What happened
On 15 July 1999, an incident involving a Saab SF340A, registration G-GNTE, was investigated at Aberdeen Airport. While the initial report details were subject to correction, the investigation focused on the condition of specific aircraft components during service.
The investigation
Technical examinations of the S2 ACP components revealed the presence of parallel axial grooves within the bores of all air holes. These grooves, measuring up to 0.05 millimetres in depth, spanned the entire length of the bores. Investigators also identified numerous very small cracks situated at the base of these axial grooves. A remnant recast layer was noted in the vicinity of two adjacent inboard fatigue origins. Despite these findings, metallurgical analysis confirmed that the material of the S2 ACP possessed a satisfactory level of hardness and microstructure.
Engineers attempted to develop a model to simulate the grooved conditions of the air holes to determine how such defects might impact the operational life of the S2 ACP, but these modeling efforts were unsuccessful.
Safety action
Following the findings of this investigation, the manufacturer implemented a mandatory safety action. All in-service S2 ACPs that have exceeded 4,000 cycles and were manufactured using the EDM process for their air holes must be replaced during their next scheduled workshop visit.