Technical Glossary

Technical Glossary

A quick guide to aviation lubrication terms

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Additives
Any compound added to oil for the purpose of improving its properties.

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Approved aviation oil
This refers to any brand name oil produced under the following specifications:

  • Mineral Grades—SAE J1966 or MIL-L-6082 (obsolete)—This SAE standard establishes the requirements for non-dispersant, mineral lubricating oils to be used in four-stroke cycle piston aircraft engines. Users should consult their airframe or engine manufacturer's manuals for the latest listing of acceptable lubricants.
  • Ashless Dispersant Grades—SAE J1899 or MIL-L-22851 (obsolete)—This SAE standard establishes the requirements for lubrication oils containing ashless dispersant additives to be used in four-stroke cycle, reciprocating piston aircraft engines. Users should consult their airframe or engine manufacturer manual for the latest listing of acceptable lubricants.

CAUTION: Some approved oils contain anti-scuffing additives that should NOT be used during engine break-in. Always refer to the label to ensure that the oil is correct for your application.

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Ashless
When burned, does not form a metallic ash, which could be a source of pre-ignition.

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Ashless dispersant
A cleanliness additive for engine oils that is widely used in aviation piston engines. Ashless dispersants do not contain metallic compounds and have no adverse affect on the break-in process. (See also "Dispersant".)

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Boundary lubrication
A condition that occurs when two surfaces are not completely separated by the lubricant. Boundary lubrication often requires the use of EP additives.

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Detergent
A cleanliness additive made of metallic compounds to protect from sludge and varnish build-up. Detergents are not allowed in aviation piston engine oils qualified under SAE J-1899 (obsolete MIL-L-22851) or SAE J-1966 (obsolete MIL-L-6082).
Note: No current aviation engine oil meeting the SAE J-1899 specification contains detergent compounds.

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Dispersant
An ashless engine oil additive that helps prevent sludge, varnish and other engine deposits by breaking up insoluble contaminant particles. These microscopic particles are kept suspended (or "dispersed") in the oil so they can be carried to the oil filter or until the oil is drained when no filter is installed.

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Hydrodynamic lubrication
A system of lubrication in which the shapes and relative motion of the sliding surfaces cause the formation of a full fluid film that has enough pressure to separate the surfaces.

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Micron (μm)
A unit of measure = 4 x 10-5 inches (1 millionth of a meter). It is used to measure oil film thickness and is also useful in specifying oil filter capabilities.

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Mineral oil
This is a lubricating base oil distilled from crude oil. This is also a term used to distinguish it from synthetic base oils. In aviation circles, this typically refers to un-additized oil often used to break-in new or newly rebuilt piston engines.

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Multiviscosity
Engine oil that meets the requirements of more than one SAE viscosity grade classification by passing tests at both high and low temperatures. Viscosity Index (VI) Improvers are employed and the designation "W" indicates winter grade.
Note: Multiviscosity aviation piston engine oils have the advantage of quicker pumping at start-up, faster flow to the upper valve train, reduced strain on the battery and starter, and a thicker oil film at high temperatures versus straight grades.

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SAE grade
The system used by engine manufacturers to define the lubricating oil viscosity requirements for their engines.

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SAE viscosity vs. MIL grade:

SAE
Viscosity
MIL
Aviation Grade
30 65
40 80
50 100
60 120

 

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Scuffing
Refers to severe adhesive wear resulting from metal transferring from one surface to a mating surface due to solid phase welding or abrasive effects between sliding surfaces.

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Sludge
A coagulated mass, often containing foreign matter, formed at low temperature in piston engines from oil oxidation residues, carbon and water.

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Synthetic oil
Lubricating fluid made by chemically reacting materials of a specific chemical composition to produce a compound with planned and predictable properties.
Note: Full synthetics have been unsuccessful in aviation piston engines due to incompatibility with leaded fuel. Synthetic blends are sometimes used. However, we have not found these to be of similar added value (cost vs. performance) in aviation piston engines as they are in automotive or heavy-duty trucking applications.

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TCP
Tricresyl phosphate or triphenyl phosphate. An oil additive that acts as an anti-scuff agent in a lubricant or a lead-scavenger when used as a fuel additive. This property in a lubricant is called for with specific Lycoming engines that require the use of LW-16702. (See also "Scuffing".)

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VI Improver
Viscosity Index Improvers are chemical polymers that expand with increasing temperature to counteract oil thinning. The higher the viscosity index, the smaller the relative change in viscosity with temperature.

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Viscosity
The measure of liquid resistance to flow at a specific temperature.

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W (e.g., 20W-50)
Grade numbers followed by the letter "W" (i.e., 5W-, 10W-, etc.) identify oils suitable for winter service. Viscosities for those oils are determined at various cold temperatures on a laboratory Cold Cranking Simulator (CCS) and Mini Rotary Viscometer (MRV). Numbers without the W (i.e., 50, 60, etc.) identify oils that are tested at high temperatures only. (See also "Multiviscosity".)

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