCAS). As many of our readers will know, TCAS is an airborne safety system that is principally transponder based (not radar), is independent of ATC and ground radar, and monitors (when installed and working properly) the corresponding airspace around a given aircraft assisting the pilots with avoiding potential mid-air collisions with other operable transponder-equipped aircraft. The system uses Secondary Surveillance Radar (SSR) to provide pilots with a caution or warning for a potential collision threat based on distance and closure rate with another such-equipped aircraft. (Note: Most aircraft using a mode S transponder are compatible with TCAS.)
Mid-air collisions, although rare, are not, unfortunately, a new thing. In 1956, a United Airlines Douglas DC-7 struck a TWA Lockheed L-1049 Super Constellation over the Grand Canyon killing all onboard – both flights were operating in accordance with established procedures at the time. It was the deadliest aviation accident in the US, until four years later, when another United Airlines Douglas aircraft (a DC-8 this time) collided with yet another TWA Lockheed L-1049 over New York, killing all 128 aboard the two aircraft, plus six more on the ground. This resulted in not only the deadliest aviation accident in the US at the time, but the world, and as the saying goes, “the FARs are written in blood”, so too was the push to develop and expand technology capable of helping prevent these disasters. Consequently, in 1981, the FAA implemented TCAS.
Why would the pilot(s) of an aircraft required by the FAA and ICAO to have an operable TCAS installed ever fly with the TCAS deferred? By deferred, we mean it is not currently working or working properly and the maintenance department, under permission (expressed in the Master Minimum Equipment List [MMEL] which is approved by the FAA) can disconnect this system, label it as inoperative, and defer fixing or replacing it until a more convenient time.
The Air Line Pilots Association (ALPA—not only the world’s largest pilot union but also the world’s largest non-governmental safety body) emphatically encourages pilots not to accept an aircraft for flight without an operable TCAS.
In some cases, and at specific airports under certain conditions, the FAA has actually approved airlines to provide an option for their flight crews (at the captain’s discretion and in accordance with safety), the option to turn their TCAS units to a degraded Traffic Alert (TA) setting in order to avoid having to respond to a Resolution Advisory (RA). The TA (a caution) alerts pilots to a potential threat. An RA, however, is a warning that an immediate threat requires immediate action be taken by the crew to avoid collision with another aircraft to the safety of the flight. Because an RA may be issued to a flight based on tight ATC sequencing of aircraft for the benefit of efficient airspace utilization (not necessarily an unsafe condition), and in the effort to maintain an orderly flow, the FAA has provided the aforementioned proviso. Yet again, why would you as an operator do this? Especially at night in a congested approach corridor (which is the very reason we need it, and the very reason ATC wants to remove the RA function—to avoid more congestion).
Also, and despite any approved option for turning the RA function off, remember that TCAS RAs are typically inhibited below 900 feet AGL (Above Ground Level) while descending, meaning the system will not provide an avoidance command (climb or descend) at altitudes lower than this.
Further, do you ever hesitate to accept a visual (especially at night) following traffic? Is the traffic you see the one that ATC is actually referencing?
We are all fallible, and regardless of whatever equipment is installed or not, is operable or not, the bottom line is to be cautious in assuming that everyone else in your operational environment is doing exactly what they should be doing, is exactly where they should be, and paying close attention.
Just something to think about.
