Near collision involving Pilatus BN2, VH-IOA and Bombardier DHC-8, VH-ZZJ, Horn Island Airport, Queensland, on 12 October 2016

What happened

On 12 October 2016, at about 1330 Eastern Standard Time (EST), a Pilatus BN2A-20, registered VH-IOA (IOA), departed from Kubin on a charter flight to Horn Island, Queensland (Figure 1). On board IOA were one pilot and two passengers. At about the same time, a Bombardier DHC-8-202, registered VH-ZZJ (ZZJ), conducting surveillance operations, was preparing for departure from Horn Island. On board ZZJ were two pilots and three crewmembers.

Figure 1: IOA track from Kubin to Horn Island

Source: Google earth, annotated by ATSB

The Kubin authorised landing area (ALA) is within the Horn Island broadcast zone, and the pilot of IOA reported that they made their departure call on the Horn Island common traffic advisory frequency (CTAF). At the time IOA departed from Kubin, a faster company aircraft, registered VH-WOT (WOT), departed from Badu Island (Figure 1) on track to Horn Island. In addition to WOT, there was a training aircraft conducting circuits on runway 08 at Horn Island. While tracking to Horn Island, the pilot flying IOA heard a radio broadcast from WOT, which indicated they would join the runway 08 circuit as number 2 to the training aircraft. The area controller^[1] then passed IOA traffic information about a DHC-8 (ZZJ) aircraft taxiing at Horn Island for departure from runway 08. The pilot flying IOA planned to join a straight-in approach to runway 14 from their track from Kubin. The pilot reported that they made all the required radio broadcasts for an arrival to Horn Island including broadcasts at 10 NM, 5 NM and 3 NM before they joined the final approach for runway 14.

The first officer on board ZZJ made a broadcast they were entering and backtracking runway 14 at Horn Island for a departure from runway 08 (position 1 on Figure 2). Just prior to the intersection of the two runways, ZZJ held short of runway 08 to allow the training aircraft to complete a touch-and-go landing on runway 08. The first officer then broadcast ZZJ was entering and backtracking runway 08. While backtracking runway 08 (position 2 on Figure 2), ZZJ’s flight crew heard a broadcast that WOT was joining the circuit as number 2 to the training aircraft and then observed a third aircraft appear on their traffic collision avoidance system (TCAS) display. The captain of ZZJ asked the first officer to contact the third unknown aircraft.

When the pilot flying IOA heard a broadcast from ZZJ, which indicated it was directed at them, they responded that they were approaching a 3 NM final for runway 14 and ZZJ would have time to depart from runway 08 if they were quick. At about this time, ZZJ was approaching the threshold of runway 08. The captain lined ZZJ up for departure and the crew completed their pre-take-off checklist items (three items). At about this time, another two aircraft, 20 NM away from Horn Island, started communicating with each other on the Horn Island CTAF.

By the time the pilot flying IOA heard ZZJ broadcast ‘rolling for departure from runway 08’, IOA was at about 400 ft on final approach for runway 14. The pilot of IOA reported that at about 300 ft, they broadcast ‘hold short’ to ZZJ. However, the captain of ZZJ reported they heard the pilot of IOA say ‘land and hold short of runway 08’.^[2] Consequently, ZZJ continued their take-off. As IOA approached 100–150 ft on final approach, ZZJ had not crossed the runway intersection and the pilot flying IOA conducted a left climbing turn away from the runways to join the downwind circuit leg for runway 08. The captain of ZZJ looked out their left window when they were in the initial climb overhead the threshold of runway 26 and saw IOA turning through east at about the same level (position 4 on Figure 2). IOA then joined the circuit for runway 08 and landed after WOT without further incident.

Figure 2: Sequence of movements

Source: Google earth, annotated by ATSB

Horn Island Airport and CTAF

The Horn Island Airport is located at the northern end of Cape York Peninsula. The airport acts as the hub for access to the outer islands in the Torres Strait. Mainland services fly into Horn Island and passengers are then transferred onto the local operators’ smaller aeroplanes and helicopters for transfer to and from the outer islands. Runway 08/26 is the main runway and runway 14/32 is shorter and narrower. Smaller aircraft, such as IOA, operate to both runways, but larger aircraft, such as ZZJ, restrict their operations to runway 08/26.

The airport apron is located adjacent to the threshold of runway 32 and there are no taxiways to separate ground movements from aircraft taking off and landing. There is also higher terrain located to the southeast of the airport, which has resulted in the following additional restrictions to airport movements published in the Horn Island aerodrome chart:

Take-off runway 14 and landing runway 32 not permitted due terrain.

Consequently, the smaller local aeroplane operators’ have adopted the local practice of departing from runway 08 and landing on runway 14, weather conditions permitting.

The CTAF boundary extends laterally to 40 NM from Horn Island, which encapsulates the outer islands, and vertically from the surface to 8,500 ft.

ZZJ TCAS settings

The flight crew on board ZZJ had their TCAS set to the 12 NM range scale while backtracking runway 08. The captain reported that they could have improved their picture of the relative bearing and distance of IOA if they reduced the scale to 6 NM while backtracking runway 08. However, their normal procedure is to set the 6 NM range scale for controlled airspace and set the 12 NM range scale for departure from a non-controlled aerodrome to improve situational awareness of inbound traffic.

Right of way provision

Civil Aviation Regulation (CAR) 162 lists the rules for prevention of collision. CAR 162 paragraph 8 states:

An aircraft that is about to take-off shall not attempt to do so until there is no apparent risk of collision with other aircraft.

The captain of ZZJ reported that they relied too heavily on their TCAS and did not place sufficient importance on the ‘see’ element within the ‘see-and-avoid’ philosophy of operations at non‑controlled aerodromes. Consequently, they started their take-off before sighting IOA.

Safety analysis

Prior to the near collision event, the flight crew of both aircraft were aware of the presence of the other aircraft and their approximate position. When ZZJ was backtracking runway 08, the captain was conscious of the fact that they were occupying the main runway that was being used by other aircraft in the circuit.

The broadcast from IOA, that ZZJ could depart before IOA landed, supported the motivation of the captain of ZZJ to avoid delaying their departure from runway 08. The captain of ZZJ also relied on an approximate position of IOA from their TCAS when they elected to take-off. However, as reported by the captain, the range scale set on the TCAS was 12 NM, where a 6 NM range scale could have provided a more accurate picture of the relative position of IOA.

During the take-off, the flight crew probably also misheard the broadcast from the pilot in IOA, to hold short of runway 14, and therefore did not reject the take-off. However, it was not determined if the radio broadcast was before or after ZZJ reached their decision speed (V1) to safely reject the take-off.

Findings

These findings should not be read as apportioning blame or liability to any particular organisation or individual.

• The captain of ZZJ was motivated to expedite their departure from the main runway, which in conjunction with the broadcast from IOA that they had time to take-off before IOA landed, contributed to them starting their take-off before they sighted IOA.
• The flight crew probably misheard the broadcast from the pilot of IOA to hold short of runway 14 and did not reject the take-off.
• IOA and ZZJ made the required CTAF broadcasts and were aware of the approximate position of each other prior to the near collision event.
• Concurrent operations to different runways at Horn Island are a normal local practice, which is employed to facilitate traffic movements.

ATSB comment

The ATSB notes that the horizontal and vertical dimensions for standard CTAF boundaries are published in the Aeronautical Information Publication (AIP). However, the dimensions for non‑standard CTAF boundaries are currently only published in the Enroute Supplement Chart LOW.

Safety message

This serious incident highlights that pilots and operators need to consider how best to employ and integrate the sources of information available to them in order to develop an accurate mental model of a potential traffic threat. In the ‘see-and-avoid’ environment, radio broadcasts and TCAS information can be used to hone the visual scan to sight other traffic, which might pose a threat.

Further information on safety around non-controlled aerodromes is available from the ATSB website.

Further information on operations at non-controlled aerodromes is available from the Civil Aviation Safety Authority’s website.

__________

1. IOA and ZZJ were both operating under instrument flight rules, which is why IOA received traffic information on ZZJ while operating outside controlled airspace.
2. Horn Island Airport experienced a temporary loss of power on the day of the incident. None of the transmissions from the incident aircraft were captured on the available CTAF recorded data.

Aviation Short Investigations Bulletin - Issue 57