Beatriz Juliana de O. M. Franco Luiz Carlos S. Góes

Instituto Tecnológico de AeronáuticaAero & Mechanical Engineering Division

Mechatronics & Aerospace Systems Dynamics

Managing the Reliability of Unmanned Aerial

System

Discuss Subjects

The importance of UAS ReliabilityWhy to manage the Reliability of UASWhat should be managedWhat management tools are relevant

andHow they can be implemented in UASConclusion: What results are expected

to Reliability Management in UAS

The importance of UAS Reliability

Unmanned Aerial System tecnhology is set to play a major role in the future of the aerospace industry.

Reliability is at the core of achieving routine access once it allows to assess the risk posed by UAS operation to people and property on the ground, or other aircraft in the air.

No UAS is not 100% reliable. UAS Reliability depends on the design features of its platform configuration and cost that pays for it.

Admit to UAS can fail, impacting its safety or mission capability. Failures increase the maintenance costs and their effects restrict UAS availability, especially in military application

This work outlines methods of managing UAS Reliability over its life cycle, serving as support to research groups that develop UAS as test-beds.

Why to manage the Reliability of UAS Developing a management system for UAS must take into account the failures probability of different components of UAS, and how these failures affect the overall UAS Reliability, as shown below.

Exemple: Sensor fault may cause false or missing actions (alarm, turning-on and off of devices). The changed data resultant from sensor fault is an error. When the error produces missed data in value of the UAS variable, affecting UAS mission, is considered that a failure takes place. If the failure is not detected and accounted for, the UAS may crash. These flaws must be identified and managed through and analysis of risk mitigation. This analysis tell us what happens when a fault occurrs, describing how and how much it matters.

Fault Error Failure

Internal State

External State

UAV Component

Information

UAV behavior deviates from specification

Fig 1. Fault vs. Error vs. Failure

What should be managed Reliability is the ability that UAS has to carry out and to maintain its mission in routine circumstance, as well as to survive in hostile and unexpected environment. In other word, reliability is the probability that an item can perform its intended function for a specified interval under stated conditions.

Mean Time Between Failures (MTBF) (ususally in terms of flight hours) Failure Rate (failures per unit time) Probability (expressed as a decimal or percentage)

Maintainability is a measure of the ease and rapidity of UAS to be retained or restored to a specified condition after failing when maintenance action is performanced by personnel having specified skill levels, using prescribed procedures and resources, at each prescribed level of maintenance and repair. Mean Time to Repair – average of repair times Maintenance Manhours Per Flight Hour Crew Size – Average number of individuals required to accomplish the maintenance action

What should be managed Availability describe how a given UAS is able to perform its mission compared to the number of times that is tasked to do, taking into account combined aspects of its reliability, maintainability and logistic support. Inherent Availability (AI) is the ideal state to check out the UAS availability, representing a function of reliability (MTBF) and maintainability (MTTR). In this type of analysis includes only corrective maintenance actions Achieved Availability (AA) is similar to the Inherent Availability, but includes in its analysis corrective and preventive maintenance actions (MTTR A);

Operational Availability (AO) is the availability of real experiences that takes place with the UAS in the field. In this type of analysis includes corrective and preventive maintenance actions, logistic time, waiting time and administrative time.

Safety is defined as the absence of mishaps. Mishaps, in turn, mean significant damage to UAS platform that need actions of extensive repair so that UAS can operate again. Safety is expressed in terms of Mishap Rate (MR);

Human Errors or mistakes cause system failure or increase the risk of failure for the safe operation of UAS. A careful analysis of human-UAS recognizes both humans and UAS can fail, and what are their effects on UAS mission. It is possible to construct a set of analogues to reliability parameters for obtaining good UAS design with respect to human mistake, such as:

What should be managed Susceptibility is measured as the probability of hit, which includes the probability of acquisition, detection,

identification, tracking, launch, guidance, and detonation. Vulnerability is a weakness in a system’s design or performance affecting its ability to be survivability. Survivability is the ability of an UAS to avoid or withstand a hostile environment without suffering an abortive or

catastrophic impairment of its ability to accomplish its designed mission;

+ =

Susceptibility

to being hit

Vulnerability

Given a hit

Survivability(Effectivenes

s)

Fig 2.: Survivability is an essential element of UAS Mission Effectiveness

What management tools are relevant to UAS Managing the Reliability of UAS consists in performing various tasks

at each stage of the life cycle, as shown in table 1.

STAGES TOOLS APPLICATION IN UAS

FRACAS Analysis of UAS field data

Aid in choosing the best design of the UAV platform

FMEA Aid to validate the design parameters of the UAS

DESIGN Failure Modes and Effects Analysis from UAS similar or previous versions

FTA Qualification and Validation of the UAS design

RELIABILITY Calculate RAMS parameters based on the UAS life data

FAULT PREDICTION Estimate the reliability or failures probability in UAS systems via RDB

MANUFACTURE ESS Remove latent failures of UAS

FMEA Identify faults in the UAS manufacture

FRACAS Data collection, analysis and corrective actions

OPERATION Highlight critical points that need improvements

FTA Assess the impact of changes introduced by UAS hardware and software

Investigate the causes of field failures, or accidents with UAS

MAINTENANCE FTA When we want to focus on a specific system failure

Assist in management activities for major COTS components of UAS maintenance

FMEA Aid the fault diagnosis process

FRACAS Record of failure analysis and corrective and/or preventive actions in UAS

How they can be implemented in UAS’ life cycle phase How to estimate the Reliability of UAS or its failure

probability? Fault Prediction is a process that can be used to

quantitatively estimate the reliability of an UAS design prior to its actual operation. Once the UAS platform was designed, we can estimate its reliability and compared it with acceptable reliability levels defined in design. In this work, we used to the number of components via Reliability Block Diagram (RDB), where reliability is calculated through series and / or parallel systems, using the reliability or failure rate as parameter, as shown in fig. 3 .

Fig. 3: For a series system of UAS operating successfully, all components must be operate successfully.

How they can be implemented in UAS’ life cycle phase How to not introduce UAS flaws or to minimize their occurrence?

Fault Prevention aims at preventing or minimizing the failure occurrence in the system during the UAS development and operation, and avoiding its fault reoccurrence

in field, driving to an improvement of UAS reliability and safety.   FMEA FRACAS

General “proactive” “reactive”

Purpose affect the UAS design before launch correct problems after launch

Errors may occur – the potential errors must be enumerated

have occurred – observederrors are simply counted

Error rate is assumed is measured

Issues withtechnique

Is it complete? Modelscan be wrong.

All errors counted?Culture inhibits reporting errors.

Can be combined with FTA FTA

Evaluate qualityof the technique

difficult – completeness,reasonableness ofmitigations is qualitative

simple - measure error rate

FMEA and FTA are compatible tools of UAS risk analysis, being that the choice of proper tool depends on risk nature to be evaluated. However, FMEA considers only single failure in its analysis while FTA considers multiples failures, requiring a greater skill level than FMEA. When an UAS is designed, the modes it might fail can be captured in a FTA and FMEA. After the UAS is launched to field operation, the modes in which the UAS has failed can be captured through FRACAS and this knowledge can be used to update the FTA of the UAS in study.

Table 2:Use of FTA, FMEA and FRACAS to detect faults in UAS

How they can be implemented in UAS’ life cycle phase How to find out hidden flaws in UAS prototypes or new design?

Environmental Stress Screening (ESS) aims at exposing, identifying and eliminating weak components, workmanship defects, flaws or defects, and other conformance anomalies which cannot be detected and removed by visual inspection or electrical testing but which will cause UAS failures in the field.

O purpose of ESS is to compress a system’s early mortality period and reduce its failure rate to acceptable level as quickly as possible, as shown in figure 4

t

Infant Mortalit

y

Useful Life

Wear-out

Reliability measure

Durability measure

λ

ESS

Fig 4: Bathtube Curve for UAS electronic items

How they can be implemented in UAS’ life cycle phase

How to develop UAS that maintains a safe operation even when some faults take place in real flight conditions?

Fault Tolerance: Monitoring faults in feedback control system components is known as Fault Detection and Isolation (FDI).

FDI unit is a basic element of the fault tolerant control schemes. Fault tolerance aims at controlling the UAS flight and mission in an uninterrupted operation thus increasing the UAS survivability and safety even after a fault takes places.

Fault

Fig 5: Fault Tolerance

noise Disturbation

UAS

Estimation

Residuals

Detection

Isolation

Accommodation

Keep on Mission

Corrected Estimate

N

Y

State or Output

Conclusion: What results are expected to Reliability Management in UAS No UAS is 100% reliable or safe.

Components failures must be expected on an UAS; The impossible really does happen!

The Reliability must be considered from the beginning of the UAS design.

It’s possible to introduce concepts about reliability management applied to the process of decision in the design, operation, and maintenance phases for UAS team starting from an overview of the system up to an analysis more specific of the system.

Obviously, the presented directives will have resulted different depending on the type of analyzed UAS, especially for parameters of Reliability, Availability and Safety

It is expected great most of UAS developers will be able benefit with the directives presented considering the impact of reliability in the design, development, operation and maintenance of UAS.

Any Questions?