What happened
On the afternoon of 13 August 2023, two aircraft flying under visual flight rules (VFR) experienced a serious loss of vertical separation while approaching Runway 07 at Lanseria International Airport (FALA). The first aircraft, a Pilatus PC12 with registration ZS-TWF, was performing a repositioning flight from O.R. Tambo International Airport. Simultaneously, a Cessna 172, registration ZS-CPL, was arriving from Welkom Aerodrome.
As the aircraft were maneuvering in the circuit, the air traffic controller instructed the Cessna 172 to perform a left orbit. The pilot of the Pilatus PC12 reported that they could not visually identify the other aircraft but could see it on their Traffic Collision Avoidance System (TCAS). To ensure safety, the Pilatus PC12 pilot climbed to 6,500 feet to maintain separation. However, the vertical distance between the two aircraft dropped to only 300ft, which is below the required 500ft standard.
The investigation
An investigation by the SACAA AIID established that while both pilots and the air traffic controller were properly licensed, a technical discrepancy existed in the secondary surveillance equipment of the Cessna 172. The investigation focused on the altitude reporting capabilities of the aircraft's transponder.
Testing revealed that the Mode C transponder on ZS-CPL was providing inaccurate altitude data. While the pilot's cockpit altimeter indicated 5,500 feet, the transponder was transmitting an altitude of 6,000 feet to air traffic control and the TCAS of the following aircraft. This discrepancy was traced back to an error in the pitot-static system.
Findings
- The loss of vertical separation was caused by an instrument error on the transponder fitted to the Cessna 172 (ZS-CPL).
- The pitot-static system error resulted in the transponder reporting an altitude that was 300ft different from the actual aircraft altitude.
- The error was attributed to the improper calibration of the transponder.
- The pitot-static system had been serviced approximately 5.7 hours prior to the incident.