TCAS Traffic Advisory Triggered During Near-Midair Collision Avoidance

Casualties unknown • DE

A Tu-154M and an A319 experienced a serious airprox incident near the RILAX intersection while operating under the control of Zurich Approach.

What happened

On February 14, 2005, at approximately 07:00 UTC, a Tu-154M performing a positioning flight from Warsaw to Zurich and an A319 arriving from Cologne/Bonn entered the same airspace near the RILAX intersection. Both aircraft were under the control of Zurich Approach.

The A319 was cleared to descend to FL 130. The Tu-154M was initially cleared to maintain FL 170, then later cleared to descend to FL 150. During the sequence, the air traffic controller (ATC) assigned the Tu-154M a new heading of 150 degrees and a speed reduction to 210 KIAS.

While the A319 was stabilized at FL 130, the Tu-154M continued its descent below the authorized altitude. The aircraft eventually closed to a vertical separation of only 300 ft and a horizontal distance of 1.5 NM. The crew of the A319 received a TCAS Traffic Advisory (TA) and observed the Tu-154M descending toward them. The crew of the Tu-154M also observed the other aircraft on their TCAS display. The ATC was alerted by a Short Term Conflict Alert (STCA) and immediately instructed the Tu-154M to stop its descent and climb back to FL 140.

The investigation

The investigation examined the ATC instructions, radar recordings, and cockpit communications. Investigators analyzed the TCAS logic, specifically why a Traffic Advisory was triggered despite the aircrafts' lateral distance beginning to increase. The investigation also reviewed the Tu-1154M crew's perception of their assigned altitude.

Findings

  • The ATC assigned the Tu-154M to descend only to FL 150, but the crew continued descending to FL 133.
  • The Tu-154M crew believed they had received an instruction to descend to FL 110. This may have been caused by a misunderstanding of a complex instruction involving multiple repetitions of the number "one" (heading 150, vectors 14, speed 210).
  • The investigation noted a lack of effective Crew Resource Management (CRM) on the Tu-154M, as the crew failed to utilize "closed loop" communication or verify the descent altitude with other crew members, allowing the unauthorized descent to continue.
  • The TCAS TA was triggered because the system predicted that the aircraft would breach the 1.0 NM horizontal separation threshold at the time of co-altitude.
  • The crew of the Tu-154M failed to comply with the ATC instruction to level off at FL 150.

Probable cause

The primary cause of the airprox was the Tu-154M crew's failure to adhere to the assigned altitude of FL 150, continuing a descent that led to a loss of separation with the A319.

Frequently asked questions

What happened in the null aircraft accident near DE?

A Tu-154M and an A319 experienced a serious airprox incident near the RILAX intersection while operating under the control of Zurich Approach.

What aircraft was involved and where did it happen?

The accident on null involved a aircraft, at DE.

What was the probable cause of the accident?

The primary cause of the airprox was the Tu-154M crew's failure to adhere to the assigned altitude of FL 150, continuing a descent that led to a loss of separation with the A319.

Loading the flight search…