Introduction
On 28 April, 1999, a near-miss occurred just outside Port Macquarie Airport [PQQ] when separation was lost between two aircraft, both flying the same route between PQQ and Taree, but in opposite directions:
Beech 1900, VH-IMA, inbound to PQQ
Beech 1900, VH-IMH, departing PQQ
The separation between the two aircraft was calculated to have diminished to only 1000 meters horizontally, and 200 feet vertically. The aircraft passed each other at 8,000 feet and 8,200 feet respectively, and in clouds. The crew of IMH initiated a climb from 8,000 feet only about five seconds before the two aircraft passed. Furthermore, the horizontal separation was only due to track-keeping deficiencies, and there should not have been any horizontal separation at all between the two 1900s.
The following is a brief analysis of the event, identifying both active and latent errors, and the defence layers that they combined to overcome.
Factual information about the incident events has been drawn from the incident report issued by the ATSB (20011). This report also refers to two other aircraft, but they have been left out of this analysis for simplicity reasons.
Accident causation chain
Few aviation incidents or accidents are the result of a single cause. Indeed, this near-miss was the result of several active and latent failures that combined to overcome the defence layers present (Weigmann & Shappell, 20032). These failures combined to form a sequence of events [see this link also for more information about active and latent failures] that were required for the incident to occur (Miyagi, 20053). Had just one of the events not been present, the incident could possibly have been averted.
Active failures
There were four active failures involved in this incident.
IMA's lack of altitude broadcast
As part of their arrival to PQQ, the crew of IMA descended from FL110 to 9,000 feet, and made a radio call advising the other aircraft of this. A few minutes later, IMA commenced another further descent, this time to 8,000 feet. While the crew of IMA stated they made a descent broadcast before making this second descent, it was not received by the other pilots, and was not recorded on any frequency. Clearly IMA did not make a descent broadcast. This was one of the most significant failures in this event, as it meant that the other crew thought IMA was still at 9,000 feet, when in fact it was 1,000 feet lower.
IMH's lack of altitude broadcast
As a result of hearing IMA’s crew broadcast that they were descending from FL110 to 9,000 feet, IMH’s crew decided to maintain 8,000 feet, but chose not to broadcast this information. When IMA then descended to 8,000 feet without transmitting an audible broadcast, IMA and IMH were flying at the same altitude in opposite directions.
Lack of contact between IMA and IMH
The crews of IMA and IMH were listening to the appropriate frequencies and heard transmissions from each other, but they never directly communicated to arrange separation. Instead, they simply relied on assumptions about previous broadcasts and indirect communication in an attempt to ensure separation, which resulted in the near-miss incident.
Delayed controller contact
Due to area air traffic officers experiencing heavy workloads in the minutes leading up to the incident, there was considerable delay in coordination between services. This resulted in a delayed traffic warning being generated which could not be relayed to the crews before separation had been breached. If it had not been delayed, there would have been more time for IMA and IMH to take evasive action, and separation would most likely have been assured.
Latent failures
In order to completely understand the incident however, it is important to look beyond the obvious active failures to ask "why?" and determine what latent failures allowed the active failures to take place.
Overly complex airspace
The airspace in the vicinity of the incident was very complex. Within only a few minutes, the aircraft involved passed through three different airspace types, each with its own frequency. While not directly affecting safety when the airspace was designed, the overly complex design was a latent failure which enabled this near-miss to occur. The three frequencies were also almost completely saturated; fifty-four transmissions were recorded in the three minutes and thirty seconds prior to the incident.
Lack of TCAS
Neither aircraft involved in this incident was equipped with TCAS. Had the aircraft been so equipped, IMA and IMH would have likely received traffic alerts and separation would have been guaranteed. Even without generating a traffic alert, TCAS would have enabled the pilots to maintain improved situational awareness (Craig, 20014). While TCAS is a requirement for larger commercial aircraft, it was not legally required to be fitted to the aircraft involved in this incident (Civil aviation safety authority, 20015).
Possible organisational factors within aircraft operators
While not immediately apparent from the published accident details, there is a possibility that organisational factors in the aircraft operators may have contributed to the incident. For example, the pilots may not have received sufficient training to enable them to effectively ensure separation, or a weak safety culture may have implicitly encouraged the pilots to be somewhat careless with their radio transmissions. Further investigation would be required to confirm whether these organisational factors influenced the incident.
Possible organisational factors within Australian ATS provider
In a similar way, it is important to ask why the controllers' workloads were so high that they could not effectively communicate traffic conflict warnings to the aircraft involved. Perhaps the working conditions or staffing levels may have contributed to the high workloads. Further investigation into these areas would similarly shed light on this possibility.
Layers of defence
Separation via radio
Firstly, the crews in the four aircraft should have been able to maintain mutual separation via radio contact. This defence layer was breached as a result of the complex airspace surrounding PQQ, and the lack of effective radio communication from the crews of IMA and IMH. Possible organisational factors may have contributed.
Separation via visual sighting "see and avoid"
Secondly, even if mutual separation failed to be arranged, it may have been possible for the crews to visually spot the other aircraft and manoeuvre to avoid the near-miss. Unfortunately, due to the cloud present and the poor lighting at the time of the incident, this defence layer was also breached.
Separation via traffic conflict warning
The final layer of defence in this incident was a timely traffic conflict warning by the sector controller. The warning was delayed by the belated communication between the two air traffic officers. If the conflict warning was issued earlier, it would have allowed IMA and IMH to maintain adequate separation. Possible organisational factors may have contributed.
Conclusion
Establishing the many factors that cause an aviation incident can be a difficult task. It is necessary to look beyond the immediate, active causes, and investigate the hidden factors that created the opportunity for this incident to occur. The high workload and poor radio communication of the participants were major factors, and there were also latent failures involved, such as the complex airspace and possible organisational factors. The latent and active failures combined to create an incident chain, which managed to overcome the layers of defence. Lessons learnt from this incident can hopefully be used to prevent such events from reoccurring.
