in a shipping case or container, some dehydrating (moisture  removing) agent must be used to remove the moisture from  the air in and around the engine. preserved to a varying extent, depending upon how long it  is to be inoperative. There are three types of engine storage:  active engine, temporary, and indefinite. An engine in active  storage is defined as having at least one continuous hour of  operation with an oil temperature of at least 165 °F to 200 °F  and storage time not to exceed 30 days. Temporary storage  describes an aircraft and engine that is not flown for 30 to  90 days, and indefinite storage is for an aircraft not to be  flown for over 90 days or is removed from the aircraft for  extended time.  There are a number of substances (referred to as desiccants)  that can absorb moisture from the atmosphere in sufficient  quantities to be useful as dehydrators. One of these is silica  gel. This gel is an ideal dehydrating agent since it does not  dissolve when saturated. As a corrosion preventive, bags of silica gel are placed around  and inside various accessible parts of a stored engine. It is  also used in clear plastic plugs, called dehydrator plugs,  that can be screwed into engine openings, such as the  spark plug holes. Cobalt chloride is added to the silica gel  used in dehydrator plugs. This additive makes it possible  for the plugs to indicate the moisture content, or relative  humidity, of the air surrounding the engine. The cobalt- chloride-treated silica gel remains a bright blue color with  low relative humidity; as the relative humidity increases, the  shade of the blue becomes progressively lighter, becoming  lavender at 30 percent relative humidity and fading through  the various shades of pink  [Figure 8-18],  until at 60 percent  relative humidity it is a natural or white color. Some types  of dehydrator plugs can be dried by removing the silica gel  and heating the gel to dry it out, returning it to its original  blue color.  [Figure 8-19]  When the relative humidity is less  than 30 percent, corrosion does not normally take place.  Therefore, if the dehydrator plugs are bright blue, the air in  The preservation materials discussed are used for all types  of engine storage. Corrosion-preventive compounds are  petroleum-based products that form a wax-like film over the  metal to which they are applied. Several types of corrosion- preventive compounds are manufactured according to  different specifications to fit the various aviation needs. The  type mixed with engine oil to form a corrosion-preventive  mixture is a relatively light compound that readily blends  with engine oil when the mixture is heated to the proper  temperature. The light mixture is available in three forms: MIL-C-6529C  type I, type II, or type III. Type I is a concentrate and must be  blended with three parts of MIL-L-22851 or MIL-L-6082C  (SAE J1966) grade 1100 oil to one part of concentrate. Type  II is a ready-mixed material with MIL-L-22851 or grade 1100  oil and does not require dilution. Type III is a ready-mixed  material with grade 1010 oil for use in turbine engines only.  The light mixture is intended for use when a preserved engine  is to remain inactive for less than 30 days. It is also used to  spray cylinders and other designated areas. The desired proportions of lubricating oil, and either heavy or  light corrosion-preventive compound, must not be obtained  by adding the compound to the oil already in the engine. The  mixture must be prepared separately before applying to the  engine or placing in an oil tank. A heavy compound is used for the dip treating of metal  parts and surfaces. It must be heated to a high temperature  to be sufficiently liquid to effectively coat the objects to be  preserved. A commercial solvent, or kerosene spray, is used  to remove corrosion-preventive compounds from the engine  or parts when they are being prepared for return to service. Although corrosion-preventive compounds act as an insulator  from moisture, in the presence of excessive moisture, they  eventually break down and corrosion begins. Also, the  compounds eventually dry because their oil base gradually  evaporates. This allows moisture to contact the engine’s metal  and aids in corroding it. Therefore, when an engine is stored  8-19 Figure 8-18.  Dehydrator plug “pink” showing high humidity  (Sacramento Sky Ranch).  Corrosion-Preventive Compounds Dehydrating Agents