Shelf life is the period during which material may be held in storage and still be considered fit for use. Cure date is the date of manufacture of an elastomeric item — seals, O-rings, hoses — from which its storage life is measured.
The essential point for anyone holding aviation stock: both clocks run from manufacture, not from first use. Material that has never been opened can expire on the shelf.
Why time-control exists
Some materials degrade whether or not they are used. Elastomers harden and lose elasticity. Adhesives and sealants lose bond strength. Chemicals separate or become unstable. Batteries self-discharge. None of that is visible on inspection, which is precisely why the control is a date rather than a look.
For aerospace elastomeric seals the widely referenced industry document is SAE ARP5316, an aerospace recommended practice covering storage and shelf life. It is an industry standard rather than a regulation, and it is worth knowing that it measures from time of manufacture and that its treatment of different elastomer families is not uniform — several materials are treated as having no shelf-life limit under proper storage, while others are limited. Manufacturer instructions and customer contracts may impose stricter limits than any general standard.
The different clocks
- Cure date. Date of manufacture for elastomeric material. Commonly expressed as a quarter and year rather than an exact day, which is why storage-life calculations conventionally start from the end of the cure quarter.
- Manufacture or batch date. The equivalent starting point for chemicals, adhesives and sealants.
- Shelf life. The permitted storage period from that starting point, after which the material must be re-tested, re-certified or discarded depending on the item and the governing document.
- Opened or working life. For some consumables a second, much shorter clock starts when the container is opened or when components are mixed.
- Retest or extension. Some material can have its life extended following specified testing. Some cannot. Assuming extension is available is a common and expensive mistake.
Storage conditions are part of the control
A shelf-life date is only meaningful if the storage conditions it assumes were actually maintained. Typical requirements include controlled temperature, controlled humidity, protection from direct sunlight and ultraviolet light, protection from sources of ozone such as electrical equipment, and appropriate handling for electrostatic-sensitive devices.
Material stored outside its required conditions may be unusable well before its nominal expiry — and unlike a date, that is not something a stock report will tell you.
Where it bites operationally
- At quotation. Material quoted as available may expire before the order ships, particularly on long-lead or staged deliveries.
- At allocation. Stock committed to an open order can expire while allocated, converting a routine sale into a customer conversation.
- At receiving. Material can arrive with less remaining life than the buyer assumed — or occasionally already expired, having spent its life in transit or at the supplier.
- At picking. Discovering expiry here is the most expensive outcome, because the commitment has already been made.
- At audit. Expired material found in a live stock location is a finding, not a housekeeping issue.
The organizational lesson is consistent: expiry needs to be visible before the commitment, not at the point of physical issue.
Common misunderstandings
Unused does not mean unexpired
The single most common error. New surplus material can be decades old. Physically pristine stock can be entirely unusable.
Cure date is not the date you received it
It is the date the material was manufactured. A seal received today may already have spent most of its permitted storage life somewhere else.
Expiry is not always terminal
Depending on the material and the governing requirement, some items can be re-tested and returned to service and some must be discarded. Treating every expiry as automatic scrap writes off material unnecessarily; treating every expiry as extendable is worse.
Not all elastomers are limited
A blanket "rubber expires in five years" rule is not accurate. Limits vary by material family, and under recognized industry practice several families are treated as unlimited when stored correctly. Work from the applicable standard and the manufacturer's instructions rather than a rule of thumb.