# Reliability and Maintenance Analysis of Unmanned Aerial Vehicles +
## 3 How Reliable Does a Drone Have to Be?  
During a typical mission, some failures are more critical than others: the loss of longitudinal stability, the loss of payload data, or the turning off of the position lights are not of the same criticality level. Therefore, it is necessary first to establish various levels of increasing gravity associated with the fault [25]. Moreover, according to the specific kind of mission and the specific kind of UAV, it is necessary to subdivide failures into subcategories [26]. Then, for each scenario, suitability and preventively forecasted, it is necessary to define a minimum acceptable level of reliability. Finally, even for the aircraft, it is necessary to define the criteria for the level of reliability, in terms of a level which is strictly linked to the type of failure [27; 28; 29; 30; 31].  
* _Catastrophic failures_: for these kind of failures, a crash of the drone is certain while injuries or even the death of persons on the ground is possible.
* _Severe failures_: heavy damages are expected and the probability of repairing the drone is low.
* _Moderate failures_: cause a moderate degradation of the drone's functions, which could lead to aborting the mission; however, they are not cause of severe damage.
* _Soft failures_: cause light degradation of the drone's functions, but do not lead to the cancellation of the mission.  
The _intrinsic reliability_ of an apparatus (in our case, a drone) is the reliability studied _a priori_[32]; however, unfortunately, a reliability study is often realized _after_ the design phase. This approach can lead to many problems during the management of the project, because a reliability study highlights a series of sensible points, and produces a series of recommendations that are usually forwarded to the designers. These should allow them to carry all the necessary modifications to the project in order to improve it [33; 34; 35; 36; 37; 38]. The perspective is completely different in the case of intrinsic reliability: in fact, the knowledge of the failure distribution of a system gives rise to the possibility, directly during the design stage, of taking specific action to reduce criticality and upgrade the critical parts or subsystems in advance, thereby increasing the overall level of reliability [39]. This, surely, increases the level of responsibility of the designers, but at the same time, decreases the risk of criticalities (also called Single Point Failures or briefly SPF) that might happen in future studies [40; 41; 42; 43].  
This way of seeing the design phase, i.e., in which a reliability study is effectively used to help the project, means that the figure of the Quality Assurance Responsible is frequently present from the beginning. Reliability analyses primarily aim to find the minimum limits for the requirements that allow UAVs to have at least a magnitude of better reliable [44].  
Another benefit of these analyses is that they help us to understand which components or parts of a specific subsystem are the most unreliable, and which are the most critical to the system [45; 46; 47; 48; 49; 50]. In Figure 1 a failure rate comparison between five different aircrafts is shown: Northrop Grumman RQ-4 Global Hawk in gray, PR-3 in orange, General Atomics RQ-1 Predator in blue, AAI RQ-2 Pioneer (for the drone category) in light blue and General Dynamics F-16 jet fighter in yellow.