**5. Lubricant specifications**

Lubricant compatibility as a global problem requires standardization; however, the decision was made to not require a single supplier or a single highly specified composition. This would eliminate competition and would stop the development of better compositions. Instead, a series of detailed specifications were adopted for lubricants based on sets of properties. In the end, commercial jet aircraft lubricants are primarily governed by standard SAE-5780. Military jet aircraft generally fall under two standards based on different low temperature require‐ ments. U.S. Air Force (USAF) aircraft typically use the MIL-PRF-7808 [10] specification because of the need for pumpability at lower temperatures. Naval aviation uses MIL-PRF-23699 [11], which has better high temperature properties. In most areas, MIL-PRF-23699 and SAE-5780 specifications are quite similar and many currently available products meet both of these specifications. These specifications undergo regular review and revision. In addition, MIL-PRF-7808L is now available in two grades with different viscosities: grade 3, a 3 cS viscosity fluid, and grade 4, a 4 cS viscosity fluid. The two grades require changes in the basestock composition, although similar chemistries are used. MIL-PRF-23699 is available in three types: standard, corrosion inhibiting (CI), and high thermal stability (HTS). The three MIL-PRF-23699 types differ primarily in the additive package used. A summary of the different lubricants is given in **Table 1**.


**Table 1.** Common lubricant specifications used in aircraft engines.

**Corrosion Inhibition (SAE ARP 4240)**—Determined by ball corrosion testing, this attribute is only required for MIL-PRF-23699 oils of C/I class. Oils of the C/I class are primarily used in

**Storage Stability Tests**—This test ensures that all of the lubricants remain miscible during

**Acid Assay (FED-STD-791, Method 3500)**—In this method, the component acids used in the preparation of the ester are measured. This is needed to ensure that different batches of the

**Trace Metals**—The trace metal content of new oils is controlled to provide a good baseline for

**Spectrographic Oil Analysis Program (SOAP)**—The concentration of several wear metals (Fe, Ag, Cr, Al, Mg, Ti, Mo, and V) are measured regularly by atomic emission either using a rotating carbon electrode or by inductively coupled plasma (ICP-OES). As an engine is used, the concentrations of wear metals increase. If the normal wear pattern of the engine is known, changes in the metal concentrations that are not consistent with the pattern can be an indicator of impending problems. Excessive wear metals in the oil are an indication that some type of maintenance is needed and the metals present may provide an indication of the area of concern.

Lubricant compatibility as a global problem requires standardization; however, the decision was made to not require a single supplier or a single highly specified composition. This would eliminate competition and would stop the development of better compositions. Instead, a series of detailed specifications were adopted for lubricants based on sets of properties. In the end, commercial jet aircraft lubricants are primarily governed by standard SAE-5780. Military jet aircraft generally fall under two standards based on different low temperature require‐ ments. U.S. Air Force (USAF) aircraft typically use the MIL-PRF-7808 [10] specification because of the need for pumpability at lower temperatures. Naval aviation uses MIL-PRF-23699 [11], which has better high temperature properties. In most areas, MIL-PRF-23699 and SAE-5780 specifications are quite similar and many currently available products meet both of these specifications. These specifications undergo regular review and revision. In addition, MIL-PRF-7808L is now available in two grades with different viscosities: grade 3, a 3 cS viscosity fluid, and grade 4, a 4 cS viscosity fluid. The two grades require changes in the basestock composition, although similar chemistries are used. MIL-PRF-23699 is available in three types: standard, corrosion inhibiting (CI), and high thermal stability (HTS). The three MIL-PRF-23699 types differ primarily in the additive package used. A summary of the different lubricants is

marine applications.

44 Recent Progress in Some Aircraft Technologies

extended storage periods of up to 3 years.

the spectrographic oil analysis program.

**5. Lubricant specifications**

given in **Table 1**.

lubricant are similar to the original qualified lubricant.

Some of the specifications for each of the lubricants are included in **Table 2**.


**Table 2.** A summary of selected performance specifications for MIL-PRF-7808L [10] and MIL-PRF-23699 [11] lubricants.
