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How A TCAS Triple RA Event Happened In RVSM Airspace

Live from the Flight Deck

Traffic in RVSM airspace is only separated by 1,000 feet vertically. Here's what can happen when an RA goes off, and what you should know about ATC separation criteria...

Air Traffic Separation + RVSM Review

Controllers must separate aircraft by 5 miles laterally. For vertical separation, aircraft must be either 2,000 feet apart (non-RVSM) or 1,000 feet apart (RVSM airspace). According to the FAA's Instrument Procedures Handbook, "controllers can accomplish this separation by issuing instructions to the pilots of the aircraft involved. Altitude assignments, speed adjustments, and radar vectors are examples of instructions that might be issued to aircraft."

'RVSM' Airspace, or Reduced Vertical Separation Minimum, reduces the vertical separation of aircraft from 2,000 feet to 1,000 feet when they're flying at altitudes from FL290 (29,000 feet) to FL410 (41,000 feet).

The Basics Of TCAS

TCAS Systems (Traffic Alert and Collision Avoidance System) monitor the airspace around an aircraft for other aircraft equipped with a corresponding active transponder, independent of air traffic control, and warns pilots of the presence of other transponder equipped aircraft which may present a threat of mid-air collision (MAC).

TCAS II is the internationally recognized system by ICAO, and we'll focus on its use for this article. TCAS divides traffic alerts into two distinct categories: Traffic Alerts (TA) and Resolution Advisories (RA). When TCAS issues a TA, it's an advisory to begin visually scanning for an aircraft nearby. RA's are much more serious. When an RA is issued, you're approximately 20-30 seconds away from a near impact. The RA will immediately instruct pilots to climb or descend. RAs will never give turning instructions. Pilots are supposed to react immediately, without scanning for traffic visually or requesting ATC instructions.

Here's a chart explaining different RA annunciations that pilots will hear with a TCAS II system:

Wikimedia

Report: Triple "RA" Event

We found the following NASA ASRS report published by an airline flight crew. This short report details what can happen when busy RVSM airspace is affected by an RA event, triggering others nearby:

Cruising at FL370, we had an RA to climb due to traffic climbing below us, from FL360. The traffic at FL360 had an RA to climb from traffic below them at FL350. Because of this, we had a subsequent RA to climb as well. We climbed to about FL375 before receiving a "clear of conflict" annunciation and returned to our altitude.

Inside busy RVSM airspace, a single TCAS RA could lead to a cascading event of traffic conflicts just like this.

How You Can Prevent RA's in RVSM

ATC agencies have recommended that pilots reduce their vertical speed below 1,500 fpm (ideally 1,000 fpm) during the last 1,000 feet of a climb before level-off. Statistics show that this reduces the likelihood for TCAS triggering an unnecessary RA event.

What else? Have you ever experienced an RA before? Tell us in the comments below.

Swayne Martin

Swayne is an editor at Boldmethod, certified flight instructor, and an Embraer 145 First Officer for a regional airline. He graduated as an aviation major from the University of North Dakota in 2018, holds a PIC Type Rating for Cessna Citation Jets (CE-525), and is a former pilot for Mokulele Airlines. He's the author of articles, quizzes and lists on Boldmethod every week. You can reach Swayne at swayne@boldmethod.com, and follow his flying adventures on his YouTube Channel.

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