The UK's Air Accidents Investigation Branch (AAIB) has recommended that the CAA considers mandatory TCAS II for aircraft operating in airspace where London City and Heathrow traffic interacts.

This follows what the AAIB described as a "serious" incident when a Citation 525 taking off from London City in July 2009 was about 100ft to 200ft and half-a-mile away from a Boeing 777 heading to Heathrow with 232 people on board.

The AAIB report said the control tower at London City had cleared the German-owned Citation 525 to climb to 3,000ft but when the flight crew acknowledged the instruction, they said they would be climbing to 4,000ft. This "read back" mistake was not noticed by the controller at the tower, the AAIB said.

Meanwhile, the Turkish flight had been cleared to descend to 4,000ft as it approached Heathrow Airport in west London.

If the aircraft had come close during bad weather "the only barrier to a potential mid-air collision" would have been built-in collision avoidance systems as the aircraft would not have been able to see each other, the AAIB said. The report found that when the aircraft came close, the Turkish flight crew had not followed the commands of three on-board collision avoidance warnings and the Citation jet did not have TCAS II on board.

A pilot sitting on the observer seat of the Turkish flight saw the Citation 525 carrying two crew members and one passenger "pass west of them at an estimated 100 to 200ft below," the report said. In his account the Citation 525's captain said he had the passenger aircraft in sight "all the time" and had at first thought that his jet would be "well above" it.

The AAIB made five safety recommendations that:

• The CAA considers whether TCAS II should be mandated for aircraft operating in those parts of the London TMA where London City airport SID interact with traffic positioning to land at Heathrow airport.
• NATS works to reduce significantly the risk of an accident resulting from a level bust by an aircraft departing London City or on the base leg turn positioning to land at Heathrow.
• London City amends all SIDs so that they terminate at an altitude of 3,000ft.
• London City removes step climb procedures from its SIDs.
• The directorate general of civil aviation of Turkey ensures Turkish Airlines TCAS training complies with ICAO guidelines.

Public debate

The incident, described as a "near miss," ignited a public debate which led to PrivateFly's Adam Twidell being interviewed on UK television. "The press wanted to know why all private jets do not have TCAS II as a mandatory requirement. Current CAA regulations are for aircraft above 5,700kgs," Twidell explains.

The AAIB points out that the Citation 525 did not have TCAS II. This meant that the "increased safety benefit of RAs was not available."

In May 2008 EBAN carried an article written by John B Robinson, operations consultant to the British Business and General Aviation Association. He asked what the reaction would be if a VLJ was flown by a single pilot holding a private pilot's licence with a current instrument rating and 250 hours total flying experience was not equipped with TCAS/ACAS and was involved in a collision with another aircraft.

Robinson pointed out: "Under the present regulations the VLJ would have been operating perfectly legally. Concern has been voiced in some aviation committees, and the national press, over VLJ aircraft operating in the same airspace as commercial and similar air traffic but without the safeguard of a TCAS. At present there is no requirement for aircraft under 5,700kg maximum take-off mass to be fitted with TCAS but Eurocontrol has for some time made it known that a study is to be undertaken to assess the impact of VLJs on the network and evaluate the risks and safety issues of such aircraft as VLJs operating in European airspace."

Robinson added: "Now that Mode S transponders are a mandatory fit for operations in European airspace it can be argued that it is not necessary to fit TCAS to a VLJ as a TCAS II equipped aircraft will be able to receive the signals from the Mode S transponder and take appropriate avoiding action. This can be done without the non-TCAS aircraft being aware that it is causing a possible conflict. However what is worrying is the case that if this non-TCAS aircraft does see the other it could take action that would put it in more danger due to not knowing the intent of the TCAS II operator."

Robinson suggested: "It would go a considerable way to assuaging the concerns of operators and crews who regularly use congested airspace if all aircraft within that airspace were fitted with TCAS II for safety purposes."