What happened
On the evening of 10 June 2002, two freight aircraft were climbing toward their cruise altitudes on a shared flight path from Napier to Paraparaumu. The first aircraft, a Piper PA23-250D Aztec (registration ZK-DIR), had been cleared to a non-standard altitude of 9,000 feet with a minimum climb rate of 50 and 500 feet per minute. Shortly after, a Piper PA34-200T Seneca II (registration ZK-MSL) departed the same aerodrome. While initially subject to a climb restriction, the restriction on the ZK-MSL was later removed by air traffic control.
As both aircraft progressed, they drifted into close proximity. At one point, the two planes were flying at approximately the same altitude with a lateral separation of less than one kilometer. The pilots were aware of each other's presence but were unaware of how close they actually were. The loss of separation was only identified once the aircraft appeared on radar at the Ohakea Control Centre. Once the breach was detected, positive separation was quickly restored, and both flights continued to their destinations without further incident.
The investigation
An investigation by the Transport Accident Investigation Commission (TAIC) examined radar data, air traffic control recordings, and pilot reports. The inquiry looked into the climb profiles of both aircraft, the impact of local weather, and the effectiveness of the air traffic control monitoring. Investigators analyzed the performance of the Piper PA23-250D Aztec and the Piper PA24-200T Seneca II, noting that while the ZK-MSL maintained a steady climb, the ZK-DIR experienced a significant decrease in climb performance as it approached its target altitude.
Findings
- The primary cause of the loss of separation was that neither pilot adhered to their assigned air traffic control climb clearances.
- The ZK-DIR pilot's climb rate slowed to an average of approximately 330 fpm, while the ZK-MSL pilot's climb rate increased to roughly 680 fpm after passing 4,000 feet.
- The air traffic controller, while following standard procedures, did not maintain continuous monitoring of the aircraft as they approached their cruise altitudes, failing to notice the changing climb rates.
- Light turbulence and wave activity encountered during the flight contributed to the varying performance of the two aircraft.
- The investigation concluded that using specific rate-of-climb requirements does not provide a sufficient safety buffer for low-performance aircraft to ensure separation minima are maintained.
Safety action
Following the investigation, a safety recommendation was issued to the Airways Corporation of New Zealand. The recommendation prompted an amendment to the air traffic services operating manual, mandating more frequent monitoring of separation for low-performance aircraft to prevent similar occurrences.