TCAS Resolution Advisory led to near-collision between Cessna and DHC-8

Casualties unknown • de Havilland DHC-8-102 C-GONH, CA

A malfunctioning transponder on a Cessna 172N caused a TCAS resolution advisory to direct a DHC-8-102 into a descending path toward the smaller aircraft.

What happened

On a night flight intended for sightseeing over Montréal, a Cessna 172N, registration C-GIXH, was operating within the Ville-Marie circuit. While the pilot was navigating the circuit, an Air Canada Regional Inc. DHC-8-102 took off from Dorval / Montréal International Airport. During its initial climb, the DHC-8 flight crew received a resolution advisory (RA) from their traffic alert and collision-avoidance system (TCAS). Following the RA instructions, the crew descended, which actually brought the aircraft closer to the Cessna. The crew subsequently spotted the Cessna directly ahead at a lower altitude and immediately arrested the descent to avoid a collision. The two aircraft passed each other with a vertical separation of approximately 200 feet.

The investigation

Investigators examined the flight paths and the functionality of the onboard avionics. It was established that the Cessna's Mode C transponder was transmitting an inaccurate altitude. Because TCAS relies on the altitude data transmitted by other aircraft, the DHC-8 crew followed an advisory that was based on this erroneous data. The investigation also looked into the actions of air traffic controllers and the maintenance practices of the aircraft owner, Cargair Ltd.

Findings

  • The pilot of the Cessna 172N failed to maintain the assigned heading and altitude, causing the aircraft to cross the path of the DHC-8 earlier than anticipated.
  • The Cessna 172N Mode C transponder transmitted an incorrect altitude, which triggered the misleading TCAS resolution advisory for the DHC-8.
  • The inaccurate altitude transmission from the Cessna's Mode C was the primary cause of the misleading TCAS advisory.
  • Air traffic controllers did not instruct the pilot to shut off the malfunctioning Mode C, despite being aware that the altitude readout was invalid.
  • The operator, Cargair Ltd., was aware of the intermittent Mode C issue but did not notify the pilot before the flight, which could have prevented the aircraft from entering Class C airspace with a defective transponder.
  • The use of aircraft landing lights and the vigilance of the DHC-8 crew were critical factors in the successful visual avoidance of the collision.

Probable cause

The collision was avoided due to the crew's visual detection, but the near-miss was caused by the Cessna 172N transmitting erroneous altitude data via its Mode C transponder, which prompted a TCAS resolution advisory that directed the DHC-8 into a descending path toward the Cessna.

Frequently asked questions

What happened in the 2001-07-18 Air Canada Regional Inc. accident near de Havilland DHC-8-102 C-GONH, CA?

A malfunctioning transponder on a Cessna 172N caused a TCAS resolution advisory to direct a DHC-8-102 into a descending path toward the smaller aircraft.

What aircraft was involved and where did it happen?

The accident on 2001-07-18 involved a Air Canada Regional Inc., operated by Between Cargair Ltd.Cessna 172N C-GIXH and, at de Havilland DHC-8-102 C-GONH, CA.

What was the probable cause of the accident?

The collision was avoided due to the crew's visual detection, but the near-miss was caused by the Cessna 172N transmitting erroneous altitude data via its Mode C transponder, which prompted a TCAS resolution advisory that directed the DHC-8 into a descending path toward the Cessna.

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