What happened
On 12 August 2022, a Beech Aircraft Corp 95-B55, registered VH-ALR, was performing an RNAV GNSS instrument approach into Parafield Airport, South Australia. While operating in turbulent weather and instrument meteorological conditions, the aircraft descended below the required segment minimum safe altitude. This descent triggered an automated minimum safe altitude warning on the air traffic control console. Upon receiving a safety alert from the tower controller, the pilot immediately climbed the aircraft back above the minimum altitude. The pilot then continued the approach, which required a steeper than normal descent rate to reach the runway, and landed without further incident.
The investigation
The ATSB examined the circumstances surrounding the altitude excursion and the pilot's performance. The investigation confirmed that the aircraft's altimeters were fully functional and providing accurate altitude information. The pilot reported that while they were adequately rested, the combination of turbulence and the complexities of the instrument approach led to increased workload. The investigation also looked into the pilot's use of a paper copy of the instrument approach procedure chart, which was being held by hand without the aid of a kneeboard or chart holder.
Findings
- The pilot's workload increased while approaching the intermediate fix due to the necessity of monitoring lateral and vertical navigation in turbulent, cloudy conditions.
- The use of a handheld paper chart made it difficult to reference check altitudes and segment minimum safe altitudes effectively.
- There was a potential for misinterpreting the altitude/distance scale on the procedure diagram, specifically regarding the relationship between the CDFA path commencement and the altitude/distance scale.
- The pilot's decision to continue the approach after the climb necessitated a descent rate that was higher than normal.