Senegalese investigators have detailed several occurrences involving incorrect altimeter readings on the British Aerospace 125 jet involved in a fatal mid-air collision with a Boeing 737-800.
None of the seven occupants of the Senegalair BAe125 survived after it crashed into the sea off the West African coast nearly 1h after the impact on 5 September 2015.
Investigation authority BEA has concentrated on an apparent 1,000ft discrepancy between the aircraft's altitude, as stated by the crew, and that transmitted by the jet's transponder.
The inquiry believes the BAe 125, as a result, was travelling in the opposite direction to the Ceiba Intercontinental 737 at the same altitude of 35,000ft.
While the BAe 125's flight-data recorder was not recovered, the inquiry has uncovered evidence of previous similar altimeter problems on the 36-year old aircraft (6V-AIM).
These issues, in particular, included another serious conflict, with an Arik Air 737, on 23 July 2015 – six weeks before the Ceiba collision.
The Arik jet had been flying at 31,000ft and its crew expressed concern to air traffic control that the BAe 125 was approaching from the opposite direction at the same altitude.
Information from the 737's collision-avoidance system indicated that the BAe 125 – en route from Roberts Field, Liberia, to Dakar – had passed the waypoint SESEL at 31,000ft.
But the controller assured the Arik crew that the BAe 125 was at 32,000ft. Unconvinced, the Arik pilots asked to climb to 39,000ft, but this was denied owing to traffic above.
"We have traffic at SESEL at the same level as us," the Arik crew again told the controller, at which point the 737 was cleared to descend to 29,000ft.
The controller contacted the BAe 125's pilots, who confirmed the aircraft's altitude as 32,000ft.
They were instructed to maintain this altitude to waypoint KOMOR, but the Arik crew informed the controller that the BAe 125 was not at 32,000ft but rather at 31,400ft, adding: "He should check his RVSM capability."
RVSM capability refers to an aircraft's ability to operate in reduced vertical separation minima airspace, where adjacent flight levels are just 1,000ft apart, requiring aircraft to have highly-accurate altimeters.
As the aircraft crossed, the confusion continued, with the BAe 125 crew stating that the Arik 737 was 2,000ft below, and the Arik crew responding by pointing out that they had descended to 29,000ft – which meant the BAe 125 was, indeed, at 31,000ft and not the 32,000ft its pilots claimed.
Not until radar information identified the BAe 125 as being at 31,000ft did its crew seek to confirm the extent of the discrepancy of the altimeter reading.
The BEA inquiry highlights altimeter problems on the aircraft which occurred on 10 July and 31 August, in the weeks preceding the mid-air collision.
It also discloses that, on the day of the accident, the BAe 125 twice triggered radar alarms alerting controllers to differences between the authorised altitude and that detected by radar surveillance. The alarms occurred within 5min of one another, as the jet was flying between the waypoints NURAS and DISNO, some 5h before the collision.