Investigators probing the mid-air collision over Washington between a regional jet and a military helicopter have provided greater clarity on the circumstances of the accident, but are still trying to understand each crew’s level of situational awareness.
The PSA Airlines CRJ700 had been inbound to Washington National on 29 January when it was hit by a Sikorsky UH-60 Black Hawk, whose crew had earlier been advised of the jet’s circling to land on runway 33.
About 11s after the CRJ descended through 500ft, its cockpit-voice recorder picked up an automated advisory, warning of the presence of traffic, says the National Transportation Safety Board.
Almost immediately after this advisory, states investigator-in-charge Brice Banning, an “audible” transmission from the tower was heard instructing the Black Hawk crew to pass behind the CRJ.
Some 16s after this instruction to the Black Hawk, he says, there was a “verbal reaction” from CRJ crew. Flight-data recorder information indicates the jet’s pitch started increasing, with the collision occurring just 1s later.
The inquiry has not indicated whether, prior to this instance, there is any other evidence of evasive action from either aircraft.
“We’re defining what is inside of each [collision-avoidance] system and what was their [output],” says NTSB board member Todd Inman. “Because of the unique nature of military aircraft we need additional specialisation.”
Commercial aircraft are fitted with airborne collision-avoidance systems which issue traffic advisories about potential conflicts, as well as resolution advisories which order crews to take avoiding action. But at low altitude conflict-resolution commands are inhibited to prevent the risk of terrain impact.
The inquiry also stated that, while all communications from tower controllers were transmitted on both VHF and UHF radio bands, the CRJ only transmitted on VHF and the helicopter only on UHF.
This meant that both aircraft crews could hear all the air traffic control transmissions, but not those of one another.
Inman stresses that this situation, with military aircraft communicating on one frequency band and civil aircraft on another, is “completely common”.
