What happened
On January 28, 2018, at 0553 UTC, a serious air proximity incident occurred within the Nagpur Area Control Centre (ACC) involving two commercial aircraft. The first, an Airbus A320 operated by M/s Indigo (flight IGO334), was cruising at FL330 en route from Hyderabad to Raipur. The second, a Boeing 777-300 operated by M/s Emirates (flight UAE353), was overflying Nagpur at FL300, traveling from Singapore to Dubai.
While the Indigo aircraft was cruising, the flight crew requested a descent. In the absence of the active Radar Controller, the Planning Controller authorized a descent from FL330 to FL2/50. This instruction directed the aircraft to pass through FL300, the exact altitude maintained by the Emirates flight. As the Airbus A320 descended, a Level Burst (LB) alert appeared on the radar screen at FL319, followed by a Short Term Conflict Alert (STCA) at FL309. Both aircraft subsequently reported receiving TCAS Resolution Advisories (RA). The Emirates aircraft descended to FL296, while the Indigo aircraft initially descended to FL307 before climbing to FL311.
The investigation
The AAIB India investigation examined radar data, ATC tape transcripts, and CCTV footage. The inquiry established that the Radar Controller 1 had frequently left the active ATC channel without proper briefing or handing over procedures. During these absences, the Planning Controller—who was not rated for Radar Control—was managing aircraft in the radar environment.
Furthermore, the investigation found that the cleared flight level was not updated on the aircraft's data block by the controller. The inquiry also noted that the incoming Radar Controller 2 handed the headset back to the original controller immediately after taking over, even as the conflict was unfolding. The investigation also identified the use of non-standard language during inter-unit coordination and improper handover/takeover procedures.
Findings
- The Planning Controller, acting as the Watch Supervisory Officer, authorized a descent for IGO334 through the flight level of UAE353.
- The Radar Controller 1 had left the active ATC channel five times in a 22-minute period without following standard handover protocols.
- The Planning Controller was not rated for Radar Control (RSR), yet was handling aircraft in the radar environment.
- The cleared flight level was not entered into the data block for the Airbus A320.
- The Radar Controller 2 handed the headset back to the previous controller immediately after the conflict was detected.
- Non-standard phraseology was used by the controller when instructing IGO334 to "Re-cleared FL305" while the aircraft was already executing TCAS RA maneuvers.
- No predicted STCA (Yellow) warning was issued by the radar system prior to the red alert.