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Planning your next engine overhaul? Read the fine print. The modern Gas Turbine Aero Engine (GTAE) which is designed specifically for aviation propulsion is incredibly reliable when compared both to its reciprocating cousins- and to its grandparents. The first successful GTAEs of the bloodline - which were all being created and tweaked safely behind home defenses during the last gasps of World War Two - were so temperamental that a ten-hour TBO interval was often a rare ...

AvBuyer   |   1st May 2009
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Planning your next engine overhaul? Read the fine print.

The modern Gas Turbine Aero Engine (GTAE) which is designed specifically for aviation propulsion is incredibly reliable when compared both to its reciprocating cousins- and to its grandparents. The first successful GTAEs of the bloodline - which were all being created and tweaked safely behind home defenses during the last gasps of World War Two - were so temperamental that a ten-hour TBO interval was often a rare luxury. In fact- many of these first-generation engines- regardless of which side built them managed to effectively ‘shoot down’ the aircraft that they were bolted on to- rarely with an aggressive shot being fired.

Moving into the present day- when the average GTAE is compared to the automotive engine- their reliability shines like a high intensity laser beam. Let’s assume that a regularly serviced V8 four-stroke engine in a limousine can achieve a 300-000 mile/10 year interval before it requires an overhaul. Its GTAE counterpart on-wing will most likely operate for the equivalent of 2-550-000 miles and 12.5 years before it is due for its overhaul. Moreover- some of the latest generations utilize some really exotic metals and processes that can almost double these figures.

Regardless of the power delivery system employed by a GTAE (Turboprop- Turbojet or Turbofan)- the engine has four distinct sections- which are the Compressor Section- the Combustion Section- the Turbine Section (also known as the Power Section) and the Exhaust Section. There is no need to for us to discuss how a GTAE works- other than to remind you that it is a machine that is based on Newton’s law- amongst others- that translate the conversion of a fuel-air mixture into heat. The greater the differential between the ambient temperature- and the temperature increase achieved by the engine- the more energy or power will be created.

As an example- the Pratt and Whitney PT6A-series engine heats 15ºC ambient air up to 939ºC while a Honeywell TFE731-series engine heats the air up to 996ºC. Pure Aluminum melts at 660ºC- which is the reason why Inconel (which melts somewhere between 1-390 to 1-425ºC)- and Titanium (which melts at 1-670ºC) are used extensively throughout the hot section of a GTAE.

Both engines’ compressors also raise the ambient air pressure (14.7 PSI) by about 100 PSI which is where the cabin pressurization air comes from (when it has been cooled down). The fastest turning Gas Generator speed within the PT6A is 39-000 RPM. The Low Pressure Rotor (N1) of the TFE731 turns at a maximum RPM of 21-000- and the High Pressure Rotor (N2) turns 30-540 RPM. By comparison- a standard 18 inch car wheel rotates at approximately 2-000 RPM at 70 MPH. After all of this- the resulting power for each engine is up to 4-400 Lbs-Ft of Torque on the largest PT6A and 4-750 Lbs of Thrust on the TFE731.

The overhaul of a GTAE is understandably a costly and time consuming enterprise that can result in an invoice amount ranging anywhere from $250-000 to $1-000-000+ per engine.

The actual labor that is expended on an overhaul is not proportionate to the actual cost; most GTAEs require approximately 600 man-hours of off-wing/in-shop labor. It is the precision measuring- non-destructive testing (ultra-sound- magna-flux- x-rays- etc.)- welding- plasma spraying- and other re-coating processes- along with grinding- shot peening- disk rebalancing- service bulletin upgrades- with the lion’s share of cost taken by the parts replacement and finally testing- that contribute towards the high-costs.

You will have to excuse me for stepping back in time- but I will draw on an engine that I am most intimate with after spending many hours tearing them down- and rebuilding them - the General Electric CF700. Although the CF700 is a ‘fan-engine’- it is a ‘legacy engine’ because it was certified in 1964- and has the ability to gulp as much as 492 USg’s of Jet A per hour on the ground at full throttle. The CF700 is also a marginal (at best) Stage 3 noise maker- and is described by its Type Certificate Data Sheet as follows:

Turbofan: 8-stage axial flow compressor- 2- stage turbine- annular type combustion chamber- with free-floating single stage fan aft of turbine. It has a 4-500 lb static thrust output- and weighs 829 lbs (dry- including accessories.)

The CF700 has eight main sub-assemblies which are: Front Frame; Compressor Stator; Compressor Rotor; Mainframe; Gearbox/Accessories; Combustion Section; Turbine Stator; Turbine Rotor; and Aft Fan. When each sub-assembly is broken down into individual components- the parts count escalates close to 3-000 separate items. There are 193 separate parts in the front frame- 75 parts in the compressor stator- 1-320 in the compressor rotor- 36 in the mainframe- 52 in the gearbox (excluding accessories)- eight in the combustion section- plus 28 for the bearing and first stage nozzle assembly just aft- 14 in the turbine stator- 292 in the turbine rotor- and 301 in the aft fan. Then of course- there are hundreds of nuts- bolts- washers- screws- retainers and pins that hold all of these parts together.

Once the engine is inducted into the overhaul facility- and the paper trail on the overhaul has begun- each section of the engine will be separated from its neighboring part and then further stripped down into individual components as described above. All consumable parts like seals- O-rings and lock washers will be discarded. The remaining parts will be cleaned and possibly even stripped of their coatings- before being inspected and measured both dimensionally and ‘true’ (warping).

By now there will be a growing pile of discarded parts in the scrap bin. However the items that have passed muster thus far- will now go on to be structurally tested and examined- and in some cases even tested for their hardness.

Once the good parts have been set aside for processing and renewal- a list of repairs will be drawn up- for items that have failed but that may be salvaged if they are welded- braised- or in some cases machined to an allowable undersize tolerance. Often engineering approval will be sought from either the engine manufacturer- or even an FAA Designated Engineering Representative who specializes in GTAE repair.

Approved facilities that strive to salvage engine parts through proven engineered repair schemes often provide a significant edge over comparable shops- sometimes including the OEM.

When you are trying to narrow down your candidates to one facility that will get both your money- and your engines to overhaul- one of your best decision aids will prove to be the question: “How many repair schemes do you have in your arsenal for repairing my engines?”

That question applies unless- of course- you are not operating your engines under an all-inclusive maintenance service plan. In that case- you are better off letting the company that you have been writing those cheques to over the years make all of the repair decisions on your engines. A word to the wise- though; if you have a ‘pro-rata’ service plan on your engines instead of an ‘all-inclusive’ one- it generally behooves you better in the long-term if you keep a handle on the decision-making process- so there are no surprises waiting for you when it comes time to pay your portion of the overhaul invoice.

The bulk of your many engine nuts- bolts- washers and screws of various sizes will be NDT tested for re-use as long as they don’t show any signs of wear- damage or erosion. While the critical parts from your GTAE are in process- your engine accessories should also be getting the required scrutiny and attention that is prescribed for them at this interval.

Unfortunately engine accessories are often the unforeseen “gotcha” during- or after an engine overhaul. Either the costs to properly attend to these items were not specified in the quotation and thus make your blood pressure rise when the final invoice arrives- or worse still- you may never have sent them with your engine- as you consider them only as a part of your Quick Engine Change (QEC) items like inlets- cowlings and jet-pipes- etc.- and therefore ignored them.

Woe to anyone who spends a significant portion of his/her operating budget on engine overhauls- to later have an in-flight shutdown (or worse)- all because of a pesky fuel control unit- fuel or hydraulic pump biting the dust because it was overlooked during the overhaul.

The ‘Hollywood divas’ that reside within a GTAE - the ‘big money’ expense - are the ‘Low Cycle Fatigue’ (LCF) components. These ‘high-dollar’ primadonna items can run your invoice up by tens of thousands of dollars- sometimes even by hundreds of thousands.

What is meant by the term LCF is that certain components within a GTAE are identified as being ‘critical’- i.e. if they failed during normal engine operations- their failure would have catastrophic consequences for that engine- including immediate in-flight shut down and even the possibility of injury or loss of life.

Liability insurance is costly enough- but no GTAE would earn a production certificate from the relevant airworthiness authority if the OEM and its engineers didn’t identify all of the LCF components within their product. Then a defined ‘in-service life’ for these LCF items is established and set at an interval which is at the bottom-end of a wide margin that any cracks or deformation would be expected to occur.

The GE CF700 series engine has 45 life-limited components- of which 25 are considered critical LCF items. These include all of the rotating components like the compressor and turbine disks- drive shafts- torque rings and seals. The blades are life-limited- but their replacement schedule is independent from the disks that they are mounted to. In this instance- the turbine blades have a 5-000 cycle life- while the stage 1 and 2 turbine disks have a 10-000 cycle life.

Virtually all GTAE OEMs consider that 1 cycle = 1 landing - however I implore you to read your maintenance manuals- chapter five very carefully- in case there are operating conditions specified by the OEM that might require a different coefficient to be used.

Even though they are very resilient under furnace-like temperatures- the use of simple iron steels by GTAE engineers is kept to a minimum- because of their relatively high density (weight)- and also because of their limited resistance to corrosion. Instead the engineers turn to much more exotic materials like titanium- vanadium- palladium- molybdenum- ruthenium- cobalt- niobium- chromium- nickel- boron and more- and alloy them into a breed of super steels that are tailor-made for use inside a GTAE.

Inconel or Inco- as it is known commercially- is an austenitic (non-magnetic solid solution of iron) nickel-chromium based super-alloy steel that is most widely used by GTAE manufacturers- along with ceramics. For example- most turbine blades and guide vanes are forged from a nickel-steel alloy. All stress that might have been induced into the crystalline growth of this steel alloy are removed by a shot peening- stress relief method (microscopic spheres of metal- glass or ceramic are thrown at a velocity against the subject item).

Then an oxidation resistant coating (called aluminides from nickel and aluminum)- and secondly a thermal barrier coating (ceramic in modern engines)- are both applied to them through either a flame or plasma spray process. In many modern GTAEs the turbine blades now last 30-000 hours versus the 5-000 hours of the early CF700 engine. This is in a large part- due to the use of ceramic thermal barrier coatings.

The subject of engine materials and their processing does deserve a deeper explanation than that provided within the scope of this article. But it is hoped this brief insight might have at least opened your eyes to where a large portion of your money is spent on the overhaul of your GTAE.

It would be remiss of me if I failed to discuss the issue of dynamic balancing. Since all GTAEs rotate at such high revolutions per minute- any imbalance of its rotating components would prove to be catastrophic. Before the compressor and turbine sections are bolted back together- each ‘disk stack’ must be balanced to within a near-perfect tolerance. This is accomplished with a dynamic balancing machine that is purpose built to hold the turbine or compressor shaft/disk in a fixture- which then allows the entire assembly to be spun at high speed.

Any difference in mass around the design axis of the part being tested will be picked up by the accelerometers of the machine- and then its location and frequency is displayed on the machine’s monitor. The machine calculates by how much the mass of the subject disk or shaft must be adjusted to bring it into balance. It is unusual for weight to be added; usually the part is ground down in the area of imbalance to affect balance.

Unfortunately not all overhaul facilities were created equal. All of them will have had to meet the minimum standards set forth by the aviation authority that provides oversight of them; however each overhaul company has its own philosophy on the level and type of services that it provides.

Some may be entirely turn-key and subsequently accomplish all reprocessing of your engine components in-house. Others may act much like a clearing-house whereby they disassemble- clean- inspect and reassemble your GTAE; but all repairs- coatings and rebalancing is accomplished through a vendor. Either way you will end up with properly overhauled engine.

The biggest effect will be manifested in the amount of money that you will have to pay at the end of this process. As I stated earlier- asking the candidate overhauler about the number and complexity of repair schemes available- in-house- will assist you greatly in deciding on which is the best- most economic choice for your engines.

There are many factors that will have an appreciable effect on your bottom line- and therefore must be mixed into your ‘best of three’ (or more) matrix that you are hopefully creating as your decision-making tool. Some of these include:

• Approvals
• Disposition towards repair parts
• Inventory
• Local and state tax laws
• Warranty policy
• Turn-time
• Removal and reinstallation costs
• Shipping costs- if applicable
• Loan engines- if applicable
• Number of similar engines overhauled
• Expected cost
• Repair and processing that is in-house
• In-house engineering
• Fixed or guaranteed costs in the quote
• Variable costs- including vendor bill-backs that might be involved
• Client references

One last point to conclude with applies if you are short on downtime- or you are nervous about the financial outcome of having you GTAEs overhauled. you might consider either a flat-rate overhaul exchange for your existing engines- or enrollment onto an all-inclusive formal maintenance service contract.

Lastly you might even employ the method that most smaller airlines use: a lease of your engines from a specialist leasing company- thus eliminating your financial exposure altogether.

Jeremy Cox is vice president- JetBrokers- Inc. If you have any questions regarding engine overhauls- or would like to receive some free advice- you are invited to contact him at JetBrokers- Inc. at +1.636.449.2833- or email: jcox@jetbrokers.com

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