As aircraft age, more of their value shifts into their engines, which means that operators must keep an eye on the different factors that influence engine values over time.
Depending on the phase of an engine’s life, options to the owner include: repair or overhaul to put value back into the asset; green-time sale; and tear-down and part-out of used serviceable material in order to extract remaining value from the engine.
An engine’s maintenance value accounts for the value of life-limited parts (LLPs) and the value associated with the on-wing time before requiring a performance restoration. Core value, on the other hand, is the value of non-LLPs and the engine data plate, and can be significantly lowered by a lack of proper documentation.
When calculating the maintenance remaining on an engine, OEM mean time between overhauls data is the starting point.
The derate on an engine will also affect the intervals between the shop visits, as the higher the derate the lower the thrust. Therefore, as with a lower thrust rating, there will be less deterioration affecting the engine hardware, thus allowing longer maintenance intervals.
The region of operation of an engine is important due to the effect of the environment on the engine. For example, the Middle East lowers the MTBO due to more thrust having to be used to achieve the same amount lift as somewhere cooler, due to the air density.
For an engine in service another factor to be taken into account is the LLP limiter; this affects the maintenance remaining as the engine will have to go in for a shop visit to replace an LLP that hits its limit. The limiter is therefore the maximum amount of cycles before the engine has to have a shop visit. So, even if the maintenance interval is higher, the limiter is used to calculate the maintenance remaining as it will be the primary maintenance driver.
An engine may have 100% maintenance remaining, but, unless a full LLP replacement is carried out, the engine does not return to full-life.
For a more detailed look at the business of engine appraisals, pick up the forthcoming Engine Yearbook 2018.