What happened
During operations at Horn Island, the flight crew of ZZJ, a Cessna 172, was backtracking on runway 08. At the time, the captain was aware that the runway was being used by other aircraft within the circuit. Following a radio broadcast from the pilot of IOA suggesting that ZZJ could depart before IOA landed, the captain of ZZJ elected to proceed with the takeoff to avoid departure delays.
While the crew used TCAS to monitor the relative position of IOA, the range scale was set to 12 NM, which limited the precision of the aircraft's position data. During the takeoff roll, the crew of ZZJ failed to reject the takeoff, likely due to mishearing a broadcast from the pilot of IOA instructing them to hold short of runway 14. It remains unclear if this miscommunication occurred before or after the aircraft reached its decision speed (V1).
The investigation
The investigation examined the cockpit communications and the use of surveillance technology during the event. It was noted that both aircraft were making the necessary CTAF broadcasts and were aware of each other's approximate locations prior to the near collision. The investigation also looked into the settings of the TCAS, noting that a 6 NM range scale would have offered a more accurate representation of the distance between the two aircraft than the 12 NM scale in use.
Findings
- The captain of ZZJ was motivated to expedite the departure from the main runway.
- A radio broadcast from IOA suggested that ZZJ had sufficient time to depart before the arrival of IOA, which influenced the decision to take off.
- The flight crew of ZZJ likely misheard the instruction for IOA to hold short of runway 14, leading to the failure to reject the takeoff.
- The use of a 12 NM TCAS range scale prevented a highly accurate view of the relative position of the other aircraft.
- Concurrent operations on different runways are standard practice at Horn Island to manage traffic flow.