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The idea of engine monitoring is not new. For years, military aircraft have used cycle counters, and
pilots in military and commercial sectors have manually recorded cockpit parameters in order to enable
maintenance and engineering personnel to detect signs of trouble. Changes in maintenance
philosophies in the 1970's, from hard-time to on-condition, were accompanied by a requirement for a
more sophisticated monitoring capability. Technological advances, made possible by the rapid
development of digital electronics, have enabled comprehensive, automated monitoring to become the
norm.
In recent years, increasing priority has been given to the development of life usage monitoring by both
commercial and military operators. This action has been motivated not only by the continuing need to
prevent failures but also to reduce life cycle costs through more effective utilization of engine parts. In
military operation, the diversity of mission profiles amplifies the need and complicates the task of life
usage tracking. However, the same basic requirements for monitoring life usage apply to both
commercial and military operators. Monitoring includes on-board data collection, on-board processing,
ground-based processing and data mangement.
Because of user interest and need, SAE Committee E-32 has developed this Guide to Life Usage
Monitoring and Parts Management for Aircraft Gas Turbine Engines.
The effectiveness of Engine Life Usage Monitoring and Parts Management systems is largely
determined by the aircraft-specific requirements. This document addresses the following areas:
a. Safety
b. Life-limiting criteria
c. Life usage algorithm development
d. Data acquisition and management
e. Parts life tracking
f. Design feedback
g. Cost effectiveness
It primarily examines the requirements and techniques currently in use, and considers the potential
impact of new technology to the following areas:
a. Parts classification and control requirements
b. Failure causes of life-limited parts
c. Engine life prediction and usage measurement techniques
d. Method validation
e. Parts life usage data management
f. Lessons learned
g. Life usage tracking benefits
1.2 General Considerations:
1.2.1 Life Usage: The failure of an engine part may be due to inherent causes such as the accumulation
of damage due to cyclic and steady-state stresses resulting from temperature, speed, differential
pressure, and vibration. In many cases, the effects of these stresses can be estimated and,
therefore, the amount of life used or life remaining in a part can be approximated with reasonable
confidence.
For these inherent failure causes, the approach generally used to determine the initial design life
estimate of an engine part is to:
a. Submit the proposed design to heat transfer, stress, and life analyses
b. Subject sample parts to rig testing
c. Subject production standard engines to full-scale simulated service endurance tests
These steps are supplemented by further analyses in the laboratory and flight test investigations to
confirm or modify initial design estimates of failure resistance and operating environment,
respectively.
Life usage monitoring depends on two key aspects:
a. Life Prediction, whereby the finite life is determined against a life criteria (creep life, LCF life,
etc.)
b. Life Measurement, by which the rate of life consumption is determined
Analytical techniques are widely used to predict the service life of gas turbine engine parts.
However, the actual life of parts in service depends upon the severity of cyclic or steady-state
operation or both. In the absence of quantitative life usage data, the initial life usage assumptions
are necessarily conservative to assure engine integrity. These initial assumptions are later
re-evaluated using data acquired primarily from lead-the-fleet sampling programs. This includes,
but is not limited to, subjecting service run parts to rig tests.
The simplest method of measuring life usage is to record engine running time. A slightly more
sophisticated approach is to record both running time and mission profile, which recognizes that
some flights are more severe in terms of fatigue than others. More complex methods include use
of airborne data acquisition systems, which provide complete usage records for a fleet of aircraft,
and microprocessor-based Engine Monitoring Systems capable of calculating life usage in real
time. Clearly, the chosen degree of sophistication depends on the required life measurement
accuracy and this, in turn, will typically be determined by perceived cost savings and safety
requirements. The optimum is to be able to measure, on an engine-by-engine basis, all
parameters which impact life consumption and, using accurate life algorithms, determine usage for
individual fracture critical components. To gain a worthwhile benefit, the life prediction and life
measurement systems adopted need to have a similar degree of accuracy.
1.2.2 Parts Management: During the life of a gas turbine engine, occasional removals from service are
required to facilitate scheduled and unscheduled repair/overhaul.
In order to minimize engine down-time during repair, parts are often replaced with new or repaired
parts conforming to equivalent or improved design standards. Consequently, parts that are
removed for rework or inspection are sometimes re-allocated to other engines undergoing repair.
Thus, after several engine rebuilds, the constituent parts can be very different from the initial
complement supplied by the manufacturer. For modular engines, the ability to exchange complete
modules tends to compound this further.
This degree of interchangeability demands the use of well organized asset management systems
to track the utilization of life-limited parts. These systems may range from simple card index
systems to computer-based information management systems capable of interfacing with airborne
engine monitoring systems via specialized data transfer equipment.strRefField
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