Investigators in Cote d'Ivoire have concluded that an Antonov An-26 crew continued an approach below minima without making visual contact with the runway before the twin-engined aircraft crashed into the sea off Abidjan.
Analysis of the accident indicates that the terrain-warning system had been silenced to avoid nuisance alarms.
The approach was conducted "without adequate monitoring" of the aircraft's descent flightpath and, under standard operating procedures, the crew should have executed a go-around, says the inquiry into the 14 October 2017 crash.
Three crew members and one of the seven passengers were killed in the accident involving the Moldovan-registered aircraft (ER-AVB), operated by cargo charter carrier Valan.
It had been arriving at Abidjan following a service from Ouagadougou in Burkina Faso. The flight, operating under the identity 'Kondor 26', was being conducted on behalf of French armed forces.
Although the crew briefed for an ILS approach to runway 21, this was subsequently changed to a VOR Z approach to runway 03, which requires flying in from the south over the sea.
Investigators state that the crew discussed an absence of VOR signal on board, so the captain decided – without informing air traffic control – to use area navigation and GPS for the non-precision approach.
The crew was given the weather conditions – reduced visibility and light rain – but the cockpit-voice recorder did not capture evidence of an announcement updating minima.
Some 3min after the tower gave landing clearance to the crew, radar contact was lost. Wreckage was subsequently located offshore, the aircraft having crashed into the sea short of the threshold.
Cote d'Ivoire's ministry of transport, which completed the probe in March through its investigation authority BEA, says the crew underestimated the adverse weather conditions and there was "insufficient monitoring" of the aircraft's altitude and speed.
The crew, carrying out their first flight to the airport, "did not notice" that the aircraft had deviated from the normal 3° glidepath for the runway.
Although the enhanced ground-proximity warning system was functioning, the aural alerts were "probably deactivated" before the flight, says the inquiry, "because they were issued too early in relation to the actual altitude of the terrain".
Investigators state that crew resource management was probably "unbalanced" and adversely affected by the authority of the captain – who was supervising a training exercise – over the rest of the crew.