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service-life extension and fewer mishap losses, in turn driving down requirements to acquire attrition
reserves.
Availability. With the removal of the pilot, the rationale for including the level of redundancy, or for
using man-rated components considered crucial for his safety, can go undefended in UA design reviews,
and may be sacrificed for affordability. Less redundancy and lower quality components, while making
UA even cheaper to produce, mean they become more prone to in-flight loss and more dependent on
maintenance, impacting both their mission availability and ultimately their LCC.
Acceptance. Finally, improving reliability is key to winning the confidence of the general public, the
acceptance of other aviation constituencies (airlines, general aviation, business aviation, etc.), and the
willingness of the FAA to regulate UA flight. Regulation of UA is important because it will provide a
legal basis for them to operate in the National Airspace System for the first time. This, in turn, should
lead to their acceptance by international and foreign civil aviation authorities. Such acceptance will
greatly facilitate obtaining overflight and landing privileges when larger, endurance UA deploy in support
of contingencies. Regulation will also save time and resources within both the DoD and the FAA by
providing one standardized, rapid process for granting flight clearances to replace today’s cumbersome,
lengthy (up to 60 days) authorization process. A third benefit of regulation is that it could potentially
lower production costs for the military market by encouraging the use of UA in civil and commercial
applications. This overview presents reliability from several perspectives commonly used in reliability
analysis.
Reliability is the probability that an item will perform its intended function for a specified time under
stated conditions. It is given as a percentage which represents the probability that a system or component
will operate failure-free for a specified time, typically the mission duration. It relates closely to Mean
Time Between Failure (MTBF).
Mean Time Between Failure. describes how long a repairable system or component will continue to
perform before failure. For non-repairable systems or components, this value is termed Mean Time To
Failure (MTTF).
1 UA Reliability Study, Office of the Secretary of Defense(Acquisition Technology, and Logistics)
UAS ROADMAP 2005
APPENDIX H – RELIABILITY
Page H-2
Availability is a measure of how often a system or component is in the operable and committable state
when the mission is called for at an unknown (random) time. It is measured in terms of the percentage of
time a system can be expected to be in place and working when needed, or mission available rate (MAR)
in percent.
Class A Mishap Rate is the number of accidents (significant aircraft damage or total loss) occurring per
100,000 hours of fleet flight time. In cases where a UA fleet has not accumulated this amount of flying
time, its MR represents its extrapolated losses to the 100,000 hour mark. It is expressed as mishaps per
100,000 hours. It is important to note that this extrapolation does not reflect improvements that should
result from operational learning or improvement in component technology.
Maintenance cancellations/aborts were broken out into failures of the aircraft’s major subsystems. Use of
these failure modes lead to a higher fidelity representation of the aircraft’s reliability. In order to make
uniform comparisons between systems, the following definitions were used to categorize areas of system
failure leading to mission aborts or cancellations.
Power/Propulsion (P&P). Encompasses the engine, fuel supply, transmission, propeller, electrical
system, generators, and other related subsystems on board the aircraft.
Flight Control. Includes all systems contributing to the aircraft stability and control such as avionics, air
data system, servo-actuators, control surfaces/servos, on-board software, navigation, and other related
subsystems. Aerodynamic factors are also included in this grouping.
Communication. The data link between the aircraft to the ground.
Human Factors/Ground Control. Accounts for all failures resulting from human error and maintenance
problems with any non-aircraft hardware or software on the ground
Miscellaneous. Any mission failures not attributable to those previously noted, including airspace issues,
operating problems, and other non-technical factors. Because operating environments are not uniform as
a variable affecting the data, weather was excluded as a causal factor in this study.
Data and Trends
Figure H-1 shows the Class A Mishap Rate per 100,000 hours versus cumulative flight hours for the
　
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