With a reported 21 level bust incidents in the skies above London City Airport since 2004, should the U.K. be doing more to prevent future incidents occurring?
Five safety recommendations have been drawn up by the U.K.’s Air Accident Investigation Branch following a potential mid-air collision over a densely populated part of London last July involving an executive jet and a commercial airliner.
A German-owned Cessna Citation 525, taking off from London City Airport, came within less than 200 ft. below a Turkish Airlines Boeing 777-300ER that was vectoring for an approach into Heathrow Airport.
The Citation, carrying two crew and one passenger, had been cleared to initially climb to 3,000 ft. by air traffic control at London City. But miscommunication occurred when the controller failed to notice that the crew incorrectly read back their cleared altitude as 4,000 ft.
The two aircraft came within 200 ft. of each other after the Citation climbed steeply to 4,000 ft., just after the Turkish Airlines jet carrying 232 passengers had been cleared to descend to the same altitude.
Despite the Boeing 777 generating three traffic collision avoidance system (TCAS) conflict resolution advisories in short succession, the Turkish Airlines crew did not follow the commands in time. An incident was avoided by the crew of the Citation, which was not fitted with TCAS II, when they saw the passenger jet close by and changed the aircraft’s heading.
Due to the close proximity of obstacles surrounding London City, departures from the airport require an unusual minimum gradient of almost 8% - with no maximum - and a ‘step climb’ to an initial 3,000ft. To ensure vertical separation, aircraft vectored for approach into nearby Heathrow are not cleared below an altitude of 4,000 ft in this region. The trouble is, despite this standard instrument departure (SID) being in place, a reported 21 aircraft have climbed above step altitude of 3,000 ft. since 2004, a third of which have resulted in a loss of separation, or a level bust.
Evidence from nearby Stansted Airport shows that removing the step climb element of the SID led to a reduction in level busts, so the AAIB has recommended that London City amend all SIDs to remove the step climb procedure and terminate at an altitude of 3,000ft.
The AAIB also wants NATS, the U.K.’s main air navigation service provider, to demonstrate that appropriate mitigation measures are in place to avoid future incidents. NATS says that since the incident last year, instructions to maintain an altitude of 3,000 ft. after takeoff are now given separately and require a separate read back to ensure there has been no miscommunication between crew and ATC.
The AAIB report suggests that if the German Citation had not been in visual contact with the Boeing 777, a serious mid-air collision could only have been prevented by the use of TCAS.
However, despite three separate TCAS warnings, the crew on board the Turkish Airlines Boeing 777 did not respond in time to prevent this level bust. As a result, the AAIB is recommending that Turkish Airlines ensure TCAS training complies with Airborne Collision Avoidance System standards.
Finally, despite the recommendations and new procedures already adopted, there is still room for human error and the risk of a mid-air collision in this busy part of London is still a very real one. The AAIB’s final recommendation to the Civil Aviation Authority is to consider whether TCAS II should be mandatory for all aircraft operating in parts of London where London City’s SIDs interact with inbound Heathrow traffic.