Investigators have determined that a Delta Air Lines Boeing 757-200 was involved in three losses of separation within a few minutes of departing Atlanta, after the crew failed to activate the twinjet's transponder.

Despite being instructed to contact departure control after being cleared to an initial waypoint, the pilots did not establish communication with air traffic control until around 8min after take-off from runway 27R.

Radar displays showed only a primary target, with no identifying data, and analysis of this information indicated that the 757 lost lateral separation with a Beech 55, a Pilatus PC-12 and an Atlantic Southeast Airlines Bombardier CRJ100. The respective closest proximities were 1.44nm, 0.81nm and 2.36nm.

"The [aircraft] flew through one controller's airspace and entered another controller's airspace without co-ordination, before radar and radio contact was established," says the US National Transportation Safety Board.

Procedures required Atlanta tower controllers to check that departing flights were showing a radar data block before transfer to terminal airspace control. But the NTSB says that the tower controller - distracted by a situation at the 27R approach - "did not notice" that the data block had not been automatically acquired.

As a result the radar showed no altitude information about the 757, which was using a standard instrument departure - designated UGAAA 2 - involving a climb to 10,000ft.

Although the aircraft took off at 13:19 local time, the crew did not make contact again until almost 13:27. In the intervening 8min, departure control had asked the tower about an unaccounted flight strip for the Delta jet, but attempts to locate the aircraft among primary radar targets proved "futile", says the NTSB.

The 757's first radio contact was not with departure control, but with Atlanta tower, to ask whether it was required to stay at 10,000ft, to which the tower replied that the crew was supposed to be communicating with departure control.

When the pilots made contact with the departure controller, they were asked to verify that the transponder was switched on. Six seconds later the aircraft's data block appeared on the radar display, showing that the jet was 20nm east of the airport. Owing to its proximity to inbound Atlanta traffic, the 757 was quickly cleared to 14,000ft and handed off to Atlanta's en-route centre.

Visual conditions prevailed during the 11 March 2011 incident.

Source: Flight International