What happened
On 21 November 2017, a Cessna 172S aircraft, registration PK-BYK, operated by Balai Pendidikan dan Pelatihan Penerbangan Banyuwangi, was conducting a cross-country training flight from Banyuwangi to Sumbawa. On board were two student pilots. During the flight, the crew requested and received permission to climb from 7,000 feet to 9,000 feet to avoid weather.
Simultaneously, an ATR 72-600, registration PK-WHS, operated by PT. Wings Abadi Airlines, was performing a scheduled passenger flight from Sumbawa to Lombok at 8,000 feet.
As the PK-BYK approached Namp Island, the student pilot flying observed the PK-WHS emerging from clouds in the opposite direction. Observing the nose of the larger aircraft pitched slightly upward, the student pilot incorrectly assumed the ATR 72-600 was climbing. To avoid a perceived collision, the student pilot decided to descend without ATC clearance. This maneuver caused the vertical separation between the two aircraft to drop below required minima, reaching a distance of approximately 2 nautical miles at 8,000 feet.
The investigation
The KNKT investigation focused on the flight paths, communication logs, and the decision-making process of the student pilots. Investigators examined the radar data from Bali approach, which showed the separation reduction. The investigation also reviewed the coordination between the Bali approach and Lombok tower controllers.
It was established that the student pilots were using one radio to communicate with air traffic control while using the second radio to communicate on a company frequency. Furthermore, the investigation found that the pilot's decision to pitch the nose down to visually search for traffic may have created a visual illusion, reinforcing the false belief that the oncoming aircraft was climbing.
Findings
- The student pilot's assumption that the ATR 72-600 was climbing led to the unauthorized descent.
- The crew was not fully aware of the established right-of-way rules, which would have dictated the appropriate maneuver.
- A lack of experience in managing head-on traffic from the opposite direction contributed to the error.
- The pilot's decision to pitch down the aircraft for visual acquisition likely caused a visual illusion regarding the other aircraft's flight path.
- There were gaps in coordination, as the Bali approach controller did not notify the Lombok tower of the PK-BYK's altitude change to 9,000 feet.
- The student pilots were not maintaining a continuous listening watch on the primary ATC frequency, as they were communicating with each other on a separate company frequency.
Safety action
Following the investigation, AirNav Indonesia updated its briefing materials for controllers to ensure all altitude changes are communicated to receiving units. Additionally, controllers in the Lombok area were reminded to maintain a strict listening watch on all communications.
The flight school (BP3B) issued safety notices to its instructors and students, emphasizing the necessity of complying with ATC clearances, adhering to right-of-way rules, and maintaining a proper listening watch on assigned frequencies. The school also implemented a requirement to ensure SD cards are inserted in Garmin G1000 units to ensure flight data is properly logged for safety analysis.