ENGINES
Regular Maintenance Engine maintenance falls into a number of categories of scheduled maintenance events, including routine inspections and overhauls, as well as unscheduled events including repairs, component replacements and troubleshooting. Basic maintenance of engines includes visual inspec- tions of the exterior and visible interior of the engine and associated systems. Specific inspections of the lubricating system’s oil, filters and chip detectors or magnetic plugs for evidence of metal or carbon is also a must do, as they can be indicative of more serious problems with the internal bearings and components, rotating seals, etc. Inspection of fuel and ignition systems is also of utmost importance when it comes to basic maintenance. A regularly-sched- uled check of the filtration system to ensure that clean fuel is being delivered to the burner nozzles is a must. With all of these maintenance tasks and regulations, how do the gremlins get their foot in the door (uh, engine)? Gremlins come in many shapes and sizes and in most instances, they work together to cause an engine malfunc- tion or failure. While a modern helicopter engine is reliable, an engine
failure event can occur. This can be due to a wide variety of reasons, and sometimes for more than one reason. An engine can fail because of a mechanical fault in its
manufacturing process. The alloys used to make the parts can be damaged by an irregular cooling process, forma- tion of air molecules and introduction of contaminants. Compressor or turbine blades might crack, and when under extremely high revolutions, these spinning pieces of metal posses amazing amounts of energy that can shred the whole engine to bits. Another form of engine failure is a flame out due to total fuel exhaustion. Fuel exhaustion can come about in a variety of ways, like a fuel leak not being detected until too late, an error in the fueling of the aircraft, or simply not paying enough attention to the fuel state. Fuel contamination (usually by water) can also lead to an engine flameout. In some situations, flames can be seen shooting from the
engine, accompanied by loud bangs. This is known as a com- pressor stall and it occurs when the airfoil of the compres- sor stalls due to irregular air flow and patterns. There is not enough pressure for the hot combustion gases to be forced to
Metal Fatigue – Combustion Liner Crack - Photo Courtesy of Vector Aerospace
flow out, and these might reverse direction or buildup until discharged by a loud bang. While things can always go wrong with an engine, most
engines can keep and have kept functioning for thousands of flight hours without the faintest problem. Modern engines are miracles of engineering and the engineers who designed and built them deserve all the credit.
Failure Scenario A failure scenario is the complete identified possible sequence and combination of events (conditions) leading to a failed system state. It starts from causes (if known), leading to one particular end effect (the system failure condition).
Rather than the simple description of symptoms that
many product users or process participants might use, the term failure scenario/mechanism refers to a rather complete description. This includes the preconditions under which failure occurs, how the thing was being used, proximate and ultimate/final causes (if known) and any subsidiary or result- ing failures that result. Now that we have identified what a failure scenario is, let’s discuss what gremlins we are likely to find in that scenario.
Buckling - Photo Courtesy of Vector Aerospace
Component/Part Failure Even parts with life limits can fail before their time. A part failure mode is the way in which a component fails function- ally on the component level. Often a part has only a few fail- ure modes — thus a relay might fail to open or close contacts on demand. Many different kinds of failure mechanisms can cause this, and often multiple factors play a role simultane- ously. These include corrosion, welding of contacts due to an abnormal electrical current, return spring fatigue failure, unintended command failure, dust accumulation and blockage of the mechanism, etc. Seldom is there only one cause (gremlin) that can be identi- fied that creates system failures. The (end) or immediate effects of failures can also be diverse depending on the function in the system. The relay failure might result in a failure (actually a fault) to activate a locking actuator. This is called the sub-sys- tem effect or sub-system functional failure mode. Finally, this might result in severe damage to the system (the end effect) due to the loss of a protection device. An effect from a lower level system can cause a higher-level system failure. We call this a cascading failure, which is a failure in a system of interconnected parts (in our scenario, the helicopter’s engine), in which the failure of a part can trigger the failure of successive parts.
Mechanical Failure Some types of mechanical failure mechanisms (gremlins) are buckling, thermal shock, wear, corrosion and various types of fatigue. The way the product is loaded and the loading history are also important factors which de- termine the outcome. Of critical importance
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HelicopterMaintenanceMagazine.com October | November 2013
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