More than the sum of the parts
When an airline decides to either switch or extend its aircraft fleet, it needs to ensure that it has the right amount of stock in place to support however many aircraft it is taking on. This initial provisioning can cover everything from small parts to complete spare engines, from rapidly-used consumables to repairable rotables. The task then is to maximize the usage of those parts.
Some of those parts are designed to be used for a designated number of cycles or hours, their designers having calculated that it would be prudent for them to be changed after that time. Known as life-limited parts (LLPs), they must be changed when that limit is reached.
Often, life-limited modules and parts are among the most costly parts of an engine, according to MTU Aero Engines. Typically, engine LLPs “include, but are not limited to disks, spacers, hubs, shafts, high-pressure casings, and non-redundant mount components,” notes the Legal Information Institute, an independently-funded project of the Cornell Law School.
Advanced monitoring of the health and usage of LLPs is now a regular discipline, enabling close tracking of a part’s performance in the operating environment of the airline. Having this knowledge of the exact number of hours and cycles has led to another strategy which can be used for engine lifecycle optimization.
The procedure – offered as an option by GE Aviation – is basically about mathematics. However, it results in more economic use of the LLPs and reduces maintenance costs and downtime.
First, when an airline’s fleet approaches the mid-life mark for its life-limited parts, the carrier’s maintenance department or third-party provider will bring in half of the carrier’s engines for an overhaul. Note that this is a tactic in the lifecycle optimization strategy, not a necessity.
Instead of leaving the ‘half-lifed’ LLPs on the aircraft, these are all exchanged for new LLPs. While the aircraft with their new LLPs are released for service, those original LLPs are then stored, with all the relative documentation attached.
The next phase is for the other half of the aircraft’s engines – with their LLPs at their life limit – to be brought in for overhaul. Obviously, as these LLPs have completed their serviceable life they will be removed and disposed of in an environmentally-friendly way.
The LLPs from the first set of engines – with only half of their useful lives spent – are then brought out of storage and fitted to the second batch of engines. During all this, of course, the first batch of engines is continuing revenue service with the second (new) set of LLPs each one received.
To put numbers to the process, if the life limit of the parts is 25,000 hours, those LLPs stored after the first batch of engines were serviced will have 12,500 hours left on them. These engines then received new LLPs with their full 25,000 hours of life on them. So they would each be able to operate for 37,500 hours on the line.
For the second batch the equation works with a set of LLPs being used from the engine’s operation right up to their limit of 25,000 hours. Then they are fitted with the LLPs from the first batch of engines, each of which has 12,500 hours of life left, taking the overall operating life of the engine, once again, to a total of 37,500 hours. This lifecycle optimization strategy thus allows extended operation of each engine with only one major overhaul.
As noted, GE Aviation has made this part of its service portfolio. It is also available through some of its Authorized Service Providers, such as StandardAero.
While the formula is, as noted, basically about mathematics, the better use of parts means that description extends to the sums of money spent. Which is exactly what lifecycle optimization is meant to do.