What happened
On November 23, 2017, two aircraft departing from runway 18 at Riga International Airport experienced a serious loss of separation within the Riga TMA. The first aircraft, a Norwegian Air Shuttle Boeing 738 (registration LNN-II), departed for Stockholm/Arlanda following a standard instrument departure. Shortly after, an Air Baltic Bombardier DH8D (registration YL-BBCT) was cleared for takeoff from the same runway.
As both aircraft climbed, the horizontal distance between them decreased to 2.6 nautical miles, while the vertical separation dropped to 700 ft, violating the required minima of 3 NM horizontally and 1,000 ft vertically. The incident triggered a Short-Term Conflict Alert (STCA) on the approach controller's radar.
The investigation
The TAIIB investigation focused on the sequence of departures and the adherence to separation standards. The investigation examined the Tower Controller's actions, the Air Baltic crew's communication, and the regulatory framework in place at the time.
Investigators reviewed the Tower Controller's compliance with Local Instruction No 114, which supplemented the standard Tower Controller Manual. This local instruction required more extended separation for certain departures, specifically stating that a succeeding aircraft should only be cleared for takeoff once the preceding aircraft has passed a point 3 NM from the line-up position and reached an altitude of 2,000 ft. Radar data confirmed that at the time of the Air Baltic takeoff clearance, the Boeing 738 was only 2.4 NM from the line-up position and at an altitude of 1,900 ft.
Additionally, the investigation looked into the communication gap between the Air Baltic crew and the Approach Controller. The crew failed to establish radio contact with the Approach Controller by the required 1,500 ft altitude, only making contact while passing 2,600 ft.
Findings
- The Tower Controller failed to follow the requirements of Local Instruction No 114 when issuing the takeoff clearance.
- The Air Baltic crew did not establish radio contact with the Approach Controller by the mandatory 1,500 ft altitude.
- The Tower Controller Manual (DI-GSV/TWR-01/3) had not been updated to include the specific amendments found in the separate Local Instruction No 114, leading to potential confusion.
- The primary cause of the incident was the decision to clear the second aircraft for takeoff in contradiction to the established local separation rules.