Mid-air Collision Risk Avoided in London TMA Following Altitude Misinterpretation

Casualties unknown • London TMA (terminal control area), GB

A Cessna Citation 525 and a Boeing 777-300ER narrowly avoided a collision in the London Terminal Control Area after a pilot misread an ATC altitude clearance.

What happened

On 27 July 2009, a Cessna Citation 525, registration D-ITAN, departed London City Airport following a standard instrument departure procedure. The procedure required the aircraft to climb to and maintain 3,000 ft before further climbing. However, due to a misinterpretation of the air traffic control clearance, the crew believed they were cleared to climb directly to 4,000 ft. This error led the aircraft to climb through its required level-off altitude.

Simultaneously, a Boeing 777-300ER, registration TC-JJA, was operating a commercial flight on approach to Heathrow Airport. As D-ITAN climbed through 3,000 ft, it passed TC-JJA on a nearly reciprocal heading, with a vertical separation of only approximately 164 ft. The crew of D-ITAN observed the larger aircraft and managed to take evasive action, but the encounter posed a significant risk of collision.

The investigation

The investigation examined the communication between the London City Tower and the crew of D-ITAN, as well as the TCAS (Traffic Collision Avoidance System) performance of both aircraft. It was established that while the controller corrected a minor omission in the pilot's readback, the incorrect altitude was not identified.

Data from radar installations and the flight path of TC-JJA were analyzed. The investigation found that TC-JJA experienced three TCAS Resolution Advisories (RAs) in rapid succession. The crew of TC-JJA did not follow the initial descent commands, and the controller's attempt to assist by suggesting a climb to 5,000 ft actually conflicted with the active 'increase descent' RA. Furthermore, the investigation noted that D-ITCA was not equipped with TCAS II, which prevented the coordinated avoidance maneuvers that would typically occur between two such equipped aircraft.

Findings

  • The primary cause was the crew's misinterpretation of the ATC clearance, leading them to climb through the 3,000 ft step-climb altitude.
  • The controller failed to notice the incorrect altitude readback by the crew of D-ITAN.
  • The high rate of climb required by the departure procedure reduced the window of time available for the controller to detect the altitude deviation.
  • The TCAS encounter was not resolved by the automated systems because D-ITAN lacked TCAS II capability, and the crew of TC-JJA did not respond to the initial descent RAs in a timely manner.
  • The use of incorrect phraseology by the crew of TC-JJA delayed the controller's awareness of the TCAS event.

Probable cause

The incident was caused by the crew of D-ITAN incorrectly interpreting an ATC clearance as a direct climb to 4,000 ft, thereby bypassing the mandatory 3,000 ft level-off altitude. This was compounded by the controller's failure to identify the erroneous readback and the lack of coordinated TCAS II functionality between the two aircraft.

Frequently asked questions

What happened in the 2009-07-27 Citation 525 and Boeing 777 300ER accident near London TMA (terminal control area), GB?

A Cessna Citation 525 and a Boeing 777-300ER narrowly avoided a collision in the London Terminal Control Area after a pilot misread an ATC altitude clearance.

What aircraft was involved and where did it happen?

The accident on 2009-07-27 involved a Citation 525 and Boeing 777 300ER, registration D-ITAN and TC-JJA, at London TMA (terminal control area), GB.

What was the probable cause of the accident?

The incident was caused by the crew of D-ITAN incorrectly interpreting an ATC clearance as a direct climb to 4,000 ft, thereby bypassing the mandatory 3,000 ft level-off altitude. This was compounded by the controller's failure to identify the erroneous readback and the lack of coordinated TCAS II functionality…

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