EIGHT SIMPLE WAYS TO DECREASE THE LIFE OF A TURBINE ENGINE
Along with the fire from the candles on my birthday cake making the “breaking news” from the Sky-Cam on all the local channels, the other thing that confirmed I have gotten older is when I was asked for an opinion on helicopter turbine engine maintenance issues because I “have been doing this for such a long time.” I remember when I had long hair instead of longing to have hair, and an old guy told me to pay attention because time flies faster than you think. He was right. I don’t know when I became the old guy who “has been doing this for a long time” — but the calendar and a follicle-challenged head don’t lie. Now that we have established that I am now the old guy who has been doing this for a while, let’s see what maintenance issues I can share with you.
Before I share my tribal knowledge, you should know that good maintenance requires a good informational foundation. First and foremost we should be more than casually familiar with the OEM’s operation and maintenance manuals (Ops & Maint manuals). Our companies spend a considerable amount of money keeping these manuals current and the OEMs have spent a considerable amount of time obtaining and printing this wealth of maintenance information in these manuals. Although tribal knowledge that is based upon our experiences and shared between us is good, it should be used in conjunction with and not instead of the instructions in the Ops & Maint manual.
Next, another bit of advice for both you and your pilot. Get to know your helicopter when it is operating as advertised. Learn and know the idle temp, and what happens to it when the bleed air accessories are utilized, anti-ice, cabin heat, and air operated particle separators. All helicopters have an acceptable vibration signature. Learn it before there is a report of a “hi-freq vibe.” We will discuss more about vibrations later. Knowing the engine oil pressure and temperature both at idle and during normal operation is another subject we will talk about in a bit. You both should know the N1/Ng rpm during normal flight operations. A change means something and requires investigation.
Low oil pressure or high oil temperature will cause bearings and gears to wear prematurely. If there is metal generation and it appears to be insignificant (based upon the Ops & Maint manual), keep the metal, record the engine time of occurrence and follow the Ops & Maint manual procedures. These are just some bits of tribal knowledge that we will expand on later.
There are eight major maintenance issues. I call them the evil eight.
Any one of these evil eight will by itself inflict significant harm upon the unsuspecting turbine engine, resulting in a reduction of its operational life. An attack by any combination of the evil eight will hasten the meeting of the turbine engine grim reaper (aka the turbine repair shop). As helicopter maintenance professionals, it is our job to recognize these eight operationally harmful events, and either prevent their occurrence or at least mitigate their activities.
So here are the evil eight:
Exceeding recommended operating temperatures
Surprised by this list? Good. Now that I have your complete attention, let’s examine these from our maintenance perspective. Where better to begin than with pilots?
Pilots are at the top of the list when it comes to affecting the life of a turbine engine. From the moment they press the start button until the twist grip or throttle lever is moved to fuel cut off, even in the best of circumstances, the pilot is using up the operational limits of the engine components and accessories. It is part of their job. Our job is to give them a safe helicopter so they can do their job.
What are these limits? Engine life is based on a combination of flight hours and/or engine event cycles. Every engine OEM, based upon its research, calculates a limited number of flight hours and cycles to its engines. Each flight hour consumed moves the engine closer to a scheduled maintenance and/or overhaul event. Event cycles are separate from flight hours but calculated using the same research. An engine start where it reaches idle is considered an event cycle and the beginning of the hour’s time clock. Some engine OEMs also assign cycles or percentages of cycles based upon excursions into certain N1/Ng rpms. As I said earlier, in the best of circumstances the pilot is using up the assigned life of the engine. If you toss in a hot start or an exceedance of the recommended operating temps, all things that can be within the pilot’s control, and the date with the maintenance department just got closer.
How we maintenance folks deal with this begins with trust. There must be a symbiotic trust between us and them (the pilots). When it comes to the helicopter, the pilots and us should be BFFs — best friends forever.
Trust is like a three-legged stool. Every day the pilots trust that we have given them a safe helicopter as they play “you bet your life” with the helicopter we give them. That is the beginning ingredient or the first leg of the trust element. We trust they will return the helicopter in one piece, undamaged, and that the last whining we will hear is the turbine engine before it is shut down. That is No. 2. That now ends our trust in the pilot, right? Nope, not hardly. The third leg is communication between us and them. The pilots are our best troubleshooting tool and they need to understand and be comfortable with that. Who knows how that engine is running better than the pilot who has been working it for the last couple of hours? If we give them a little insight into what we need from them regarding engine operation and listen to them during the post-flight conversation, our jobs as rotor doctors (helicopter maintenance professionals) will be that much easier. Thus communication is the final ingredient to total trust. Here are some suggestions for information the pilot can provide:
1.) The engine idle temperature just prior to shut down. A rise in this temperature will indicate a probable cooling air leak.
2.) The engine oil pressure and temperature and any change to the steady state readings during flight and at idle.
3.) Any noticeable change in the vibration signature that they feel in the controls any time the engine is operating.
4.) Any possibility of a temperature exceedance either during flight or at start. Some helicopters have turbine temperature recording devices. I have always felt that this information was recorded not because the pilot can’t be trusted to report it, but because the pilots are usually pretty busy flying the helicopter and can’t always know exactly the length of time and the actual temperature that was reached at the time of an exceedance. This information is vital as the Ops & Maint manuals are very specific, meaning one second in duration and/or one degree of exceedance can make significant difference in the required maintenance and the cost of the repair.
5.) Available power during flight. Was there a higher engine temperature required to achieve the same torque or power. How much? Perhaps the compressor needs a wash or there is some FOD or a malfunctioning bleed valve.
6.) What is the N1/Ng rpm during normal operation? They should know that this information is important to us. A change in N1/Ng can be the harbinger of an eroded compressor or turbine.
7.) The other engine issue to which a pilot can be the contributor has to do with engine surging. I will explain more when we get to that section.
8.) Any abnormality in the behavior of the helicopter that the pilot notes is our responsibility.
The pilot is in control of this event most of the time. I say most of the time because they are the ones controlling the fuel flow on the start, even in the case of engines equipped with full authority digital engine controls (FADEC). The pilots have the ultimate control, or do they? A FADEC system has its own set of specific checks as does a conventional system and we won’t get into the particulars of either. Suffice it to say the items listed below are generic to all systems. Here are some items that are out of the pilot’s control:
1.) Has the helicopter just completed a maintenance event?
a.) Is the rigging correct?
b.) Is the fuel control properly adjusted?
c.) Are all the fuel system pneumatic lines tight and the system free from leaks?
2.) Is the battery fully charged? A low battery will not sustain the starter rpm through an entire start sequence, causing a hot start.
3.) Have the inlet covers been removed?
4.) A hot start requires accurate reporting from the pilot. As I said earlier, one second and/or a single degree will have a significant impact.
5.) Remember that damage from a hot start might be more noticeable downstream in the turbine. As the fuel/air ratio is out of balance, the flame will be seeking air further into the turbine section to sustain itself. Thus the tip of the flame will be moved from its normal location in the combustion section while it is searching for air and expensive turbine parts to sauté.
EXCEEDING RECOMMENDED OPERATING TEMPERATURES
Turbine temperature equates to available power. Too hot means diminished engine performance, an increase in fuel consumption and a decrease in engine life.
All right, so our pilot returns and reports that he ro she exceeded the temperature limits in flight. Once again this is when good relations between the pilot and maintenance come into play. If this was a happy running engine before the flight, then the pilot will have to help us with all the circumstances involved: did he or she try to lift too much or got into a tight spot and needed an arm full of collective to get out? is he or she working in a dusty, dirty environment? We need to know what he or she noticed before and after the exceedance. If the helicopter has an exceedance reporting system then you will have accurate numbers to work with.
Performance degradation occurs most often over time. If the pilot were in a normal flight mode and reports escalating temps compared to earlier flights, there is reason to suspect a problem. They are our BFFs when it comes to troubleshooting.
1.) How much have the temps increased?
2.) Has the N1/Ng changed over a period of time?
a.) An increase in N1/Ng indicates a possible eroded or dirty compressor, malfunctioning bleed valve (sticking open), and/or an air leak in bleed air supported aircraft accessories.
b.) A decrease in N1/Ng indicates a possible eroded turbine section.
3.) Did the helicopter fly through some rain, smoke or dust?
a.) Rain could turn dirt in the particle separator to “mud” restricting the airflow.
b.) Smoke could coat the compressor blades with an oily residue and reduce its ability to compress the air properly.
c.) Dust can erode the compressor blades, once again compromising the compressor’s ability to perform effectively.
Sometimes we might need to go for a flight to get a better feel for the problem.
Ok, so we have put the pilot in informational download, gone for a maintenance check flight and referred to the Ops & Maint manual. The combination of this information will help guide our investigation. This is when restraint and a good troubleshooting plan come into play. We need to resist the temptation of going to the most difficult possible reason for the engine problem and begin removing engines and components. In trouble shooting use the K.I.S. method: “keep it simple.” Start with the easiest stuff first. Begin with a check of the reporting system. In other words, we need to be sure that the aircraft instruments are giving us correct readings. We proceed to inspecting the engine’s systems depending upon those results.
Corrosion prevention is strictly a maintenance responsibility. Keeping our helicopters clean inside and out should be de rigor in our maintenance routine. Frequent rinses for the airframe and engine, plus a regular wash of the internal components of the engine, make for a happy running engine and a well-functioning helicopter. The air that the engine ingests and the compressor blades beat into submission is then converted to kinetic energy to support the lift efforts of the main rotor blades. This air is full of stuff just waiting to attack the helicopter as well as the vulnerable engine components. The purity of the water we use, as well as the frequency of its use on the helicopter and the engine, is a subject that can and will be an article on its own in an upcoming issue of Helicopter Maintenance magazine. Right now suffice it to say a clean helicopter is a happy helicopter
Running with the same crowd as the corrosive elements in the air are tough little pieces of dirt and sand that work in tandem with the corrosive guys to exact as much damage as they can upon our helicopter and its components. The problem with the erosive ingredients is they remove any corrosive-resistant protective coatings. This gives their corrosive buddies an easier target for them to create mischief. Our defense against this destructive group is a good filtration system in the engine inlet. As a collective group, the OEMs have attempted to make the engine components out of more resilient materials with marginal success. Our job is to keep the filtration system clean and keep a vigilant watch on the wear limits spelled out in the Ops & Maint manual.
Unlike the song made famous by the Beach Boys, there are no “Good Vibrations” for a turbine engine. With turbine engine rpms running the gamut from 50,000 to 6,000 and several ranges in-between, rotational balance is critical.
Unless we work for a repair station that overhauls engines, we don’t have control over the rotational balance of the engine components. Our job is to recognize when it begins to exhibit an unbalanced behavior. As a former purveyor of turbine engine overhauls, I have always suggested to the operator that after installation of an overhauled engine or engine module, they should do a vibration survey prior to releasing the helicopter back to service. Be sure to keep the information obtained from the vibration analysis equipment for comparison when you suspect an engine vibration event.
Without equipment, how do we recognize a vibration prior to installing equipment? A quick way to learn about the vibes without equipment is by touching the hard lines on the engine and other components during a ground run, as well as by feeling the sheet metal on the helicopter while it is running. A “high freq” feels like an electric shock to the tips of our fingers. Make note if there is a sudden and frequent requirement for external navigation light bulbs, or a rash of sheet metal cracks, or we are suddenly replacing hard line support clamps. Verifying our touch information via vibration reporting equipment is best. Remember that vibration survey we did when things were working right? Now we have a baseline from before for a comparison which will help us identify the offending component sooner rather than later.
Ok, what about metal degeneration? Well, with all of those spinning gears and compressor and turbine wheels, the engine has a bunch of bearings that need the loving care of a good lubrication system. The compressor wheel and turbine wheel assemblies should be checked for balance also. Now is a good time for a quick review of the use of oil:
1.) Provide lubrication
2.) Remove heat
3.) Carry away dirt and debris.
Bearing life is jeopardized if our lubrication system is not providing these services. All engine OEMs provide filtration and cooling systems to help the oil in its job. The other neat thing the OEMs have done is manufacture the gears and bearings out of ferrous material. If a bearing or a gear begins to fail and shed metal, it will be attracted to a magnetic capture and reporting system in the engine. These devices are known as chip plugs. The plugs are basically an open electrical circuit that sits strategically within the oil flow system. When a piece of ferrous material flows by the chip plug, it is attracted to the center section of the chip plug. It completes the electrical circuit once the metal debris bridges the gap between the center of the plug and the external grounding portion. It doesn’t take much metal to complete the open circuit which illuminates a bulb on the flight panel in the cockpit. The light projects through a translucent face with the word “CHIPS.” Trust me, the light is bright enough to get any pilot’s attention. Pilots are instructed to “land the helicopter as soon as practical” and investigate the cause for the light. Our job as the maintenance folks is to make sure they don’t have to experience an illuminated chip light.
We make sure there is the correct level of oil in the externally-mounted oil tank. There are markings on the tank sight glass to help with this inspection. We need to follow the OEM instructions on the frequency of oil changes. We monitor the filters for debris and make sure that the filters and oil cooler are kept clean. Along with correct oil temperature, proper oil pressure is also a must. In most turbine oil pressure systems, we should not have to constantly adjust oil pressure. Once it is set we should not have to change it. A decrease in oil pressure is an indicator of compromised system seals. An 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 significant. 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 affects the rest of the engine.
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 compressing it. The turbine then takes this compressed air and with the aid of an extremely hot flame (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 flight.
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 flow. This disruption is called engine stall or surge. There are three audible warning signs that the engine is experiencing an airflow 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 firecrackers 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 flames 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 flow 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 configuration 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-flight 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 fly. Trust me, time flies 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, field 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 specific 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.
Mike is also a member of Helicopter Maintenance magazine’s Advisory Board.