How can a drone fall out the sky? Find out here!

This is the first of two blogs looking at why drones sometimes wrong. First in this two part blog is hardware:

Firstly, what do we mean by hardware? Well anything that you can physically see and touch on your drone is hardware; the motors, the casing, the antenna, the landing gear, even the screws holding it together are considered hardware. Hardware has three distinct failure characteristics; Wear in, Random, Wear Out. Let’s take a drone rotor as an example.

When you first use a brand new rotor, the chances of a failure of that rotor can be more than a hundred times more likely when compared to any other time. This could be for a number of reasons, perhaps due to errors on the production line causing minute cracks under the surface, perhaps it got dropped during transportation causing hidden weaknesses or it might even be due to installation errors. The number of factors that could cause the rotor to be defective prior to it’s first use is so great that the chances of failure are dramatically increased and it’s not until you have inspected and thoroughly tested that rotor can you be more confident that it is working as time goes on. This is known as the wear in period and is the reason why any changes or modifications to flight critical hardware should be followed by a rigorous test process in a low risk environment prior to exposing yourselves or your clients to the risk of harm.

A graph showing the failure rate decrease with time as the drones equipment is 'worn in'

Once you are satisfied the prop is fit for purpose the failure rate dramatically decreases, the chance of a failure remains a risk however the failure rate is now much lower and remains constant with time (unlike previously where it decreased with time and confidence). The length of this period of low constant failure rate is determined by the design life of the rotor. But a number of other factors can influence it for example how often it is used, how hard it is used, what environment it is exposed to etc. but generally it will last for as long as it is designed for, provided it is used within its designed envelope (that’s important because if you’re using a rotor on a motor it wasn't designed for and at speeds it was not designed for, you are potentially stressing the rotor which will in turn increase the chances of failure).

A graph showing the steady state period whereby the failure rate remains constant with time

Finally when you approach the end of it’s design life the chances of failure once again rise rapidly and this is due to general wear and tear on the rotor. It’s been used so many times that the material is getting aged, weathered and tired so the more you use it, the more likely it is to fail. This is known as the wear out period and is the reason why we change aged or damaged rotors and batteries after a set number of cycles. It's also why it's important to keep logs and monitor performance of your aircraft, you and only you are responsible for it.

A graph showing the failure rate increase with time as the drones equipment is 'worn out'

Some things to consider asking yourself before you next fly, have you tested these new motors in a safe environment? When did you last change your rotors? When did you last inspect your airframe? How many flights have you made on that battery? Have you been using the aircraft outside of it’s design envelope? Is this aircraft safe?

Please feel free to comment and note this is a very basic introduction to the concept of failure theory in hardware and the post is aimed at drone operators and clients for information. If you wish to know more information on quantifying the probability of failure of your drone and what actions you can take to reduce that probability then please get in touch directly.

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