Abstract: The tendency of liquid hydrocarbon-based fuels, such as kerosene, diesel l, and jet fuel toward the formation of fuel-insoluble solids during thermal stress in an aircraft fuel system is assessed by an accelerated test method comprising passing a predetermined quantity of a sample of fuel through a heated test section maintained at a predetermined temperature, at a predetermined fuel flow rate. A metal test strip, weighed before the test, is clamped in the heated test section during the test. It is weighed again after the test and the weight of solids buildup during the test is determined. It is related directly to the tendency of the fuel to form fuel-insoluble solids during thermal stress. A filter is weighed before the test. It is then connected to the outlet of the test section, and liquid fuel leaving the test section during the test is passed through it. The filter is then weighed a second time and the weight increase of the filter due to its capture of fuel-insoluble solids is calculated.

Abstract: Reactions leading to insoluble sediments formation in distillate fuel are accelerated by forcing oxygen into solution in the fuel at pressures of between about 90 and 110 psig and then stressing the fuel under conditions of accelerated storage at temperatures of between about 40.degree. C. to 100.degree. C. The method then makes use of gravimetric determination of the total insolubles formed. The stability of the fuel over a period of time as well as its comparative stability to other fuels can then be predicted from the amount of insolubles formed. The method can be carried out by using a specialized pressure vessel.