In the period 2006 to 2012, the Australian Transport Safety Bureau (ATSB) investigated three fatal accidents involving PZL-Mielec M18 Dromader aircraft. All three investigations identified actual and potential safety issues regarding the operation of M18 Dromader aircraft at take-off weights above 4,200 kg. Although the aircraft were all permitted to operate at increased weights, the ATSB sought an understanding of any common factors across the investigations, and whether there were any implications for the continued safe operation of the type at those increased weights. As a result, the ATSB initiated a safety issues investigation in November 2011 to examine those factors and identify any potential safety issues.
What the ATSB found
The investigation identified several safety issues indirectly arising from the operation of M18 aircraft at increased weights. Though some of these issues were minor in isolation, collectively the increase in risk was more significant and the investigation illustrated the need for careful consideration of potential risks when expanding an aircraft’s role and/or capability.
The investigation established issues regarding the:
- practicality of a 15° bank angle limitation that is imposed at increased weights, which would probably affect or preclude some agricultural operations,
- potential for pilots and operators to apply incorrect operating limitations for flights at some weights under a relevant Supplemental Type Certificate (STC),
- method used to apply service life adjustments required at higher weights,
- risk of aircraft incorporating the STC not being configured to the same standard as the flight test aircraft, and
- risk to operators if they do not recalibrate a hopper sight level gauge when changing the hopper size and/or shape.
What has been done as a result
The owner and developer of the STC reported that they were in the process of amending it and were developing a proposal for Civil Aviation Safety Authority consideration and approval. This proposal would address the maximum bank angle limitation and required aircraft modifications, as well as clarify the applicability of the flight manual limitations and airframe service life adjustment method.
The investigation highlights some of the risks involved in increasing an aircraft’s operating weight. The importance of pilots and operators understanding and applying any operating limitations that might accompany an operational enhancement is also highlighted.