As the temperature plummets and the snow flies, the hangar door may not open until spring. Here’s how to put the airplane to bed correctly.
More than ever the need for following manufacturer guidelines for aircraft and engine storage has come to the fore. This is particularly true when the aircraft is parked outside or in other than those relatively limited, dry areas of the country that also do not have industrial pollutants to deal with.
Unfortunately some of the greatest concentrations of aircraft are along the coasts where the environment is not at all friendly to airplanes not properly kept up. With the onset of the winter season we like to publish a refresher on the importance of proper preparation of the plane.
According to Lycoming “Our experience has shown that in regions of high humidity, active corrosion can be found on cylinder walls of new engines inoperative for periods as brief as two days. In engines that have accumulated 50 hours or more time in service in a short period, the cylinder walls will have acquired a varnish that tends to protect them from corrosive action; such engines under favorable atmospheric conditions can remain inactive for several weeks without evidence of damage by corrosion. Aircraft operated close to oceans, lakes, rivers and in humid regions have a greater need for engine preservation than engines operated in arid regions.”
Conversely, for those engines subject to moderate amounts of blow-by and relatively high oil consumption, accumulating moisture and acid in the oil is a given, regardless of how often the aircraft flies.
In some respects, we can do only so much to keep the damaging impact of corrosion to a minimum, but some form of storage should be considered when limited operation and frequent periods of downtime are expected. This would also be applicable to engines removed from the aircraft awaiting reinstallation.
The TCM bulletin on engine storage is Service Information Letter 99-1, which has a specific checklist of products and procedures. For Lycoming engines the publication is Service Letter L 180B.
We should also not forget the need to review the storage protocols that the airframe maker has listed. Some of these are found in the POH, with expanded information usually found in the maintenance manuals.
Particularly important for owner safety is doing a very thorough preflight check before putting the airframe back into flying status. Both corrosion and varmints have a way of doing their damage, which is not always so obvious that a cursory preflight would catch it.
For example, the engine compartment should be fully inspected, which on a Bonanza is easy, but not so much so on a Mooney. But it needs to be done to find nests and such that could block intake systems, cause overheating or start a fire.
To quote Lycoming again: “Engine temperature and length of operating time are very important in controlling rust and corrosion. The desired flight time for air cooled engines is at least one continuous hour at oil temperatures of 165F to 200F at intervals under 30 days, depending on location and conditions.”
“This one hour does not include taxi, take-off and landing time. If recommended oil temperatures are not obtainable, contact the aircraft manufacturer for availability of oil cooler winterization plates.” These plates are recommended at temperatures well above freezing in some Pipers, for example.
To Preserve or Not
It is generally accepted that engines that are not operated in flight to normal operating temperatures at least once a week should be managed under some preservation or storage program to reduce the effects of corrosion. We realize that in today’s flying environment this is unlikely, but you need to be aware of the guidelines and act accordingly.
It should be noted that ground running of engines is not a substitute for in-flight operation of an aircraft engine to dispel moisture. Ground running doesn’t get the engine hot enough, and tends to cause uneven heating at higher power. It’s likely much worse to ground run an engine that to just leave it alone.
All preservation/storage methods are concerned with one goal-keeping moisture and other corrosive agents from contacting metal surfaces by placing some type of protective coating or barrier between the metal and the corrosive environment.
Pulling the Prop Through
Note the factory is quoted verbatim here per Lycoming SL 180B: “Pulling engines through by hand when the aircraft is not run or flown for a week or so is not recommended. Pulling the engine through by hand prior to start or to minimize rust and corrosion does more harm than good. The cylinder walls, piston, rings, cam and cam followers only receive splash and vapor lubrication. When the prop is pulled through by hand, the rings wipe oil from cylinder walls.”
“The cam load created by the valve train wipes oil off the cam and followers. After two or three times of pulling the engine through by hand without engine starts, the cylinders, cam and followers are left without a proper oil film. Starting engines without proper lubrication can cause scuffing and scoring of parts resulting in excessive wear.”
Note that pulling the prop through prior to starting is a different story. Here you will be starting the engine and it will be immediately receiving lubrication. Also, pulling the prop through in this situation will check for the condition of the valves i.e. you are feeling for any flat spots in the compression, or other untoward signs of abnormal prop feel as the pistons are cycled.
Both manufacturers agree that inactivity in excess of 30 days strongly suggests the need for some special preservation methods and chemicals, especially if the aircraft is located near salt water or similar humid environment.
Lycoming’s procedure is as follows:
- Install a preservative by one of the following methods:
- Drain the lubricating oil from the sump or system and replace with a preservative oil mixture. This preservation mixture consists of one part by volume MIL-C-6529C Type I concentrated preservative compound added to three parts by volume of MIL-L-6082C (SAE J1966), Grade 1100, mineral aircraft engine oil or oil conforming to MIL-C-6529C Type II. Follow carefully the manufacturer’s instructions before use.
- An alternative method is the use of Cortec VC1-326 preservative concentrate added to the original oil at a ratio of 1 part VC1-326 to 10 parts of oil.
With the magnetos off rotate the propeller by hand through sufficient rotation to remove excess preservative oil from the cylinders. Drain the remaining preservative from the engine through the sump.
Source for preservative oil:
Storage preparation per SIL 99-1:
- Remove the oil sump drain plug and drain the oil. Replace the drain plug, torque and safety. Remove the oil filter. Install a new oil filter, torque and safety. Service the engine to proper sump capacity with oil conforming to MIL-C-6529 Type II. (Aeroshell Fluid 2F is available through the Aeroshell Web site and aviation retailers.)
- Perform a ground run-up. Perform a pre-flight inspection and correct any discrepancies. Fly the aircraft for one hour at normal operation temperatures.
- After flight remove all spark plug leads and remove the top spark plugs. Protect the ignition lead ends with AN-4060 Protectors. Using a common garden sprayer or equivalent, spray atomized preservative oil that meets MIL-P-46002, Grade 1, at room temperature through upper spark plug hole of each cylinder with the piston at bottom dead center position.
- Rotate the crankshaft as opposite cylinders are sprayed. Stop the crankshaft with none of the pistons at top dead center.
- Re-spray each cylinder. To thoroughly cover all surfaces of the cylinder interior move the nozzle or spray gun from the top to the bottom of the cylinder.
- Install top spark plugs but do not install plug leads. (For long-term storage Install dehydrator plugs MS27215-1 or -2 in each of the upper spark plug holes. Make sure each plug is blue in color when installed. Attach a red “Remove Before Flight” streamer to each bag of desiccant. Place a bag of desiccant in the exhaust pipes and seal the openings.)
- Seal all engine openings exposed to the atmosphere using suitable plugs and covers. Attach a red ” Remove Before Flight” streamer at each location.
- Tag each propeller in a conspicuous place with the following notation on the tag: DO NOT TURN PROPELLER-ENGINE PRESERVED/PRESERVATION DATE: XX/XX/XX
TCM engines prepared for indefinite storage must have the cylinder dehydrator plugs visually inspected every 15 days. The plugs must be changed as soon as they indicate other than a dark blue color.
If the dehydrator plugs have changed color in one-half or more of the cylinders, all desiccant material on the engine must be replaced. The cylinder bores of all engines prepared for indefinite storage must be re-sprayed with corrosion preventive mixture every 90 days.
Dehydrator plugs, desiccants, and other storage material can be obtained from several aircraft supply houses such as Aircraft Spruce, Chief Aircraft, and wholesale suppliers such as Aviall.
Preserved in Pink
Although corrosion-preventive compounds act as an insulator from moisture and remain intact for long periods of time, they can eventually dry out due to normal evaporation of the solvents used to support the base stock. If the engine is exposed to very high ambient moisture levels, this loss in protection can go unnoticed by maintenance personnel.
For this reason, uninstalled engines should be stored in airtight containers packed with dehydrating agents. Engines fitted to inactive aircraft should also be “sealed” as much as possible to avoid entry of moisture-laden air. Dehydrating agents (also known as desiccants) absorb moisture from the atmosphere; the primary substance used is silica gel. This gel is ideal for use as a dehydrating agent because when saturated with moisture it does not dissolve.
Silica gel is often used in small bags stuffed into exhaust and intake openings. It is also the material used in spark-plug type dehydrator plugs screwed into each cylinder. Both are designed to absorb any moisture from the air within the engine and prevent moisture from making its way into the engine from outside.
Cobalt chloride added to the gel mixture indicates by its color the relative humidity in the local environment. A bright blue color indicates low moisture levels with no water absorption. As the relative humidity increases, the shade of blue becomes progressively lighter, with lavender indicating a humidity level of about 30 percent. Absorbing higher levels of moisture will result in the silica gel transitioning through various shades of pink, until eventually turning colorless (white) when reaching a 60 percent humidity level.
At humidity levels of less than 30 percent, corrosion seldom poses any problem. As a result, silica gel capsules or bags that remain bright blue are indicative of a safe, dry condition. Any change to the color is indicative of a need for additional preservation techniques.
Often, an envelope containing a small amount of silica gel is attached to an engine that is bagged or stored in a container. The envelope is made accessible so that an occasional check of the preservation status can be made without completely unwrapping the engine.
Once sealed with dry air or placed in a vacuum, metal shipping containers or storage bags should not be opened unless the humidity indication suggests the need for re-preservation.
Ideally, engine parts should be coated with some type of corrosion-prevention compound prior to storage, and all exposed surfaces, holes, or orifices should be closed, plugged, or in some way protected from direct exposure to moisture. Propeller shafts should be treated with corrosion-prevention treatment and wrapped with some type of barrier paper or moisture-resistant tape.
Fuel systems should be purged of all fuel-Continental claims that no special preservation preparation is necessary with TCM fuel-injection systems other than to completely drain the system. (See separate section on fuel bladders.)
The original equipment manufacturer for Marvel-Schebler carburetors recommends that the float bowl be drained and that a light coating of Mil-C-4339 preservative oil be sprayed in the throat of the carb and on any ex-terior surfaces. They do not recommend that the bowl interior be flooded with the oil.
Recommendations for the Precision/Bendix line of fuel-injection servos is a little more complicated. For servos subject to inactive periods beyond 28 days, the fuel servo and flow divider should be purged of all fuel and the fuel cavities flooded with 10 weight non-detergent oil.
This oil must pass through a 10-micron filter prior to use and should be applied under gravity feed conditions. Too much pressure can rupture delicate diaphragms within the servo regulator assembly.
The air and fuel outlet side of the Bendix servo should be open to ambient air during the filling operation. You probably will not see fluid venting from the fuel outlet or the air side of the servo; however, proper preservation will still be achievable.
Uninstalled fuel components should be preserved and placed in air-tight bags with a small amount of wrapped silica gel. Pressure-injection carburetors should be drained of all fuel, the mixture placed in the full-rich position, and grade #1010 (light lubricating mineral oil) introduced into the fuel inlet. When the preservative oil begins to flow from the uncapped vapor vent opening, all drain and vent plugs should be reinstalled, safety wired, and inlet and outlet fittings capped for storage.
This discussion has not addressed uninstalled engine components, including cylinder assemblies, crankshafts, camshafts, and internal gears. However, inasmuch as nitrided parts are more susceptible to corrosion than plain steel components, it seems logical that similar procedures should be employed for these parts as well. Heavy preservatives, barrier paper, airtight containers, and dehydrator agents should be used with particular attention paid to monitoring of the level of preservation.
If you have an airplane with rubber type flexible fuel bladders, the standard recommendation is to keep the tanks full in order to minimize cracking of the membrane. We agree. There are also different materials used in fuel bladders-some made of more robust material than others.
Keeping it full also cuts back on water condensing as much as it does in partially empty tanks. That begs the question of how long the fuel is good for. The answer for avgas is one year according to Chevron. Avgas has special stabilizers to make it last a long time.
Unleaded auto fuel has no such capability, so beware of octane loss as well as formation of gums beyond a few months of inactivity. Fuel stabilizers, to our knowledge, are not approved to be added to auto fuel used in an aircraft. Check your STC or with the STC provider i.e. Petersen Aviation or the EAA. The solution is to plan on filling the tanks with avgas just prior to storage on a plane that normally runs on auto fuel.
It is important to remember that long-term preservation of engines can result in trapping large amounts of oil in the combustion chambers of one or more cylinders. For this reason, engines should not be rotated until all of the preservative oil is drained away. Failure to do so can result in damage to the piston, connecting rod, and crankshaft of the flooded cylinder.
- Remove seals and all desiccant bags.
- Remove cylinder dehydrators and plugs or spark plugs from upper and lower spark plug holes.
- Remove oil sump drain plug and drain the corrosion preventive mixture. Replace drain plug, torque and safety. Replace and safety the oil filter. Rotate propeller by hand several revolutions to remove preservative oil.
- Service the engine with lubricating oil.
- Service and install spark plugs and ignition leads.
- Service en-gine and aircraft in accordance with the manufacturer’s instructions.
- Thoroughly clean the aircraft and engine. Perform visual inspection.
- Correct any discrepancies.
- Conduct a normal engine start.
- Perform operational test in accordance with “Operational Inspection,” of the applicable Maintenance Manual.
- Correct any problems.
- Perform a test flight in accordance with airframe manufacturer’s instructions.
- Correct any discrepancies prior to returning aircraft to service. 14. Change oil and filter after 25 hours of operation.
For Lycoming engines:
- To return the aircraft to service, remove seals, tape, and desiccant bags. Use a solvent to remove tape residue. Remove spark plugs or dehydrator plugs.
- With the magnetos off, rotate the propeller by hand through sufficient rotations to remove excess preservative oil from the cylinders. Drain the remaining preservative through the sump.
- Most engines are equipped with a quick-drain oil fitting on one side of the oil sump and a standard AN plug on the other. Remove both in order to drain as much of the preservative oil from the sump as possible. Uninstalled engines should be hoisted level, then tilted as required to aid in draining of the oil.
- If the spark plugs were installed in the flooded cylinders, all traces of oil should be removed from the firing end and the external barrel with clean solvent or MEK prior to reinstallation.
- Remove all dehydrator bags, tape, plugs, and barrier paper from the induction system, exhaust ports, breather lines, and so on. Drain and flush the carburetor or other fuel components with fuel, and re-safety all drain and vent plugs as necessary. Check all fuel and oil hoses and intake ducts for security and leaks.
- After servicing the oil sump with the proper grade of oil, wash and pre-flight the engine and engine compart-ment, perform a ground run to operating temperature, and check for leaks. If all is satisfactory, install the cowling and make a logbook entry noting the reversal of the preservation process.
Keep the first flight local to be sure there were no items missed or loose items. Double check for any pest dam-age or roosting, as well as hand operate all control surfaces to feel for any signs of binding or corrosion in hinges.
That includes a visual check and light thumping on the tail and control surfaces, as well as the gear wells and en-gine compartment to assure there are no stubborn tenants or nest residues. If there is any signs of varmints or in-sect activity, more serious investigation is called for such as panel removal and the use of a borescope. If you plan on not doing all these things for storage, at least consider doing some of them such as the dehydrator plugs and desiccants as well as the use of vapor corrosion inhibitors) VCI technology for the cabin and avionics.
This article first appeared in the January 2012 issue of Belvoir Media Group’s Light Plane Maintenance magazine.