What happened
On 13 March 2017, two commercial aircraft flying toward OR Tambo International Airport experienced a near-miss near the IMGEV waypoint, approximately 23 nm west of Warden, Free State. The first aircraft, a Boeing 737-800 with registration ZS-ZWS, was descending from flight level 410 to 380. During this descent, air traffic controllers rerouted the flight direct to waypoint ETLIG, cancelling its previously assigned arrival procedure.
To meet the new routing requirements, the flight management computer necessitated a steeper descent profile. As the crew attempted to capture this profile, the Traffic Collision Avoidance System (TCAS) issued a traffic advisory, followed immediately by a resolution advisory (RA). Simultaneously, an Embraer ERJ135, registration ZS-SNX, was cruising at flight level 370. The crew of the Embraer received a traffic advisory and visually identified the descending Boeing aircraft to their left.
In response to the RA, the crew of ZS-ZWS disconnected the autopilot and manually adjusted the pitch to comply with the TCAS command. During this maneuver, the aircraft descended through its assigned altitude, reaching flight level 374. This resulted in a vertical separation of only 500 feet and a lateral distance of 1.6 nm between the two aircraft.
The investigation
The SACAA AIID investigation examined radar data, communication logs, and cockpit procedures. Investigators noted that the air traffic controller observed a clear level alert monitoring (CLAM) warning on their display, indicating the Boeing 737-800 had breached its assigned altitude. The investigation also reviewed the flight paths of both ZS-ZWS and ZS-SNX, confirming that the Embraer aircraft maintained its assigned altitude throughout the encounter.
Because both aircraft remained in active service, the cockpit voice recorders (CVRs) had been overwritten by the time the investigation commenced. Consequently, investigators could not determine the specific level of coordination between the crew members or the exact nature of the interaction with the automated systems during the advisory.
Findings
- The rerouting of ZS-ZWS to waypoint ETLIG caused the flight management computer to calculate a descent profile that required a higher rate of descent than originally planned.
- The crew of ZS-ZWS descended below the assigned flight level of FL380, reaching FL374, which violated RVSM separation standards.
- The TCAS resolution advisory was successfully followed by the crew of ZS-ZWS, which eventually stabilized the aircraft back at FL380.
- The Embraer ZS-SNX maintained its assigned altitude of FL370 and experienced no vertical deviation.
- No injuries were reported, and no damage was sustained by either aircraft.