Retest Dates on Grease?

by Norman Chance

Every aircraft maintenance technician will have a number of shop supplies they use on a regular basis. It doesn’t matter if you’re working on a DC6 in Nome, Alaska, or repairing a Gulfstream V in Dubai — some items will always be there. Toolboxes are usually stocked full of many recognizable items, ranging from cans of safety wire to a wide selection of spray lubricants, electrical contact cleaners and greases. Although these are not FAA-regulated parts, they are at least supplies used for the proper care of an aircraft. The most important issue in the past has been to make sure that first you have the correct specified consumable as directed by the airframe or component manufacturer, further identified by a Mil-Spec or trade name. Secondly, the next important requirement is that you had ample stock on hand. There is nothing worse than being the middle of aircraft maintenance at 2 a.m., only to find that tool crib is out of MIL-PRF-23827 grease.

Recently, we have seen a lot of aircraft maintenance shops that have been throwing away thousands of dollars worth of aircraft grade grease foolishly because they either could not find an expiration date or they see the word “re-test” and freak out. If they would have marked the grease as soon as they bought it, and would have developed a simple marking system for the container, all would be calm and safe. I have heard a lot of people who think that grease is automatically devoid of any use when it hits the re-test date. Even if you have a batch of grease that has expired, prudence says the products can be used for any manner of non-aviation uses around the hangar.

Grease isn’t a glamorous thing, but it is critical for proper aircraft maintenance. It has always been easy to use and is normally something DOMs don’t waste time thinking about. As a parts distributor, I have purchased, stocked and sold more aircraft grease than I care to remember. It isn’t something that I have particularly given much consideration to, but recently many repair stations, government inspectors and A&Ps have been bombarding parts distributors over an issue that has completely gotten blown out of proportion. As usual, many people have failed to do their homework and are wasting enormous amounts of money because of it. Several years ago, both Shell and Exxon Mobil started using the term “re-test date” on aircraft grease containers. For some reason, shop inspectors seem to have some sort of mental block and read this to mean the expiration date (which it is not). I suspect this term has caused many local FAA representatives heartburn as well, because the answer is not straight forward. I will make an attempt to explain what has happened, and what the best course of action is.

First, I want to tell you that this confusion has nothing to do with the FAA. They did not cause this, and this has nothing originally to do with the U.S. government. Surprisingly, this confusion started with NATO. It is important to remember that until recently, aerospace lubricants were largely military-specified items. Lubricant manufacturers would create their products to comply with large military contracts all over the globe. NATO and the U.S. government have followed the Mil-Spec route for years. Manufacturers would make large batches of aircraft grease with the Mil-Spec markings stamped right on the container as required. During WWII, allied military aircraft manufacturers specified Mil-Spec grease in their aircraft. When WWII ended, airframers continued this trend in the assembly of the new civilian aircraft they built. Since a Mil-Spec grease was used in the maintenance of an aircraft, as specified in the maintenance manual, by default that is what must be used by the letter of the law. Recently, the Society of Automotive Engineers (SAE) has slowing begun to take over lubricant qualifications in the U.S. Some SAE greases and oils are making their way into aircraft maintenance on current aircraft.

In 2004, NATO released a document called STANAG 3149. This standardization document outlines testing and qualification of lubricants according to NATO Specs. Soon after its release, the U.S. Department of Defense issued a document MIL-STD-3004A titled “Defense Department Standard Practice for Quality Assurance for Fuels, Lubricants and Related Products.” This document established, among other things, requirements for the retesting of lubricants. Since the aviation grease you and I use in the hangar is the same as is used in military bases all over the world, all of the popular lubricant manufacturers had to change the packaging to include the phrase“re-test date.” The kicker of this change is that it is not, nor was it ever, intended to affect general aviation, business or commercial aviation. The D.O.D. letter specifically states that this change was made to be used by military services and defense logistics agencies following U.S. government rules. The problem is nobody in our industry has been told what to do about this term.

For every DOM who reads this, or any member of a quality control team, you must understand that re-test does not mean expired. These are two distinctly different terms. Expired means it is no longer considered safe for use. Re-test means that the product is sent back to an authorized lab that can perform such work. In a document called “Aeroshell Lubricants and Special Products” from Shell Aviation, Shell gives the example that Aeroshell greases need to be retested after three years. If the batch passes, then it may continue to be used if retested every 18 months afterwards. Essentially, the document states that as long as the grease is retested and passes lab tests, grease can continue to be used. I was baffled to read a statement in the article that reads, “Their products are inherently stable and if stored properly, their quality and performance should not be affected by prolonged storage.” If this statement is true, then why would anyone need to retest grease every few years? It seems rather silly to me to say that, on one hand, your products can sit on the shelf for a long time with no adverse effects, yet on the other hand you need to pay to retest them? These statements seem contradictory. Upon doing research for this article, I came across a document called “Aviation Lubricant Shelf Life” from Exxon Mobil. It states that a shelf life is a recommended shelf life, and that they extend the shelf life when a certain product batch has been retested. In essence, they establish that their grease won’t magically vanish into a pile of pixie dust when a product hits an expiration date. It is stated that they can prolong a batch of grease if they chose. Exxon Mobil also goes on to state that U.S. military periodic testing rules do not apply to Exxon Mobil products. Exxon Mobil has its own rules, which in fact, supersede U.S. D.O.D requirements.

 With all this talk of re-test dates, you probably wondered why you can’t simply retest the grease at a local lab. You can’t. The tests are specific and can only be done by a handful of U.S. government-approved laboratories. Nearly all letters from grease manufacturers state that it may be more economically viable to purchase new grease than to retest the products you have on your shelf. These manufacturers have created this whole retest issue with the government operators in mind. This military decision was made with absolutely no regard for the needs of general aviation or any of its commercial siblings. This was all done for the military.

Since each grease manufacturer is different, I propose you use what is common to all aerospace lubricant manufacturers in the interest of safety. First, all Mil-Spec grease must have a manufacturer date and a batch number. These items are mandatory, as each batch of grease is issued its own certificate of analysis (COA). Some grease manufacturers will stamp the grease shipping box with the letters “DOM” for “date of manufacturer.” Either way, this is information all lubricant manufacturers will have. Secondly, have your parts professional research the actual shelf life date for each individual grease. Mobilgrease 28 has a shelf life of six years, whereas Mobilgrease 27 only has a two-year shelf life. Once you know the exact shelf life for each item, label the lubricants in your parts room and tool box. Each label should have the date of manufacturer and the expiration date as specified by the manufacturer. If, in the case of a product made from someone like Shell, simply write the retest date in the place of the expiration date.

I have been asking several people in the aviation lubricant industry and in the FAA when will somebody come forward and present a blanket document that covers the issue in definite and concise concerning this topic. I have gotten a lot of blank stares in return and witnessed a lot of shoulder shrugging over this issue. The best course of action is to research and follow exactly what the product manufacturer states. I am hopeful that in the next few years, we all will have a better option in regards to retesting lubricants using local labs. Until then, it is my recommendation that you stock little of the aviation lubricant products and diligently monitor them often.  

Norman Chance is president and CEO of Chance Aviation, an international aircraft parts distributor headquartered in Indianapolis. He has a degree in aircraft maintenance from Vincennes University and a degree in aeronautics from Embry-Riddle University. He holds an FAA A&P certificate.