The similarities between the Trim Interrupt and Flap Interrupt switches and the proximal location of the two switches unnecessarily increased the risk of mis-selection and contributed to the excessive out-of-trim condition.
The safety action nominated by Pilatus may result in less need for pilots to use the Trim Interrupt switch (due to more reliable relays) and training guidance may increase the probability of the correct switch being selected in the case of a trim runaway event. However, the two switches do remain identical and co-located, and given the Flap Interrupt switch is no longer required, there is potential for engineering controls to eliminate the mis-selection of the interrupt switches and associated possible loss of control.
Safety action taken: Pilatus advised they took actions to minimize the risk of mis-selection and possibly resulting excessive out-of-trim conditions to ALARP by design and documentation changes.
To minimise the probability of a fused mechanical pitch trim relay, Pilatus developed a design change for the mechanical pitch trim relay. The mechanical relay is replaced with a solid-state relay. Service Bulletin SB 27-023 for the modification had been technically approved on March 4th, 2019. Publication of the SB was delayed until March 16th, 2020 due to limited parts availability.
In addition, the probability of erroneous activation of the Flap interrupt switch instead of the Trim interrupt switch has already been reduced by the publication and active distribution of SIL 003 to all PC-12 operators. This document highlights the crucial importance of correct execution of the Pitch Trim Runaway emergency procedure and provides information about the intent of the emergency procedure, but also stresses the shared responsibility of the operators to ensure correct execution of the emergency procedure by proper training.
Further improvements to the guidance and information provided in the PC-12 operational documentation will be considered by Pilatus. The combination of these actions is considered by Pilatus to effectively minimize the risk of mis-selection to ALARP.
