The aerospace and aviation industry require regular inspections of jet engine compressor and turbine blades to identify defects in an effort to delay expensive part replacement and prevent massive engine failure. Not surprisingly, the blades must often meet stringent and high tolerance demands imposed upon them.
Blades are designed to generate the maximum power and efficiency at the minimum weight and cost. Any shape variation from the original design can significantly impact efficiency and fuel consumption and, in certain situations, cause blade failure.
MRO inspection challenges
The ability to quickly access blade geometry can have significant impact on the performance, and repair cost in a maintenance environment. Blades are one of the most highly stressed components in a turbine or compressor. Their weight and high speed applications make them subject to tremendous centrifugal forces. Additionally, the harsh environmental conditions of temperature and pressure after long service hours can result in corrosion, erosion, distortion, and loss of the original component shape and dimensions.
To combat these conditions, turbine designers and engineers have devised effective defenses to protect their turbine blades. These include using strong exotic materials capable of withstanding high temperatures — such as nickel and cobalt based metal alloys — while innovative blade airfoil designs have improved the defenses against corrosive properties. In addition, several effective defensive coatings have been added as “armor” for the blades.
Rust never sleeps?
Every turbine engine uses fuel contaminated with sulfur. Sulfur corrosion is similar to rust resulting from oxidation of ferrous-based metals. Just as there is no way to eradicate rust-producing moisture, no one has ever designed a filter to purify turbine engine air intake completely. And once sulfur corrosion penetrates the blade’s protective layer and reaches the base metal, there is no way to reverse the process.
If the aircraft is operated near a saltwater source, the likelihood of severe damage dramatically increases. The same is true for aircraft operating near high pollution risk areas where the aircraft’s engine may intake contaminants from airborne particles.
Blade deterioration usually affects the blades in the following ways — blade edge, affecting chord length; blade thickness variation; and blade twist — the variation in twist from root to tip. Consequently, accurate measurements and analysis of airfoil section and root parameters are important in blade inspection, and yet complicated. Another critical form to inspect is the leading edge and trailing edge profiles.
The service technician and inspector are called upon to collect and analyze the intelligence data and physical evidence to determine the condition of the blades and develop a strategic plan of defense.
While there are only limited things a pilot or mechanic can do to prevent the sulfidation attacks on the turbines, just being aware of the turbine’s vulnerability can justify shortening maintenance intervals.
Meanwhile, the MRO blade industry is continuously seeking high accuracy inspection, higher blade measurement speed, and minimized requirements from the operator — in an effort to extend operating intervals and keep maintenance and inspection costs to a minimum.
Today, mechanical touch probes on coordinate measuring machines (CMM) and mechanical profile gauges are the most common sensors employed for blade inspection, and most blade measurements today are based on this design. However, the amount of useful data that can be collected using the mechanical touch probe or a gauge is somewhat limited both in terms of hardware and measurement software.
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