Grease is probably the least talked about aviation lubricant.
What do you need to know about it to keep your airplane airworthy?
Let’s start with the basics: Grease is not thick oil. It is thickened oil. Oil companies take base stocks like those used in engine oils and then add thickener, similar to how a cook adds flour to meat juices to make a thick gravy.
And while engine oils are compatible with other brands or grades, greases may not be. So how do you know what grease to use on each part of your aircraft?
On all certified aircraft, the service manual will list what grease to use on all lubrication points of a particular airframe. It will usually list the mil spec that a grease must be qualified against. It’s important to know that this makes it the only product that can legally be used on that part of your airplane.
Some airframes may list the required greases by a Qualified Products List (QPL) instead of just the mil spec. A QPL is a directory of products, materials, or manufacturers that have been formally tested, inspected, and pre-approved to meet specific technical standards.
Another important thing to know: There will probably be more than one product needed for each airframe.
On many GA aircraft, manufacturers will call for Aeroshell Grease 5 (or its equivalent) for wheel bearings and Aeroshell Grease 6 for general airframe lubrication.
The one question I get asked the most about grease is: “Can I just use one or the other in the entire airframe so I do not need to stock two products and have two separate grease guns?”
The answer is no.
One of the big concerns with greases is compatibility.
Companies use different thickeners with their greases — and the thickeners do not mix with other types of thickeners.
For example, if you mix Aeroshell 5 with Aeroshell 14, which has a different type of thickener, you’ll end up with a soupy mix that will leak out and make a mess, while leaving the bearing with little or no protection.
What about greases that have the same thickener system, such as Aeroshell Grease 5 and Aeroshell Grease 6? Can you use them interchangeably?
Again, the answer is no.
That’s because Grease 5 has a much thicker base oil. At low temperatures, your flight controls will be very stiff and hard to move. Conversely, if you use Aeroshell Grease 6 in all of the lube points, including the wheel bearings, you may have shortened the life of the wheel bearings — especially if you do a few touch-and-goes and get the bearing near maximum temperature.
The bottom line? For best performance, always use only the product that is qualified for each lube point on your aircraft.
Application and Amount
The other important thing to know about grease is how to apply it and how much to use.
Most aircraft lube guides will give a flight hour or calendar time between applications. These are good guidelines, but may need to be changed for severe operating conditions, especially wheel bearings.
When repacking wheel bearings, always wash the bearings with a proper solvent and then carefully inspect for pits and other problems and replace if necessary.
For the next step, it is critical that you wear proper latex or rubber gloves, not only to protect your hands, but also to keep your grimy fingerprints off the bearing.
After the inspection — and keeping your gloves on — wash the bearing carefully with isopropyl alcohol (IPA) or something similar. This will remove the film left by the cleaning solvent and give a clean surface to repack the bearing with the right grease.
If you are not installing the wheel bearing immediately, do not wrap it in a paper towel. Use only an oily paper designed for storage. A paper towel will absorb the base oil and leave just the thickener and no proper protection for the bearing.
Source: Google
AeroShell 5 is made with standard mineral oil.High Evaporation Rate:
Under the high-heat friction of landings, mineral oil evaporates much faster than synthetic oils.Poor Thermal & Oxidative Stability: When subjected to heat and oxygen, mineral oils break down quickly, leading to rapid oil degradation and heavy oxidation.
The Microgel (Clay) Thickener Retains Less OilAeroShell 5 uses Microgel® (an organo-clay) as its thickening agent.High Bleeding Rate:
Clay greases function like a sponge but lack the molecular strength to hold onto mineral oil long-term. It releases its base oil (bleeding) aggressively, even while the aircraft is parked.Crusting/Hardening:
Once the oil separates and drains out or evaporates, only the dry clay thickener remains in the bearing, forming a hard, crusty residue that can cause bearing wear.
Poor Mechanical Stability (Shear Thinning)The immediate shock of landing forces a cold wheel bearing to accelerate from 0 to over 100 mph in a millisecond. ( ? )
Clay thickeners have lower mechanical stability compared to modern lithium complexes. Under extreme shearing forces, the grease structure physically breaks down and thins out, which can cause it to liquefy and fling away from the spinning rollers.
Poor Compatibility with Advanced AdditivesThe molecular structure of a clay thickener does not blend efficiently with modern chemistry. This limits the type and volume of advanced anti-wear, anti-fretting, and anti-corrosion additives that can be blended into AeroShell 5.
Compared to modern synthetic, lithium-complex greases like AeroShell 58.Summary of Performance DifferenceModern alternatives like Mobil SHC 100 or AeroShell 58 utilize synthetic oils bonded to a lithium-complex soap. This synthetic bond naturally retains its consistency, releases oil only under precise thermal demands, and will not dry out or form a hard crust.