Some part on an aircraft landing gear is defined as having a life span that cannot be extended. These parts must be disposed of at this point regardless of their condition; they are commonly known as SLI – Safe Life Items or LLP – Life Limited Parts.
The landing gear itself is a complex component and while some items can be replaced “on wing” (with the landing gear installed in the aircraft); there are many items that need the landing gear removed from the aircraft to accomplish replacement.
In the aircraft lease agreement, it will note the return considerations for such items regarding the remaining life that they must have for the lease return.
The landing gear must be assessed for the LLP/SLI items that are installed and then compared to the lease return conditions initially and ensure that it meets the conditions.
The next consideration is then the verification of the life for the LLP/SLI and this requires a consideration of the parts history – we need to know what has happened to the part in its life (since manufacture up to the present we want to know where it has been used).
This process of reviewing a parts history from when it was manufactured is known as BTB or Back to Birth. We can build a picture then of an item if we review when it was used from and to, bearing in mind it might have been used in several applications.
The history would need to be verified with records to prove the installation/removal as and when it occurred; this forms the basis of a back-to-birth review and is required for SLI/LLP items.