Why can’t you use just one grease in your aircraft?
It’s a question I get a lot from pilots. And while I understand the reasoning for this, I always recommend against the practice.
Why?
One reason is that not all greases are created equal — and the differences can cause problems.
Let’s start with the basics: Greases are not thick oil, but rather base oils that are thickened just like gravy in your kitchen.
And in aviation, there are many differences between base oils and thickeners.
The first variable is the base oil that begins the process.
For example, in general aviation, most applications use Aeroshell Grease 5 or 6 or the equivalent. These are both mineral oil-based products, but with varied viscosity base oils.
Grease 5 uses a thick base oil similar to that contained in Aeroshell 120 grade engine oil. It is primarily used in wheel bearings and can stand up to the high temperatures from touch and go training in the summer heat.
Grease 6 is similar, but is blended with a thinner base oil more like a grade 65 or even thinner engine oil.
The problem is the thick base oil in Grease 5 works in wheel bearings at cold temperatures because of the high torque that is applied to wheel bearings. But if it is used in flight control systems, it will make for very stiff controls at cold temperatures.
Conversely, Grease 6 works well in flight control systems even at cold temperatures, but would not hold up in wheel bearings, especially if subjected to multiple touch and goes at summer-time temperatures.
I have also received questions from people who look at the specification sheet for aviation greases and wish to use Aeroshell Grease 7 in all applications on their aircraft.
Again, this is not recommended. Even though Grease 7 has better low temperature characteristics than Grease 6 and better high temperature characteristics than Grease 5, it is blended with a synthetic base oil and it would probably ruin all of your airplane’s grease seals, plus leak everywhere.
The second component of grease that may not be compatible is the thickener system.
When engine oils are blended, the manufacturer just dumps the components into a tank and mixes it up.
However, with grease, it is more like a cooking show where the components are added in a certain sequence, heated, and then stirred and the mixture suddenly thickens to the desired consistency.
There are many different thickening systems and many are not compatible.
For example, if you should mistakenly mix Aeroshell Grease 14 into a bearing that contained Aeroshell Grease 5, you would end up with a soup-like product that would probably leak everywhere.
Then there are the many aircraft owners who have been using automotive or heavy-duty greases in their airplanes because of lower cost and better availability.
Why is this a problem?
It all comes back to compatibility. You can’t just mix two different greases.
This especially can be a problem should the aircraft change owners and the new owner starts using the approved grease.
That’s why it’s important that when buying a used aircraft, always go through the maintenance records to see how often the airframe has been serviced and with what products.
If it has been serviced using non-approved greases, you will need to completely clean the bearings and start over with the correct product.
How do you know what is the correct grease for each application?
The airframe maintenance recommendations for your airplane will specify what mil specification grease can be used at each lubrication point — and only a grease that is qualified against that mil spec should be used.
Remember, when repacking wheel bearings, always use latex or rubber gloves for your protection and for the protection of the bearings.
Wash the bearing in a solvent like mineral spirits before inspecting the bearing.
Before repacking with the correct grease, wash the bearing with something like isopropyl alcohol, which will remove any film and provide better adhesion for the grease.
Another tip is if you are not re-installing the bearing immediately and plan to store it for some time: Do not wrap it in a paper or cloth towel. This will wick out the base oil and cause a problem later.
For proper storage use an oil paper like that used for new bearings.
