DUAS xx24

• We believe the root cause of this occurrence was the downward looking obstacle sensors being switched on. We understand that some versions of the DJI M300 automatically switches them on below 5m and automatically switches them off above 5m. If the aircraft was hovering only 1m above water, the problem was created by a combination of the barometric pressure-based altitude reading saying one thing and the Infrared Sensing System altitude reading saying something different. Water has very low reflectivity and in addition, if there is any wave movement, what is reflected gives a false readout.
• The DJI M300 User’s manual makes it very clear that the downward looking vision system will not function properly when over water.

Page 21 of the DJI M 300 User Manual states:

Using Infrared Sensing System

The Infrared Sensing System can only be used to avoid large, diffuse, and reflective obstacles (reflectivity > 10%). Please be mindful of blind spots (Darker Grey in the image below) of the of the Infrared Sensing System. The downward Infrared Sensing System is used for positioning and assisting height setting during take-off and landing, while the Infrared Sensing System on the other five sides are for obstacle sensing.

Vision System and Infrared Sensing System Warning

The measurement accuracy of the Vision System is easily affected by the light intensity and the surface texture of the object. The Infrared Sensing System can only be used to avoid large, diffuse, and reflective obstacles (reflectivity > 10%).

The Vision System may NOT function properly when in any of the following situations:

1. Flying over monochrome surfaces (e.g. pure black, pure white, pure red, pure green) or without clear texture.
2. Flying over highly reflective surfaces.
3. Flying over water or transparent surfaces.

• The Board felt that turning the downward looking Vision System off when hovering at such a low altitude over water would have been advisable.
• The User Manual was considered a little confusing. It states that the Sensing System can only be used to avoid “…. reflective obstacles” but it also states that it may not function properly over “…highly reflective obstacles”. Well, if that is indeed the case, perhaps adding a % reflectivity number that represents the maximum would be clearer.