What happened
On the evening of October 23, 2023, a serious air proximity incident occurred at Stuttgart Airport involving a Boeing 737-8AS on final approach to runway 07 and an Airbus A321-231 preparing for takeoff.
At approximately 19:35, the Airbus A321 began taxiing for a flight to Istanbul. During the taxiing process, several other aircraft were active on the airfield, including an Embraer EMB500 and a Beech 350. The air traffic controller initially planned to sequence the departures by allowing the slower Beech 350 to depart before the Airbus A321. However, after noticing the Embraer EMB500 was ready to depart, the controller spontaneously decided to change the departure sequence to facilitate traffic flow, intending to launch the Embraer via an intersection takeoff before the Airbus A321.
As the Boeing 737-8AS approached the runway, the controller instructed the aircraft to reduce its speed. Simultaneously, the controller cleared the Embraer EMB500 for takeoff and subsequently cleared the Airbus A321 for departure. At 19:50, the Embraer EMB500 commenced its takeoff roll. Shortly thereafter, the Airbus A321 began its takeoff roll on runway 07.
At 19:51, while the Boeing 737-8AS was approximately 0.63 NM from the threshold in a descent, the controller realized the distance between the landing aircraft and the departing Airbus A321 had become critically low. The controller issued an immediate go-around instruction to the Boeing 737-8AS. The crew of the Boeing 737-8AS promptly executed the maneuver, transitioning from a descent to a climb. To further increase separation, the controller instructed the crew to perform a left turn, even though this involved vectoring below the Minimum Vectoring Altitude (MVA).
The investigation
The BFU investigation examined radar data, ADS-B data, and radio communications. The investigation focused on the controller's decision-making regarding the departure sequence and the application of reduced runway separation minima. The investigation also reviewed the visibility conditions, noting that the event occurred at night, which limited the crew's ability to visually identify the departing aircraft against the runway lighting.
Findings
- The controller made a spontaneous decision to alter the established departure sequence to manage traffic flow.
- The controller misjudged the time required to implement the new sequence and execute the various instructions.
- The controller delayed the decision to issue the go-around instruction until the aircraft were at a very close proximity.
- The controller utilized vectoring below the MVA in an attempt to rapidly establish the required separation.
- The use of reduced runway separation minima at night increased the complexity of the situation, as the crew of the Boeing 737-8AS could only identify the Airbus A321 via TCAS and anti-collision lights rather than through clear visual contact.