Aircraft parts shelf life: cure dates, sealants and how to ask for remaining life

Which items have a shelf life

Shelf life is how long an item may sit in storage before it is used. It is not a life limit, which counts hours, cycles or calendar time in service. For a buyer, shelf-life items fall into three groups:

• Elastomer seals — O-rings, packings, gaskets and seal kits, dated by their cure date: the quarter in which the rubber was cured (vulcanized).
• Chemicals — sealants, adhesives, paints, primers and similar materials, dated by the manufacturer's batch and date of manufacture, with a shelf life that holds only under the storage conditions on the label. Composite materials belong here too: the FAA's AC 20-62E notes that most have a refrigerated shelf life recommended by the manufacturer and a limit on time out of refrigeration, and that the cumulative out time must be recorded.
• Components with internal seals — AC 20-62E names pumps, valves, actuators, motors, generators and alternators, whose internal seals "are subject to deterioration from long-term storage and are susceptible to early failure in service," and says a procedure should be established to control shelf-life items.

So on almost every seal line you are quoted, the information that decides whether you can use the part is missing. That does not mean the supplier lacks it: the date may be printed on the bag and simply not typed into the email. It does mean you have to ask, and compare offers only once you have the answer.

Elastomer seals: reading a cure date

Seal makers mark the cure date by calendar quarter, not by day. 2Q24 means the seal was cured at some point between April and June 2024. Because the exact day is not recorded, seal companies such as Apple Rubber and Global O-Ring treat the last day of the quarter as the date of manufacture, so the shelf-life count starts when that quarter ends.

Add the applicable limit to the start date to get the expiry. With a five-year limit, for example, a 2Q24 seal would run until 1 July 2029. Which limit applies is the harder question.

MIL-STD-1523A is cancelled

Quality manuals, purchase terms and older certificates still cite MIL-STD-1523A, Age Controls of Age-Sensitive Elastomeric Material. It was cancelled by Notice 1 on 30 January 1995. Notice 2, dated 16 November 1999, directs future acquisition to SAE AS1933 for hose and SAE ARP5316 for seals.

ARP5316 was an Aerospace Recommended Practice. SAE has since replaced it with an Aerospace Standard, AS5316, issued in November 2017 and reaffirmed in November 2022. Its public abstract says it specifies the general requirements for data recording, packaging and storing of elastomeric seals and seal assemblies before the seal is assembled into hardware, and that packaging is an integral part of controlled storage, providing "a means of positive product identity from the time of manufacture to the time of assembly into a component." AS1933, for its part, sets an age limit and guidance for accepting hose and hose assemblies at the time of delivery; by its own abstract it sets no limit on storage time or operating life.

The limit that binds you

Seal companies publish shelf-life tables by polymer, often based on AS5316 or its predecessor ARP5316. In those tables some rubbers carry a limit in years and others none at all, provided the storage conditions are met. The limit you must work to, however, is the one in your own quality manual, the component maintenance manual, your customer's contract or the part's specification. Put that limit on the PO, not a table found online.

Two consequences for purchasing. If your procedures still reference MIL-STD-1523A, they reference a document cancelled more than thirty years ago; ask your quality department which limits it actually applies. And AS5316 is about seals before they are fitted: a seal already inside a pump or an actuator is the internal-seal case above, where the component's own storage history matters.

Sealants, adhesives, paints and other chemicals

For chemicals, the manufacturer sets the shelf life, usually counts it from the batch's date of manufacture and makes it conditional on storage. A published PRC-DeSoto data sheet for PR-1776M Class B fuel tank sealant (2009 issue) is a typical example: at least 9 months in a Semkit or can kit kept between 40°F and 80°F (4.5–27°C) in original, unopened containers, and 28 days for the premixed and frozen cartridges kept at −40°F (−40°C) or below. The figures differ by product and change between issues, so work from the current data sheet for the exact product you buy.

Premixed and frozen items

Premixed and frozen (PMF) sealant is mixed by the manufacturer and frozen to hold back the cure. The manufacturer's application guide says PMF material is shipped in dry ice, that polysulfide PMF should be stored at −40°F (−40°C) or below and polythioether PMF at −80°F (−62°C) or below, and gives a typical shelf life of 30 days for both. With a life counted in days, the shipping plan is part of the offer: ask when it will ship, how, how long transit takes and how much of the month will be left when it reaches your freezer.

Stock of unknown history

AC 20-62E is direct about this: dope, paint, sealants and adhesives "advertised as aircraft quality may have deteriorated due to age or environmental conditions, while in storage, and may require testing before use." Of surplus stock it says that "the storage time, storage conditions, or shelf life of surplus parts and materials are not usually known." A date on the can proves age, not storage history. Whether an out-of-date batch can be tested and used is a decision for your quality system under the manufacturer's instructions and the governing specification, not for the seller.

Remaining shelf life: the number to ask for

Remaining shelf life is the time between the day the item reaches you and its expiry date. It is what you actually buy. A sealant with a 9-month shelf life made 7 months ago is new, unopened and in date — and has 2 months left.

Ask for it as a minimum, in months or as a percentage of the total shelf life, and say where it is measured: at shipment or at receipt. Set the figure from your own use: the time until you use the last of the quantity, plus the time from your dock to the bench. Asking for more than you need can rule out stock that is on the shelf today, which matters when the aircraft is waiting.

Put the requirement on the purchase order, not only on the RFQ. The PO is what the delivery will be received against, and a requirement that lived only in an email thread is easy to lose. Our guide to writing an aircraft parts RFQ covers the rest of the line.

Paperwork that should come with it

In its list of information to obtain for used parts, AC 20-62E includes "shelf-life data, including manufacturing date or cure date." For shelf-life items of any condition, the paperwork should let you tie the item in your hand to a batch and a date:

• Seals: the maker's package label showing the part number or specification, the cure date and the batch or lot, and a certificate of conformance that carries the same batch. AS5316's point about packaging is exactly this: the package is what keeps the seal's identity from manufacture to assembly. A loose O-ring from an unmarked bag has no cure date anyone can prove.
• Chemicals: the manufacturer's certificate for the batch with the date of manufacture and expiry date, a container label that matches it, and the current technical data sheet and safety data sheet for the product.
• From a distributor: the distributor's own certificate plus the manufacturer's certification for the batch. If a larger lot was split into smaller packs, the new label should still carry the original batch, cure date or date of manufacture, and expiry.

Condition codes do not carry this information. "NE" or "NS" says the item is new; it says nothing about when it was made. More on what each code does and does not tell you in our guide to aircraft part condition codes.

What to check on receipt

A short check at the dock, before the item goes into stores, catches most shelf-life problems while they are still the seller's to fix:

Then store the item as its label says. The date on the paperwork holds only while the storage conditions behind it are kept, and once the item is in your stores, that part is yours.

The short version

1. Flag the line. Mark seals, sealants, adhesives, paints and similar items as shelf-life items on the RFQ.
2. Ask for dates and lot. Cure date for seals; date of manufacture and expiry for chemicals; batch or lot for both.
3. Set a minimum remaining life. In months or percent, at shipment or at receipt, from your own consumption.
4. Compare on usable life. A cheaper offer that expires before you use it is not cheaper.
5. Write it on the PO. Requirement, certificate content and, for frozen items, the shipping method.
6. Check at the dock and store it right. Label, certificate, date, packaging, temperature, expiry booked in.

Sources

• AC 20-62E (Change 1), Eligibility, Quality, and Identification of Aeronautical Replacement Parts — FAA, 2010, change 2018 (paragraphs 10.b, 11 and 12).
• MIL-STD-1523A Notice 1 (30 January 1995) and Notice 2 (16 November 1999), cancellation of MIL-STD-1523A — EverySpec.
• AS5316, Storage of Elastomer Seals and Seal Assemblies Which Include an Elastomer Element Prior to Hardware Assembly — SAE International, issued November 2017, reaffirmed November 2022 (public abstract only).
• SAE AS1933, Age Controls for Hose Containing Age-Sensitive Elastomeric Material — abstract as listed by Kikaku Master.
• PRC and Pro-Seal aerospace sealants application guide — PRC-DeSoto International (PPG Aerospace).
• PR-1776M Class B Low Weight Fuel Tank Sealant, technical data sheet — PRC-DeSoto International, issue 09/09.
• How to Calculate Shelf Life of Elastomers — Apple Rubber, March 2015.
• O-Ring Shelf Life — Global O-Ring and Seal.