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Plane Sense - Don’t Forget Life-Limited Components

When you finally make the move from a reciprocating powered aircraft to a turbine powered aircraft, you are also moving from one type of engine overhaul to another. With reciprocating engines, the overhaul requirements are fairly simple and easy to understand, and just about anyone with a Powerplant Mechanics Certificate can do the job. A turbine powered aircraft overhaul is a little more complex.

Steve Watkins   |   13th March 2012
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Steve Watkins Steve Watkins

Steve Watkins is Technical Services Manager, Western Region for Jet Support Services, Inc....
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Don’t Forget Life-Limited Components (And their impact on your maintenance budget)

When you finally make the move from a reciprocating powered aircraft to a turbine powered aircraft, you are also moving from one type of engine overhaul to another. With reciprocating engines, the overhaul requirements are fairly simple and easy to understand, and just about anyone with a Powerplant Mechanics Certificate can do the job. A turbine powered aircraft overhaul is a little more complex.

You have to take into consideration the most recently performed Mid Point Inspection (MPI) and Overhaul Inspection. You have to consider the hours that have been flown, or the calendar time elapsed since the last MPI or Overhaul. Overhauls and MPIs are all determined by hourly or calendar intervals. Some engine manufacturers even like to throw in an extra requirement based on not flying enough hours. All of these items make planning for the next engine inspection an interesting and challenging process.

Once you have determined when your next MPI or Overhaul is due, you might think you can then sit back and relax from the plethora of mind-numbingly complex considerations that were required in order to make your determination. You’d be mistaken, however…

DON’T FORGET THE LIFE LIMITED COMPONENTS!

The engine manufacturers and regulatory agencies around the world decided that there was a need to set additional limits for some of the internal parts on a turbine engine that spin around at thousands of revolutions per minute, and are subject to extreme temperature changes.

I happen to agree with these requirements, and feel they are actually more important than the Mid Point Inspection or Overhaul. These limits are referred to as cycles, and the parts involved are called Life Limited Components (LLCs). These can be everything from a major turbine hub down to a spacer located in the engine.

The Mid Point Inspections and Overhauls as well as the Calendar Inspections can be exceeded by a small amount of hours or days with proper approval from the engine manufacturer and the Regulating Agencies. LLCs, however, cannot exceed their cycle limits and are required to be replaced before this limit is passed.

The Life Limited Components are specified by the manufacturer, and finding the actual document that lists them can be as challenging as identifying the criminal in a ‘who-done-it’ mystery novel. Some manufacturers have the cycle limits spelled out in the Engine Maintenance Manual, others in a Service Bulletin, and still others in some other obscure location.

A cycle in layman’s terms means that you have started the engine, advanced the throttles, and went to a certain temperature before reducing the throttle back to idle and shutting down the engine again.

Maintenance managers need to be sure they understand the description of a cycle for their particular engine model as this can vary not only by manufacturer, but by the specific model. (As an example, the CF34 engine counts a cycle as one start, take off, and landing, but in certain circumstances, you have to add on a partial cycle based on Thrust Reverser usage. Until you have researched your particular engine, you should never automatically assume a cycle is equal to one flight.

It always seems that a Life Limited Component reaches its limitation either before or after the MPI or Overhaul limitation. When you are planning your Overhaul/MPI inspections, one of the things you have to always consider are the LLC components installed in your engine. It would be nice if you could use a mathematical formula to figure out how many cycles will be used in a given number of hours or days, but I have never had any luck in coming up with this equation, so it ends up being a “calculated guess.” Using this calculated guess, you then must decide whether to replace the LLCs before they reach their life limit, or run them to their life limit and then enter the engine for replacement.

COSTS: NEW OR CONTINUED-TIME REPLACEMENT PARTS?

Having a skilled engine program manager to determine the most economical decision with respect to a turbine engine is critical. The decision is not only when to replace an LLC, but which replacement LLCs to purchase. You have the options of installing a new or continued-time part. Continued-time parts were once installed in another engine and part of the maximum cycles were used, but the component is still usable for the remaining cycles.

The used components usually come at a discounted price, based on the number of cycles that have been used and the number of cycles still remaining before the life limit is reached.

If a new disc price is $50,000.00 and has a cycle life limit of 5,000 cycles, this new disc will cost $10.00 per cycle. If 2,500 cycles have been used in the previous engine, then the cost of the disc would be based on the 2,500 cycles remaining at $10 per-cycle or $25,000. These continued time components will usually be discounted below the new component cycle cost and can save you a substantial amount on the replacement cost.

A variable to consider when determining to buy new or continued-time components is the projected number of cycles after your current work is completed until the next time the engine will be torn down to the same point to access this component again. If you can procure a continued time component that has sufficient cycles remaining to reach the next time you have access, this will usually save a substantial amount of cost over buying a new component.

You also need to take into consideration your projected ownership of the engine. If you plan on keeping an aircraft for five years after the LLCs are replaced and you use 150 cycles per year, it does not make much economic sense to install a new LLC that has a life limit of 5,000 cycles if you can find a continued- time part that will reach the next access period.

All this may sound very complicated and more than you ever wanted to know about turbine engine Life Limited Components but, at JSSI, we deal with it every day and understand how these complex decisions can impact your maintenance budget, as well as the availability of your aircraft.

 


Read more about: Engines for Business Aircraft

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