What happened
On 4 September 2019, a near collision occurred in the airspace above the runway at Stockholm/Västerås Airport. At the time of the incident, the airport tower was closed, meaning the area was operating as uncontrolled Class G airspace. Two aircraft were operating in the vicinity using different flight rules: a Cessna 172 was flying under an IFR flight plan, performing repeated NDB1 approaches to runway 01, while a Piper PA-28 was conducting a VFR training flight following published circuit procedures.
Because the two aircraft were following different approach procedures, their flight paths crossed at the same altitude directly over the runway. Sensor data indicates that the aircraft passed each other with a minimum horizontal separation of only 150 metres. The pilot of the Piper PA-28 performed a sharp right-hand avoidance manoeuvre just before the paths intersected. The pilots were only able to detect one another at a very late stage in the encounter.
The investigation
The investigation focused on the conflict between the IFR and VFR procedures and the regulatory environment of the airport during non-tower hours. Investigators established that the airport's terminal manoeuvring area and control area are only active during tower operating hours, leaving the airspace uncontrolled when the incident occurred. The inquiry also examined the visibility and situational awareness of both crews, noting that the Cessna 172 pilot's view may have been partially obstructed by a window beam, and the Piper PA-28 pilot's view of the Cessna 172 was hindered because the latter was positioned below the horizon.
Findings
- The primary cause of the incident was that the two aircraft were following different approach procedures with flight paths that crossed at the same altitude.
- The crew of the Piper PA-28 lacked the instrument ratings necessary to fully anticipate the intentions of the IFR-compliant aircraft.
- Visual detection was delayed because the pilot of the Cessna 172 may have been focused on flight instruments rather than external observations, compounded by potential obstruction from a window beam.
- The positioning of the Cessna 172 below the horizon made it difficult for the Piper PA-28 pilot to spot the aircraft early.
- There is existing regulatory ambiguity regarding whether IFR approaches are permitted at instrument aerodromes when the tower is closed and the airspace is uncontrolled.
Safety action
Following the investigation, a recommendation was made to the Swedish Transport Agency to consult with EASA to clarify the requirements for IFR flights into uncontrolled instrument aerodromes and to ensure these rules are clearly communicated to pilots and aerodrome operators.