A business jet had to take avoiding action to prevent a potential mid-air collision with a Heathrow-bound airliner with 232 passengers aboard, an accident report revealed today.
The German-owned Citation 525 jet passed only just over half a mile away and 100ft to 200ft below the Turkish Airlines' Boeing 777 plane in the skies over London, the Air Accidents Investigation Branch (AAIB) report said.
The Citation was cleared by the control tower at London City Airport to climb, initially to 3,000ft.
But acknowledging the instruction, the crew said it would be climbing to 4,000ft - a "readback" mistake not noticed by the London City tower controller.
The Boeing 777 had been cleared to descend to 4,000ft and it was at this height that it passed the Citation, which had two crew and one passenger aboard.
The AAIB report, which described the events of the afternoon of July 27 as a "serious incident", said the Boeing 777 had not "followed the commands" from three on-board collision-avoidance warnings.
The report added that the only person to see the Citation was a pilot occupying the right observer seat who saw it "pass west of them at an estimated 100 to 200ft below".
The AAIB said the Citation captain later filed an airprox (aircraft proximity) report in which he stated the crew was given clearance to climb to 4,000ft.
He said he had the Boeing 777 in sight "all the time" and initially thought his aircraft would be "well above" the Boeing as he crossed its track.
The AAB said that if the weather had been bad the Citation would not have been able to see the Boeing and would therefore not have been able to take effective avoiding action.
In bad weather "the only barrier to a potential mid-air collision" in this case was the collision avoidance system, the AAIB added.
But the report said that the avoidance system did not resolve the incident as the Boeing crew did not respond to the in-flight alerts in time and the Citation did not have a collision-avoidance piece of equipment known as TCAS II.
The AAIB recommended that the Civil Aviation Authority considered whether TCAS II should be mandatory for certain aircraft operating in the London City-Heathrow area.
Making a number of other recommendations, the AAIB said that since the incident the instructions from London City to maintain a height of 3,000ft is given separately from the remainder of the flight take-off clearance and requires a separate readback from the crew.