What happened
On the morning of December 17, 1996, an Air New Zealand Boeing 747 (flight NZ 31) was en route from Auckland to Brisbane. While cruising at flight level 350, the flight crew requested permission from air traffic control to climb to a non-standard flight level of 370 to avoid turbulence encountered at their current altitude.
After receiving authorization, the aircraft began its ascent. As the plane transitioned through flight level 350, the crew observed contrails from another aircraft positioned above them. Simultaneously, the aircraft's Traffic Alert and Collision Avoidance System (TCAS) issued a traffic advisory. The investigation established that the vertical distance between the two aircraft during the passing maneuver was only 1,800 feet, representing a loss of required separation.
The investigation
The investigation focused on the circumstances surrounding the altitude change and the subsequent proximity of the two aircraft. Investigators examined the communication between the flight crew and air traffic control, specifically regarding the request for a non-standard flight level. The inquiry also looked into the operational environment and the management of aircraft separation during altitude transitions.
Findings
- The aircraft was operating in a non-standard flight level following a request to avoid turbulence.
- A loss of vertical separation occurred between the Boeast 747 and another unidentified aircraft.
- The vertical separation at the moment of passing was 1,800 feet.
- The incident was identified via both visual observation of contrails and a TCAS traffic advisory.
Safety action
Following the investigation, safety recommendations were issued to the Director of Civil Aviation and the Chief Executive of the Airways Corporation of New Zealand. These recommendations addressed the procedures for issuing clearances for non-standard flight levels and the protocols for utilizing flight progress boards.