increase is an indication of a blockage of one of the oil delivery nozzles or elsewhere within the system. Both incidents require investigation and correction of the cause. How long do you think a bearing will survive in an engine that’s exhibiting vibrations? Remember, oil is the life blood of the engine — keep it cool and keep it clean. Any amount of metal accumulation that has complete the chip light circuit is signifi cant. The engine will generate some metal throughout its life … just a normal course of events. Even if the found metal is considered “normal,” we should save it and record the engine time when the metal was generated. What we are looking for here is a trend. If we have the bearing or gear in the early stages of failure, we want to catch it for it fails completely and aff ects the rest of the engine.
ENGINE STALLS/SURGE As we all know turbine engines ingest enormous quantities of air. Within a very short physical space, the compressor increases the ambient air pressure to eight or nine times above normal by squeezing or compress- ing it. The turbine then takes this compressed air and with the aid of an extremely hot fl ame (3,000 degrees F), accelerates the speed something close to Mach 1 while extracting this heat energy and converting it to kinetic or working energy, and thereby producing horsepower to sustain helicopter fl ight. The shape and composition of the compressor wheel,
stator blades, turbine wheel blades and turbine nozzle guide vanes is critical. Any change to the geometry of the rotating components which move the air or the stationary guides which direct the air will cause a disruption in air fl ow. This disruption is called engine stall or surge. There are three audible warning signs that the engine is experiencing an airfl ow disruption. 1.) A kind of honking or rumbling noise. We will typically hear this during an engine start. This is normal and does not result in damage to the compressor or turbine.
2.) A group of short rapid explosions, sounding very much like a string of fi recrackers igniting. This is not normal and its cause should be investigated. There is a system malfunction and continuation of this event will lead to a more severe stall ( see No. 3 below), and cause internal damage to the compressor and/or turbine.
3.) A loud bang sometimes accompanied by fl ames exiting the compressor inlet. This for sure is not normal, and most of the time its occurrence causes an interruption to engine operation, and in all cases also causes a lot of internal damage to the compressor and/or turbine.
What can cause the air fl ow abnormality? 1.) Hovering in a downwind condition
2.) Hovering and ingesting the engine exhaust
3) Maneuvering the helicopter into an unusually nose high attitude
4.) An eroded compressor 5.) Malfunctioning bleed valve (closing too soon) 6.) A fuel system requiring adjustment 7.) An eroded turbine and/or combustion section 8.) Blocked or restricted air inlet.
Items one through three are obviously pilot induced. Nothing we can do here other than to make sure the pilots can describe the severity of the stall as well as what they were doing with the helicopter. Items four through eight are squarely in the maintenance department. It is our responsibility to ensure that the systems in the helicopter are in proper confi guration and operating as designed. As we discussed at the beginning, our job is to provide a safe helicopter to the pilot. This requires constant diligence from us regarding the helicopter’s condition. To help us with our job is an open and honest dialog between the pilots and the maintenance crew. Sounds easy, huh? It is — believe me. Post-fl ight conversation with the pilot is much easier than a post-accident conversation with an FAA or NTSB representative. With that said, this old guy will let you all get back
to work. I appreciate your letting me share my years of experience in engine and helicopter maintenance and would appreciate learning some of your tribal knowledge. You are never too old to learn. One last thing — helicopters are not the only things that fl y. Trust me, time fl ies faster than any helicopter! .
Mike has been an A&P Technician since 1971. During his career he has worked as a shop and hangar technician; fi eld tech and customer support rep, and owner of a Part 145 engine overhaul facility. He was also the VP of Business
Development at H.E.R.O.S. Inc., which is a Part 145 repair station for the Rolls-Royce 250 engine, and a Honeywell fuel system overhaul agency.
His specifi c experience is in turbo-shaft engines in light to medium helicopters. The one he is most familiar with is the Rolls-Royce (formally Allison) 250 series engines. Mike is currently employed at TRACE Worldwide Corp., as its business development specialist.
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HelicopterMaintenanceMagazine.com August | September 2019
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