What aircraft was involved and where did it happen?

The accident on 1961-08-11 involved a PIPER AIRCRAFT CORPORATION J3C-65/L-4., registration HB-ONZ, at Gross-Tisch bei Riemenstalden, SZ, CH.

What was the probable cause of the accident?

A student controller's failure to correct an incorrect altitude read-back, combined with a lack of supervisory oversight, led to a loss of vertical and horizontal separation between the two aircraft.

TCAS Alert Averts Mid-Air Collision Near Port Elizabeth

A Hawker Beechcraft 390 and a Robinson R22 narrowly avoided a head-on collision near Port Elizabeth after a controller's failure to correct an altitude read-back error.

What happened

On 2 September 2014, a Hawker Beechcraft 390, registration ZS-ETN, was conducting an instrument flight rule (IFR) descent toward Port Elizabeth aerodrome. Simultaneously, a Robinson R22 helicopter, registration ZS-RTZ, was performing IFR training maneuvers in the same airspace.

While under radar control, the pilot of ZS-ETN received a sudden traffic alert and a TCAS warning on the primary flight display. The aircraft was on a direct head-on path with the helicopter. The pilot immediately executed an evasive right turn. At the moment of the maneuver, the two aircraft were separated by only one nautical mile horizontally and approximately 100 feet vertically. The crew of the ZS-RTZ remained unaware of the near-miss until after landing, as the helicopter lacked onboard collision warning technology and the student pilot was flying under the hood.

The investigation

An investigation by the SACAA AIID established that the incident stemmed from a communication error during the helicopter's climb. The student controller had instructed the ZS-RTZ to climb to 3,000 feet; however, the helicopter pilot incorrectly read back an altitude of 4,000 feet. The controller failed to notice or correct this error, allowing the helicopter to climb to the higher, conflicting altitude.

Investigators found that a Clear Level Alert Monitoring (CLAM) amber warning had been active on the controllers' screens for four minutes prior to the encounter, yet neither the student nor the instructor controller intervened. Furthermore, the instructor was preoccupied with administrative tasks and was not actively monitoring the student's performance.

Technical limitations also played a role. The Short-Term Conflict Alert (STCA) system failed to trigger because the helicopter's speed was below the software's 90-knot minimum threshold. Additionally, the audio component of the CLAM alert had been deactivated to prevent nuisance alerts from VFR traffic.

Findings

The primary cause was a lapse in concentration by the student controller, who failed to correct an incorrect altitude read-back.
The instructor controller failed to provide adequate supervision due to being preoccupied with other duties.
The CLAM amber warning was ignored by both controllers for several minutes.
The STCA system was unable to identify the helicopter as a potential conflict because the aircraft was flying slower than the 90-knot minimum setting.
The audio alert for the CLAM system had been intentionally deactivated.

Safety action

Following the investigation, recommendations were made to Air Traffic Navigational Services (ATNS) to reduce the minimum speed threshold for STCA activation to ensure slower aircraft are properly monitored. It was also recommended that both visual and audible alerts for level monitoring be kept active at all times and that the involved controllers undergo proficiency training.