Shelf Life Challenges

NOTE: I’ve updated this article with some information that I thought would be of some value to those of you similarly ‘challenged’. (10-2007)

I learned a new word recently: Whinge. It’s an old English word meaning to moan, or to complain fretfully; to whine. When it comes to certain shelf life issues, I feel an unexplainable urge to engage in whinging.

Shelf life control should be an easy program to manage, right? Not always. The easy part is when the packaging on a part has a stated expiration date; these are most easily identified and placed on your shelf life control program. Most difficulties however, arise from the following:

• New parts that have a stated cure date, but no stated expiration date or shelf life limitation
• New parts that really have a shelf life but do not have either a stated cure date, shelf life, or expiration date, or any other indication of shelf life
• In service rotables (repaired or overhauled for example) that have shelf life limitations, but the company stocking these parts is not aware of the limitations

We’ll talk more about these, but first an introduction to the subject.

A PRIMER: Certain parts and commodities (I’ll refer to both collectively as ‘parts’) start to deteriorate or degrade from the moment they are manufactured. Engineers determine how long a part or commodity can last in storage, and thus the shelf life. The shelf life is typically expressed either in the length of time of the shelf life, or with an actual expiration date. The length of time might be expressed in quarters (a quarter being three months), 20 Quarters for example (5 years) from the time of manufacture. It’s most easy when there is a plain expiration date on the packaging. There is another class of part, that although it does not deteriorate or degrade as discussed, must nonetheless be periodically tested. These don’t have a literal ‘shelf life’ per se, but nonetheless must be tracked as are typical shelf life parts to assure none are issued ‘expired’.

Such parts known to contain shelf life include:

• Aircraft Main, or APU Batteries.
• Parts containing batteries such as Flight Data Recorders, Cockpit Voice Recorders, life jackets, Survival Equipment, Emergency Path Lighting Packs, defibrillators, and Emergency Locator Transmitters (ELT’s).
• Parts required by FAR’s to be tested periodically such as ATC Transponders (FAR 91.413)
• Certain o-rings, packings, hoses, or components containing such parts.
• Certain greases, paints, enamels and sealants.
• Pressure vessels, such as oxygen cylinders and fire extinguisher bottles have mandatory tests that must be performed periodically. Known as Hydrostatic tests, the period depends on the type of bottle.
• Aircraft First Aid kits

For these parts, you should have a shelf life control program that accomplishes the following:

• For parts in stock you should have a system to track these such that stock is rotated for First-In, First-Out, and most importantly, that no stock is issued beyond its date of expiration. Depending on the markings on these parts, you may need to affix the expiration date on the individual parts, or some sort of other conspicuous indicator. There are many variations on how you control this stock, but suffice it to say that the process should be documented in your Quality System, and be in compliance with your chosen quality standard.
• For commodities issued to the shop or to mechanics: Your stock room may issue greases, paint, and sealants to the shop, or for mechanic’s use. If these are shelf life controlled items, operators typically affix their own expiration date decals to each issued commodity. Its purpose is to assure that shop employees don’t use the commodity beyond its expiration date. Most operators have written procedures making it clear that it is the mechanic’s responsibility to assure when using such products, it not beyond it expiration date.

THE CHALLENGES: But wait! I have not yet engaged in the sport of whinging! Thus far, the aforementioned Primer section was pretty routine for those of us familiar with shelf life control. Lets now examine those challenging areas:

New parts that have a stated cure date, but no stated expiration date or shelf life limitation: Some people see a stated “Cure Date” on a part’s packaging and assume the part must be shelf life controlled. Not necessarily. I’ve seen many parts with a stated cure date, but the manufacturer helped us by also stating “No expiration date”, or some variation of that. Great huh? The problem arises when there really is an expiration date, but there is no indication of that except the cure date. Here’s a sampling of the reasons I’ve heard for leaving it off:

• The part is also used in the commercial world (non-aviation), where there is not a quality concern with shelf life, so, to standardize packaging, they leave off the expiration date.
• The expiration dates were originally controlled by military specs, and those specs are no longer being maintained by the government
• The application determines the shelf life

Exacerbating this is the fact that quite a few end users/operators will evaluate these parts and establish their own shelf life limitations. So what’s a person to do with these parts? Your first option is to call the manufacturer and ask them if any shelf life limitations exist. Your second option is to call your customer and ask them the same question. Regardless, you should be able to track the cure dates for the parts, and state as much to potential customers for them to make up their mind whether they want to proceed with the purchase. Unfortunately, there is no black and white answer to this problem, other than to whinge about lack of support from the manufacturers.

New parts that really have a shelf life but do not have either a stated cure date, shelf life, or expiration date, or any other indication of shelf life. Now, this one really chaps my hide! We have a lot of bearings in stock, and a customer’s auditor asked us why we didn’t have the bearings on our shelf life control program? Imagine, bearings of course don’t have a cure date on them, and these don’t have any indication either on the packaging or accompanying paperwork of such limitations. Surprised, I called the manufacturer, and sure enough, there is shelf life. I’m now heard to raise my incredulous voice and ask the obvious: Well, why don’t you put it on the packaging? A long list of self-serving excuses ensues, along with a cryptic faxed document explaining the shelf life policy. It is based on the bearing’s preservative grease and packaging. Unless we are blessed with the Spirit of Prophecy, we are simply not going to know these things! I’m want to whinge some more, but nah...

ROYBOY’s COUNSEL: For rubber products, there is a document published by the Department of Defense Titled “Rubber Products, Recommended Shelf Life.” If you are pressed for placing these commodities on a shelf life program, and you can find no credible information, I’d use this document. It is 45 pages long and available free at this link:

https://progate.daps.dla.mil/home/ 

1) At the home page, click on the “Specs/Stds” link

2) In the Document ID block, type in MIL-HDBK-695, and submit it, then follow the links. Royboy’s cool insider info: If you are looking for any government specs or standards, try this link first to see if it’s available free. It’s also useful to see the latest revision and it’s status; is it still active?

Another document which may help as well is titled “Storage of Elastomer Seals and Seal Assemblies Which Include an Elastomer Element Prior to Hardware Assembly.” It is 18 pages long, numbered ARP5316, and is available from the SAE for purchase at www.sae.org. Try the DoD handbook first, since it is free. Both documents contain very useful tables of cross references to standards, part numbers, and of course, recommended shelf life limitations.

In service rotables (repaired or overhauled for example) that have shelf life limitations, but the company stocking these parts is not aware of the limitations. Here’s where we could help by expanding a little on parts affected by shelf life.

• Anything with batteries in them. If you see a Cockpit Voice Recorder (CVR) or Flight Data Recorder (FDR) with an installed acoustic beacon, check the beacon, and you’ll see it has an expiration date on it. The beacon, or pinger as some call it, sets off an emergency signal when immersed in water; it runs on batteries, and thus the shelf life. The beacon is usually a cylinder about 4 inches long by 1.5 inches in diameter on the front of the CVR/FDR. As stated, the beacon will have an expiration date, and the repair/overhaul shop’s paperwork will state the same. Life vests, emergency equipment such as that found in life raft kits, Defibrillators, Emergency Locator Transmitters, Main Aircraft batteries, APU batteries, and Emergency path lighting packs, all contain, or are batteries. A visual inspection of the part and paperwork will reveal its shelf life.
• Parts required by FAR’s to be tested periodically such as Transponders (FAR 91.413). This is another potential gotcha. Transponders are typical of parts that must be periodically tested, in this case every 24 months. They must be tracked similarly to shelf life parts to assure none are issued to your customer expired. The DOT also requires pressure vessels to be tested periodically. For these parts in your serviceable stock, are they on a monitoring program?

Savvy Shelf Life Purchasers: Keen purchasers of shelf life limited parts usually put a boiler plate statement in their purchase orders that they will not accept such parts unless 80% of the shelf life remains. So much for first in, first out! Of course if there’s an AOG, and you’re the only game in town, and only 20% remains, guess what? But don’t get me started...