15.4 Engine Preservation, Storage & Depreservation

15.4 Engine Preservation, Storage & Depreservation

Quick Answer: FAA-H-8083-32B names three types of engine storage: active ("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 ("an aircraft and engine that is not flown for 30 to 90 days"), and indefinite ("an aircraft not to be flown for over 90 days or is removed from the aircraft for extended time"). Protection comes from corrosion-preventive compounds, "petroleum-based products that form a wax-like film over the metal to which they are applied," and from desiccants — cobalt-chloride-treated silica gel in dehydrator plugs and humidity indicator envelopes that read bright blue at low humidity, lavender at 30 percent, and natural or white at 60 percent. The handbook's hard rule after the final cylinder spray: "The crankshaft must not be moved after this final spraying, or the seal of corrosion-preventive mixture between the pistons and cylinder walls are broken."

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Why an Idle Engine Corrodes

The handbook's framing is worth internalizing because it explains every step that follows:

"An engine in service is in a sense self-purging of moisture, since the heat of combustion evaporates the moisture in and around the engine, and the lubricating oil circulated through the engine temporarily forms a protective coating on the metal it contacts."

Stop flying and both protections disappear at once. "The normal combustion process creates moisture and corrosive by-products that attack the unprotected surfaces of the cylinder walls, valves, and any other exposed areas that are unprotected." The moisture and the acidic residue are already inside the engine when it shuts down; what changes is that nothing is boiling them off any more, and the oil film is draining away.

Two modifiers the handbook names:

• Recent high utilization helps. "In engines that have accumulated 50 hours or more time in service in a short period, the cylinder walls have acquired a varnish that tends to protect them from corrosive action; engines under favorable atmospheric conditions can remain inactive for several weeks without evidence of damage by corrosion."
• Climate dominates. "Aircraft that operate close to oceans, lakes, rivers, and humid regions have a greater need for engine preservation than engines operated in dry low humid areas."

"Engines that are not flown regularly may not achieve normal service life because of corrosion in and around the cylinders." Corrosion, not wear, is what kills a low-utilization engine.

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The Three Storage Categories

The exam trap is the active storage row: it is a storage category even though the procedure is simply "run the engine properly." The two numbers that define it — one continuous hour and 165 °F to 200 °F oil temperature — matter because a short ground run that never reaches operating temperature does the opposite of preserving. It puts fresh combustion moisture into a cold crankcase and then leaves it there.

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Corrosion-Preventive Compounds

"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: Three Forms of MIL-C-6529C

"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."

Note the oil specifications: MIL-L-22851 is the ashless dispersant specification and MIL-L-6082C (SAE J1966) is the straight mineral specification covered in Section 9.1. The 3:1 blend ratio for Type I and the turbine-only restriction on Type III are both testable.

The Hard Rule on Mixing

"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."

Pouring a can of compound into a sump full of used oil produces an unknown concentration in an unknown base, and that is exactly the outcome this rule exists to prevent.

The Heavy Compound

"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."

Why Compound Alone Is Not Enough

"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 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."

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Desiccants and the Humidity Indicator

"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."

Silica gel appears in two forms:

1. Bags placed "around and inside various accessible parts of a stored engine."
2. Dehydrator plugs — "clear plastic 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, and that additive is what turns a desiccant into an instrument. "This additive makes it possible for the plugs to indicate the moisture content, or relative humidity, of the air surrounding the engine."

The Colour Scale — Memorize This

   RELATIVE HUMIDITY          DEHYDRATOR PLUG / INDICATOR COLOUR
   -------------------------  ----------------------------------------
   Low                        BRIGHT BLUE   <- engine is dry, corrosion
                                               held to a minimum
   Increasing                 shades of blue growing progressively lighter
   30 percent                 LAVENDER      <- below this, corrosion does
                                               not normally take place
   Above 30 percent           fading through the various shades of PINK
   60 percent                 NATURAL / WHITE  <- desiccant is saturated

The handbook's exact wording: "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, until at 60 percent relative humidity it is a natural or white color."

And the interpretation rule: "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 engine has so little moisture that internal corrosion is held to a minimum."

Regeneration. "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."

Humidity indicator envelopes. "This same cobalt-chloride-treated silica gel is used in humidity indicator envelopes. These envelopes can be fastened to the stored engine so that they can be inspected through a small window in the shipping case or metal engine container." The window is what makes a periodic check possible without breaking the seal.

Storage of the desiccant itself. "All desiccants are sealed in containers to prevent their becoming saturated with moisture before they are used. Care should be taken never to leave the container open or improperly closed." A desiccant that arrives pink is useless.

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Preserving a Reciprocating Engine: The Sequence

1. Run the engine on preservative oil. "Before an engine is placed in temporary or indefinite storage, it should be operated and filled with a corrosion-preventive oil mixture added in the oil system to retard corrosion by coating the engine's internal parts. Drain the normal lubricating oil from the sump or system and replace with a preservative oil mixture according to the manufacturer's instructions. Operate the engine until normal operating temperatures are obtained for at least one hour."
2. Safe the propeller and open the cylinders. "Always take the appropriate precautions when turning or working around a propeller. After the flight, remove all the spark plug leads and the top spark plugs."
3. Spray each cylinder at bottom center. "Spray the cylinders by inserting the nozzle of the spray gun into each spark plug hole and playing the gun to cover as much area as possible. Before spraying, each cylinder to be treated should be at the bottom center position and the oil at room temperature. This allows the entire inside of the cylinder to become coated with corrosion-preventive mixture." Bottom center exposes the maximum cylinder-wall area to the spray.
4. Respray with no piston at top center. "After spraying each engine cylinder at bottom center, respray each cylinder while the crankshaft is stationary with none of the cylinder's pistons at top dead center."
5. Do not move the crankshaft again. "The crankshaft must not be moved after this final spraying, or the seal of corrosion-preventive mixture between the pistons and cylinder walls are broken. Air can then enter past the pistons into the engine. Also, the coating of corrosion-preventive mixture on the cylinder walls is scraped away, exposing the bare metal to possible corrosion."
6. Placard it. "The engine should have a sign attached similar to the following: 'DO NOT TURN CRANKSHAFT—ENGINE PRESERVED PRESERVATION DATE ____________.'"
7. Install dehydrator plugs. "Dehydrator plugs are screwed into the spark plug opening of each cylinder. If the engine is to be stored in a wooden shipping case, the ignition harness leads are attached to the dehydrator plugs with lead supports." For an engine stored horizontally in a container, special ventilatory plugs are installed in the spark plug holes instead.
8. Treat and seal the exhaust ports. "Because the residue of exhaust gases is potentially very corrosive, a corrosion-preventive mixture must be sprayed into each exhaust port, including the exhaust valve. After the exhaust ports have been thoroughly coated, a moisture-proof and oil-proof gasket backed by a metal or wooden plate should be secured over the exhaust ports using the exhaust stack mounting studs and nuts."
9. Seal the intake. "If the carburetor is to remain on the engine during storage, the throttle valve should be wired open and a seal installed over the air inlet. But, if the carburetor is removed and stored separately, the seal is made at the carburetor mounting pad." "Silica gel should be placed in the intake manifold to absorb moisture. The silica gel bags are usually suspended from the cover plate. This eliminates the possibility of forgetting to remove the silica gel bags when the engine is eventually removed from storage." That last sentence is the design rationale, and it is a good question stem.
10. Protect the propeller shaft. "The propeller shaft and propeller shaft thrust bearing must be coated with the compound. Then, a plastic sleeve, or moisture-proof paper, is secured around the shaft, and a threaded protector cap is screwed onto the propeller retaining nut threads."
11. Seal every remaining opening. "All engine openings into which dehydrator plugs (or ventilatory plugs if the engine is stored in a metal container) have not been fitted must be sealed," including points such as the oil inlet and outlet where compound can seep out.

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Depreservation and Return to Service

"An aircraft engine and its accessories that have been in storage must undergo careful depreservation and inspection before they may be installed in an aircraft. This involves more than removing an engine from its container and bolting it to the aircraft."

Opening the Container

• "If the engine is stored in a pressurized metal container, the air valve should be opened to bleed off the air pressure. Depending upon the size of the valve, the air pressure should bleed off in somewhat less than 30 minutes." Only then are the bolts holding the two sections removed and the top section lifted clear with a hoist at the hoisting points.
• For a wooden shipping case: "carefully break the seal of the protective envelope and fold it down around the engine. Remove the dehydrating agent or desiccant bags and the humidity indicator from the outside of the engine."

Depreserving the Engine

1. Remove every cover and inspect as you go. "All covers must be removed from the points where the engine was sealed or closed with ventilatory covers, such as the engine breathers, exhaust outlets, and accessory mounting-pad cover plates. As each cover is removed, inspect the uncovered part of the engine for signs of corrosion."
2. Read the dehydrator plugs before you discard them. "As the dehydrator plugs are removed from each cylinder, make a very careful check of the walls of any cylinder for which the dehydrator plug color indicates an unsafe condition. Care is emphasized in the inspection of the cylinders, even if it is necessary to remove a cylinder." A pink plug is a work order, not a disposable item.
3. Drain the compound — and watch for hydraulic lock. "On radial engines, the inside of the lower cylinders and intake pipes should be carefully checked for the presence of excessive corrosion-preventive compound that has drained from throughout the interior of the engine and settled at these low points. This excessive compound could cause the engine to become damaged from a hydraulic lock (also referred to as liquid-lock) when a starting attempt is made." The handbook gives two removal methods: a hand pump through the spark plug hole, or the more positive method of removing the lower intake pipes and opening the intake valve by rotating the crankshaft so the compound drains out through the open intake valve.
4. Clean the oil screens. "The oil screens should be removed from the engine and thoroughly washed in an approved solvent to remove all accumulations that could restrict the oil circulation and cause engine failure. After the screens are cleaned, immerse them in clean oil and then reinstall them in the engine."
5. Remove the intake desiccant. "When the cover has been removed from the intake area, the silica gel desiccant bags... must be removed from the engine area."
6. Service the propeller shaft. "Remove the protective covering from the propeller shaft and wash all corrosion-preventive compounds from both the inside and outside surfaces of the shaft. Then, coat the propeller shaft lightly with engine oil."
7. Turbine engines. "Turbine engines require the removal of several covers on many external areas on the engine."
8. Final clean. "As a final check, see that the exterior of the engine is clean. Usually a quantity of compound runs out of the engine when the dehydrator plugs and oil screens are removed. To clean the engine, spray it with an approved commercial solvent."

Accessories Have Their Own Rules

"An engine's performance is no better than that of its accessories. Though the engine has been completely overhauled and is in top condition, any oversight or error in installing the accessories can result in improper engine operation or even irreparable damage to it."

• Check the storage time first. "Before depreserving any of the accessories enclosed with the engine, consult the storage data usually stenciled on the outside of the engine container or the records enclosed with the engine to determine how long the engine and accessories were in storage. Certain accessories that normally accompany an engine from overhaul are considered unsafe for use if their time in storage has exceeded a specified period. This time varies according to the limits prescribed by the manufacturer."
• Inspect anything carried over from the old engine. "Any accessory that has been removed from an old engine that can be installed on the new one must be given a thorough inspection to determine its condition. This inspection includes a check for general condition, cleanliness, absence of corrosion, and absence of wear as evidenced by excessive play in the moving parts."
• Replace accessories after an internal failure, regardless of time. "Some accessories must be replaced, regardless of their operating time, if the engine is being changed because of internal failure. Such accessories may have been contaminated by metal particles carried into their operating mechanisms by the engine oil that lubricates them." This is a direct consequence of the metal-in-the-oil logic in Section 14.5.
• Before installing any replacement accessory, "check it visually for signs of corrosion and for freedom of operation. Always wipe the mounting pad, flange, and coupling clean before mounting the accessory, and install the proper gasket between the mounting pad and the accessory mounting flange. Lubricate the accessory drive shaft if so indicated in the manufacturer's instructions."

The Test-Cell Connection

An engine that has just been overhauled is preserved before it ships. FAA-H-8083-32B describes how: "After an engine has successfully completed test requirements, it is then specially treated to prevent corrosion, if it is shipped or stored before being installed in an aircraft. During the final run-in period during testing... the oil system is serviced with a mixture of corrosion-preventive compound and engine oil. The temperature of this mixture is maintained at 105 °C to 121 °C. Near the end of final run-in, corrosion-preventive mixture (CPM) is used as the engine lubricant. The engine induction passages and combustion chambers are also treated with CPM by an aspiration method. CPM is drawn or breathed into the engine."

That is why a new or overhauled engine arrives already coated inside and why the depreservation steps above are not optional even on a zero-time engine.

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Independent Prep Note

Independent FAA AMT Powerplant prep by OpenExamPrep. Not sponsored by or affiliated with the Federal Aviation Administration (FAA). Technical data compiled from FAA-H-8083-32B, FAA AC 43.13-1B, and 14 CFR Parts 43 and 65.