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:

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:

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.