What happened
On June 26, 2017, an Airbus A319-132 departed from runway 07 at Stuttgart Airport under Instrument Flight Rules (IFR). During the initial climb, at an altitude of approximately 1,200 ft AGL, the aircraft's Traffic Collision Avoidance System (TCAS) issued a Resolution Advisory (RA) commanding a descent. The crew immediately executed the maneuver, initiating a descent rate of approximately -1,600 ft/min, which resulted in a loss of about 150 ft of altitude.
Shortly after the descent began, the aircraft's Enhanced Ground Proximity Warning System (EGPWS) issued a "Don't sink" warning. The pilot in command decided to prioritize the TCAS RA instructions until the aircraft reached 400 ft AGL. The conflict was resolved when the TCAS issued a "Clear of conflict" message, at which point the crew resumed their climb.
At the same time, an Aerospool Dynamics WT9 ultralight aircraft was operating in the vicinity under Visual Flight Rules (VFR). The ultralight aircraft crossed the path of the departing Airbus, creating the potential for a mid-air collision.
The investigation
The BFU investigation examined the flight paths, radio communications, and cockpit systems. Radar plots and Quick Access Recorder (QAR) data confirmed that at the point of closest approach, the two aircraft were separated by approximately 0.213 NM horizontally and 600 ft vertically. The investigation also reviewed the instructions provided by the Stuttgart Tower controller and the compliance of both pilots with those instructions.
Findings
- The primary cause of the conflict was that the pilot of the Aerospool Dynamics WT9 failed to follow the air traffic controller's instructions. Although the controller had instructed the ultralight pilot to fly south of the airport and wait until after the Airbus had departed before crossing, the pilot instead flew north, directly into the departure sector of the Airbus.
- The Airbus crew's decision to follow the TCAS RA despite the EGPWS "Don't sink" warning was considered a necessary action to avoid the collision.
- The investigation noted that the ultralight pilot likely experienced increased stress due to high ground speeds, navigation requirements, and radio communications within the controlled airspace.
- There were no technical failures found in either aircraft; the TCAS and EGPWS systems functioned correctly.