The US NTSB is investigating two separate incidents in which errors or poor judgement on the part of pilots and controllers may have led to inadequate airborne separation of aircraft.

A 22 April preliminary report by the NTSB on a Southwest Boeing 737 that flew too close to a Cirrus SR22 general aviation aircraft in central Florida on 27 March reveals that controllers "solicited" the assistance of the Southwest (SWA) aircraft, and the pilots agreed to help.

At issue was an SR22 (N1487C) in cruise flight at 11,000ft (3,353m) that had not been responding to air traffic controllers in the region, an anomaly that defence officials monitoring domestic air traffic were also aware of.

"F11 (Central Florida air traffic control) solicited the assistance of [Southwest Airlines Flight 821] to attempt to verify the condition of the occupants of N1487C," the NTSB notes in the preliminary report.

"SWA821 obliged their assistance and the F11 controller issued vectors to SWA821 toward N1487C. A separate radar scope was set up and single frequency was used by a front line manager who provided control instructions to SWA821," the report states.

The NTSB says that when Flight 821 was approximately 4.3nm (8km) from N1487C, the Southwest pilot "reported N1487C on TCAS and visually", adding that "visual separation was not applied by [air traffic control]".

"SWA821 was instructed to resume own navigation, get as close as safely possible and report any abnormalities," says the NTSB. "SWA821 manoeuvred on his own along side of N1487C. The crew reported two occupants in the Cirrus and no apparent movement from them. SWA821 was then issued vectors away from N1487 to [its destination, Orlando]."

The two aircraft came to within 30m vertically and 0.09nm horizontally during the encounter, says the NTSB. The Cirrus later landed at the Kissimmee airport near Orlando, its intended destination.

A lesser known incident occurred on 11 March in Atlanta involving a tower controller and a Delta Air Lines Boeing 757.

According to a preliminary NTSB report released 18 April, Flight 2086, the Delta 757 with 130 on board departed Atlanta for New York LaGuardia but its flight information "failed to auto acquire" on the departure controller's radar display. That means either the aircraft's transponder "failed or the pilot failed to turn it on", notes the NTSB.

Instead of the usual display, which has flight number, airspeed, altitude and destination, controllers saw only an "enhanced primary" target, which shows an aircraft position but does not positively identify which aircraft is being tracked.

"Atlanta Tower did not advise the radar departure controller that DAL2086 had not auto acquired," says the NTSB. The pilots, after climbing to 10,000ft using the approved departure procedure, were told by the tower to contact departure controllers.

"The pilot read back the instruction correctly, but did not switch to that frequency," says the NTSB. "After flying approximately eight minutes in Atlanta [air traffic control] airspace, the aircraft contacted the tower controller, who immediately instructed the pilot to contact departure, and was then radar identified 20 miles east of Atlanta level at 10,000 feet".

The departure controller had been unaware that the 757 had departed "until he noticed the airplane's flight strip had been scanned by the tower, indicating DAL2086 had departed, and he was not talking to the airplane," the report states.

As a result, the NTSB says "separation was lost" with a Pilatus PC-12 (closest proximity 0.7nm), a Beechcraft Baron twin (closest proximity 1.25nm), and a CRJ200 regional jet (closest proximity 2nm).

Source: Air Transport Intelligence news