Investigators believe there was no collision-avoidance warning before the mid-air impact involving a Boeing 737-800 and a British Aerospace 125 over Senegal, but the reason for its absence remains unclear.
The aircraft collided outside the area of radar coverage, apparently after the Senegalair BAe 125 descended into the flightpath of the opposite-direction Ceiba Intercontinental 737 at 35,000ft.
As a result the 737 lost part of its starboard winglet while the BAe 125 flew onwards, unresponsive to communication, before eventually crashing in the Atlantic Ocean.
Rockwell Collins examined the TTR-2100 collision-avoidance system from the Ceiba 737 in Atlanta, and determined that it was functioning correctly.
But the examination revealed "no evidence" that the device had issued a resolution advisory – an instruction to the crew to take urgent evasive action – against the BAe 125, says Senegalese investigation authority BEA.
The corresponding equipment from the BAe 125 was not recovered from the sea, although the inquiry says its collision-avoidance system had experienced faults on previous occasions in 2013 and 2014.
In testimony to the inquiry the 737's captain, who was the monitoring pilot, stated that there had been no preventative or collision-avoidance system warnings before the encounter.
He told the investigation that the 737 had been cruising at 35,000ft in good weather and the crew had been expecting opposite traffic 1,000ft below.
"Some time later on our displays we could see the other traffic which was [1,000ft] above us at [36,000ft]," he says, adding that maintaining visual contact was "not easy".
"When we saw the [aircraft] it was really very close and looked as if it was descending through our flight level."
Only after the BAe 125 had passed the 737, says the captain, did the collision-avoidance system sound a "traffic, traffic" warning.
An air safety report filed by Ceiba similarly states that there was no collision-avoidance alert beforehand, and initially listed the 737's winglet damage as having been caused by wake turbulence from the encounter.
Transcripts of communications in the minutes just after the accident show the 737 crew told Dakar air traffic control that the BAe 125 had been "descending in front of us" and had "passed behind and very close", before adding that they suspected it had "hit our wing".
After the collision, and having subsequently being informed that the Dakar-bound BAe 125 was supposed be at 34,000ft, the 737 crew stated: "We have traffic [1,000ft] above us."
The relevance of this remark to the BAe 125's altitude information and the effect on the collision-avoidance systems of the two aircraft is not clear.
Dakar air traffic control was unable to obtain further clarification of the comment, or any additional details, because the 737 had been in the process of handover to Bamako air traffic control and communications with its crew became unreadable.
Altimetry problems may have contributed to the 5 September 2015 collision, says the inquiry, which has been hampered by the absence of recorder data from the BAe 125 and the loss of cockpit-voice information from the 737 – erased as it continued onwards to Malabo.
Senegalese civil aviation authority ANACIM has questioned some of the inquiry's conclusions, including the claim that there was no collision-avoidance warning, stating to the probe that the collision-avoidance system of the BAe 125 had been listed as functional in 2015.