In a perfect world, a column on aviation grease would be very short and direct.
I would tell people to consult their airframe manual and only use the mil spec grease called out in the manual, and at the given amount and intervals for each lubrication point.
But, alas, the real world raises its ugly head and problems can — and do —arise.
Most of the problems stem from a belief that all grease is created equal and as long as a bearing has some sort of grease in it, it will be fine.
The most common problem is that pilots and FBOs do not want to have separate inventory and equipment for every application.
When we go back to the basics of grease, we learn that grease is not thick oil, but rather oil base stocks that are thickened.
This is much like making gravy at Thanksgiving. You combine the turkey drippings with, say, chicken stock, which forms a liquid mixture, to which you add flour or corn starch. With heat this changes the liquid to a thick sauce.
In the same way, greases start as oil base stocks and are then thickened by a specific thickening agent.
The most common greases used on general aviation aircraft are Aeroshell Grease 5 and 6. They are similar, but have very different base oils.
The base oil for Grease 5 is similar to that in Aeroshell 120 piston engine oil, compared to Grease 6, which has base oils more like Aeroshell W65.
In an aircraft, Grease 5 is generally used in wheel bearings, while Grease 6 is used for most airframe application points.
One Size Does Not Fit All
If you want to use just one grease for all applications, this will cause problems.
For instance, if you use Grease 6 in wheel bearings, you will not have adequate protection, especially if you make a couple of panic stops and the brakes and wheel bearings get really hot.
Conversely, if you use Grease 5 for everything, your control surfaces will be very stiff and unsafe, especially in cold weather or at altitude.
If you should try Aeroshell Grease 7, it has a pure synthetic base oil and will eat your seals out quickly.
The other significant part of a grease composition is the thickening agent. Some agents are compatible with other chemistry types, but many are not.
For example, say you have a helicopter that requires Aeroshell Grease 14, which has a soap-based thickener. If you use the same greasing equipment with Grease 14 and then switch over and use Grease 5 or 6 on other applications points, the small amount of contamination between the two greases will produce a soupy mess that will probably run out and not properly lubricate that application.
It is best to have dedicated application equipment for each product.
Tips to Get the Most Out of Grease
One of the problem areas with grease becomes evident with the purchase of a used aircraft. The maintenance log for the aircraft should have a record of which grease was used, where, and when.
If it does not, you may want to do some extra work. Look up in your airframe manual which grease to use, when it should be lubed, and how much should be applied.
On plain bearings, it is usually OK to purge the application until good clean product comes out.
Always wipe the excess grease so that it does not collect dirt.
If your manual states do not over grease or limits the amount to be applied, do not try to purge these applications because it can blow out the seals and allow dirt and moisture into the bearing.
On things like wheel bearings, you will need to remove the bearing and re-pack them. When re-packing bearings always use rubber or latex gloves for your safety and to keep your greasy paw prints off the clean bearings.
I usually wash the bearings in mineral spirits and then carefully inspect the bearings and races for pits and scratches. If they are OK, you need to add an additional step, which is to wash the bearing in alcohol like IPA to remove the oil film so that the grease will adhere better to the metal surfaces. Now pack the bearing and install.
Caution: If you are not going to install the bearings right away and want to keep them in inventory, do not wrap them in a paper towel or shop towel to store. If you are going to store them, you will need oil paper like what new bearings are wrapped in, then place them in a paper or cardboard box, not a plastic bag. A plastic bag will collect moisture and the paper towel will wick out the base oil from the bearing.
Finally, there is cleaning your aircraft. Most owners take pride in their aircraft and want to keep it clean. Many of them use a power washer to clean difficult spots, like around the brakes.
The problem here is that if you hit the back of the wheel bearing, it is very easy to force water and detergent past the seal, which will greatly increase corrosion in the bearing and reduce its service life.