Winterising your aircraft: Keeping the corrosion at bay
- Credit: NA
Advice for inhibiting and storing an aircraft that remains inactive during the winter months. Words Bob Grimstead
We all know that the best way to keep your aircraft’s engine healthy is to fly at least one hour every week, ensuring the oil temperature rises above 70°C (185°F) for several minutes to boil off the dissolved water and acids. But this is not always practical, especially over winter because of weather, the condition of your airfield, and other commitments, including financial ones.
I now properly inhibit and store my aeroplanes for the winter (I’m lucky that I can fly in sunnier climes) and it’s also a great opportunity to tackle those niggling things which require grounding for a few days, such as welding or painting.
I must be doing something right because, when I had my Champ’s 1940s engine rebuilt a few years ago, the engineers found no corrosion whatsoever and very little internal wear despite its being near the nominal 1,800-hour life.
First, let me offer some advice I learned the hard way: if you can’t fly your aircraft weekly, don’t simply run the engine on the ground or?worse still?pull the propeller around a few times every week or so. The moving parts within the engine (predominantly the piston rings and camshaft) will scrape off whatever thin oil film is left on the stationary parts (mainly the cylinder walls and cam followers) and the intake and exhaust valves will open and close, letting in whole litres of cold, moist air to corrode (rust) those surfaces you have just wiped clean of oil!
You can perform simple maintenance tasks including: draining and refilling the engine oil and changing the filter; checking and filling the fuel tank(s); checking and inflating tyres; checking, charging and changing the battery; and general cleaning and lubrication of the airframe and controls where nothing needs to be undone or disconnected.
If you are uncertain about any of these tasks, ask your engineer to supervise the first time. And don’t forget to make appropriate logbook entries.
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It takes a day to prepare an aeroplane for winter, and another to ready it for flight again in spring. I usually split the work over several days because it’s more efficient. I work in sequence, according to the value of the aircraft’s components, starting with the engine, followed by the propeller, avionics, battery, wheels and tyres, and the interior.
First, wash off the bugs and stains
First, I wash it all over, using a mild solvent to get the squashed bugs off the leading edges and petrol (fuel) to remove oil smears and stains. This gets rid of any salt deposited by our island-nation’s always damp air, neutralises corrosive industrial pollutants, and enables a thorough check of the whole airframe, particularly for areas that might need a lick of paint or re-lubricating before winter proper sets in and lower temperatures render painting impractical. At this stage I lubricate all moving parts.
Remove all dust and dirt from your transparencies, especially if you are going to cover them, as unclean perspex can cloud if the cover shifts. I put an old, soft, flannel sheet between cover and plexiglas as additional protection.
For your engine, first change the oil. Old oil (after more than a few days or flying hours’ use) absorbs water, acids and other corrosive chemicals which cause internal engine damage if left for very long. Get that oil good and hot with a minimum half-hour flight (ground-running is not sufficient), then quickly drain it all out into a suitable container.
Volkswagens, Rotaxes and four-cylinder Continentals hold less than five litres, so a cut-open five-litre plastic oil jug (or metal can) makes an ideal receptacle. A Lycoming might need something larger. On a tailwheeler if the sump plug is in the centre of its lower crankcase (as with a Lycoming or Volkswagen) rather than at the rear you will have to lift the tail and get the sump horizontal to drain it. Leave it overnight to get every last drop out. The Maule’s and Fourniers’ tails stay raised, but I don’t bother with the Champ (more on that later).
By the way, if you have a bayonet-style quick-drain fitting on your sump, don’t let it ‘ping’ shut under its internal spring pressure. The central brass rod is merely peened over at its end, and repeated snapping shut gradually un-peens this ‘shop head’.
The surprisingly common result is that, during your first flight after an oil change, presumably when the quick-drain barrel warms and expands just enough, that spring drives out the central shaft, followed by all your oil.
Remove your oil filter or screen and check it for debris. Tiny black carbon granules are no cause for concern, but any metal flakes, particularly if they are magnetic, are very bad news. These generally come from the camshaft and followers and mean a complete, costly engine overhaul.
If it’s alright, clean the screen in petrol and re-fit it (with fresh copper washer/s), or replace the spin-on filter with a shiny new one, and wire-lock them. Re-fit the sump plug with a new washer or gently close the quick-drain, and place a large drip tray under your engine before re-filling with oil as, inevitably, some will spill out.
My aeroplanes have the three most common types of engines: a Lycoming O-360 in the Maule, a Continental C85 in the Champ, and Volkswagen-derived Rectimos in the Fourniers. The priorities are slightly different for each manufacturer, so I’ll cover each separately (I don’t have any maintenance experience of radial engines).
Lycomings are difficult to protect...
Lycomings are the most difficult to protect because their camshafts run along the top of the engine and any lubricant quickly drains away down into the crankcase, leaving the case-hardened cam lobes and cam-followers (‘valve-lifters’ in American parlance) to attract condensation.
This causes minute rust spots which then become tiny corrosion pits and break through the thin layer of case-hardening. Next time you start your engine the bare cam lobes scrape across the dry cam-followers until the engine has run long enough and the oil has become thin enough to lubricate them by splash-feed.
This bare metal-to-metal scuffing causes very rapid wear, leading to spalling and flaking away of the hardened surface; the cam lobes wear down so engine power and aircraft performance quickly deteriorate. This is usually the first symptom until metal flakes show up in the screen or filter.
Lycoming suggests a special (costly) inhibiting oil, but I’ll share the results of my aviation lubricants research with you. In the eighties, Shell developed a semi-synthetic multigrade oil with additives especially devised to minimise wear on under-utilised engines and, in 1993, an extra additive made the oil more ‘sticky’ when cold, so it doesn’t easily run off bare metal surfaces, thus acting as a corrosion inhibitor.
This oil is Aeroshell 15W50?equivalent in viscosity to both W80 and W100 oils?and, yes, at £7 or so per litre it is around ten per cent more expensive than ‘ordinary’ monograde aviation lubricants, but nothing like as costly as an engine overhaul. Except when breaking them in, I use this oil all year around in both my certified aero engines and I hear that Lycoming mandates its use if running on UL91 fuel.
Lycomings have a wet sump, meaning the oil stays in the bottom of the crankcase when not being pumped around the engine. To cover the camshaft and followers completely in oil for winter, I simply fill the crankcase to its brim?or specifically the breather, which is right on top at the back of the engine.
That’s why I put the Maule horizontal. It takes about 24 litres of oil, two boxes of a dozen quart bottles. Filling the sump is easy: put a clean container under the breather outlet (I use the first emptied quart bottle) and then pour in all the others through the filler until oil dribbles out of your breather. Then close up the breather outlet?a small cork works although you can buy special bungs ? and replace the dipstick/filler cap. Your crankcase is now a sealed container; the hygroscopic oil cannot absorb moisture and your engine internals are weatherproof.
Caring for a Continental
Continental camshafts are at the bottom of the crankcase, so as long as you have some oil in there the cams and followers should be pretty well covered. But they have dry sumps, meaning the oil is held in a separate, usually rugby-ball-shaped vessel, so my Continental process is a little different.
After adding the four or five quarts of fresh Aeroshell 15W50 and replacing the filler cap, I start the engine and allow it to run for about forty seconds, so the new oil will be pumped and sloshed all around the internals without actually warming up or thinning very much or being scavenged back into the oil tank. Then I stop the engine and also seal the breather to keep out moisture and condensation.
VWs are different
Volkswagens are different again. Their camshafts are at the bottom of the crankcase, but they have wet sumps. So although it might seem possible to fill them right up like a Lycoming, the front crankshaft oil seal is a scroll which only keeps the oil inside when the engine is actually turning, so if you pour in much more than four litres the overfill trickles out overnight.
Therefore, as with the Champ, I start the Fournier’s engine, let it run for thirty seconds (they’re smaller engines) then stop it, hoping the fresh oil has been liberally flung around the internals. Bung up the breather and your crankcase is sealed. Incidentally, don’t use Aeroshell 15W50 in a VW engine during the flying season; motor oils are the only appropriate lubricant for a VW, and only those with an exact percentage of a particular additive called ZDDP, vital for your Volkswagen. I use Comma or Halfords’ ‘Classic’ 20/50 multigrade or Castrol ‘GTX for older vehicles’.
Now you need to seal up the cylinders against external air, which means bunging up your exhausts and carburettor intake. An aerosol can cap works well as an exhaust bung so long as there isn’t a tiny hole in it (or block that up).
For the Fourniers’ perforated silencers, the adhesives on all the duct or PVC tapes I have tried seem to be soluble in oil, so I simply accept the leakage. I use tightly folded cloths to cover the intakes. Sometimes I insert a bag of silica gel desiccant.
Fill up the cylinders
Next, I remove the top sparking plug from each cylinder and fill it with fresh engine oil. As an approximate guide to how much is needed, a Lycoming O-360 displaces 5.9 litres, so one cylinder takes a maximum of quart and a half, as do the identical cylinders of a flat-six O-540.
O-320s take one and a third quarts and O-235s just under a quart apiece. All the four-cylinder Continentals displace around 200 cu in or 3.27 litres, so their cylinders need about three-quarters of a quart.
The six-cylinder O-300s and C145s again utilise identical cylinders. A 1700 VW cylinder holds just under half a quart, while a 1200 cylinder holds one third. And if you don’t know what engine your aeroplane has, shame on you!
Of course, at any time one of your four cylinders will have an open exhaust valve while another will have an open intake valve, so quite a lot of oil will seep out of the combustion chambers – thus the drip tray.
But in the process your valve seats, faces and stems should become wetted with nice, cold, and comparatively thick and sticky 15W50, protecting them as well as you possibly can. There’s an alternative for Volkswagens: pop off the rocker box covers and undo the two nuts (or bolts) holding down the rocker shaft each side. This ensures that all four valves on each cylinder bank are closed.
Now you can re-fit your sparking plugs to the oil-filled cylinders. You could use desiccant plugs packed with silica gel in the part-full ones but I don’t bother. Just re-fit all the sparking plugs to make each cylinder a sealed container and so minimise corrosion. Experience tells me that any minor valve face and seat corrosion (a particular problem with Volkswagens) wears off as they rotate against each other in the first few flying hours anyway.
And replacing valves, seats or even a whole cylinder on any engine costs a tiny fraction of a full overhaul, so the crankcase and its internals are the important components to protect. Whatever you do, if you fill the cylinders with oil, be sure to placard the engine ‘Do not turn’.
Don’t use WD40!
Finally, spray the whole exterior of each engine with a proprietary rust-inhibiting fluid. I use Ballistol, simply because a gunsmith friend recommended it, but there are probably other, better sprays. Don’t use WD40 because, as its name implies, it is only a water-dispersant lasting forty days, and not a proper corrosion inhibiter.
Now for the propeller. If it’s aluminium, I wash it with water and then petrol, and coat it all over with a thin layer of grease to prevent corrosion (which shows up as tiny, dusty, white spots). A constant-speed propeller should have a few pumps of the appropriate grease squirted into its hub to replace any inside that might have absorbed moisture.
Special care for wooden propellers
Wooden propellers have different issues. Again, wash off all the nasty, potentially acidic bugs, grass and other flora. Polish the brass leading edges if you have them. Wood expands in the winter, so unless yours is secured with Bellman washers it’s important at the very least to cut the wire-locking and unwind each bolt a full turn to prevent the front crush-plate from cutting into the wood fibres.
This of course means that you will have to get your inspector involved in the re-torquing and wire-locking process next spring. Because of this, I reckon it’s just as easy to remove the whole thing, and I take it home to store in my nice dry garage, sort of replicating summer atmospheric conditions so the wood doesn’t expand too much, eventually cracking the paint and causing other problems.
Because wood has a moisture content and this will migrate over time, it is important to store your propeller on its back (driven) face rather than in the orientation it has on your aeroplane, and never with one blade up and the other down.
Looking after the electrics
If you wish, you could also remove the radios for dry storage at home.
The battery is next. ‘Wet’ lead/acid batteries are much more tolerant of long periods of inactivity than gel cell ones ? a couple of hundred quid for an annual replacement Gill ‘dry’ battery taught me that! All batteries deteriorate more rapidly when left uncharged, so I disconnect and fully charge them.
Sometimes I leave one on a trickle-charge throughout the winter, being careful to ensure it stands on its own in the open, on a non-combustible surface (e.g. the concrete hangar floor, well away from the aeroplanes) just in case a spark should ignite the hydrogen gas given off during charging.
Then wheels and tyres. Pump up the tyres to a slight overpressure to allow for winter osmosis, then put blocks under the axles or brake callipers to support the aircraft’s weight in case one or more of the tyres should deflate. Not only might this cause wingtip damage, but prolonged resting on a flat tyre will ruin it?and a 600 by six tyre and tube can cost £200.
If there is any chance of flooding, put the wheels into wide flexible plastic animal feeding buckets. This also helps foil rats and mice, who will nibble your rib-stitching and wiring?plus their urine is a deadly corrosive to steel and aluminium (and it stinks!)
That’s another reason to raise your tailwheel?putting it on an old cable drum again foils those rodents.
Leave only a drop of fuel in the tanks
Siphon or drain any remaining onboard fuel out of the tank(s), leaving a little to keep the bottom of each tank and fuel gauge sender covered to protect them from corrosion.
Then dispose of this fuel, because avgas in an open container (with its mandatory vents, a fuel tank counts as an open container) has only a three-month life, whereas in a sealed jerry can it has a twelve-month life. Mogas only has a one-month ‘shelf life’.
If your hangar is damp you might care to remove your upholstery, or even the complete seats to avoid mildew. I just put the cushions with the radios in my study. I do spray the wings and tail with mould inhibitor, although I am still undecided on its effectiveness.
Finally I fit the tailored cover. If this doesn’t envelop the fuel fillers, seal them, and the tank vents and pitot tube, with duct tape, and tape up any uncovered cockpit vents. Cowling intake bungs cut from foam are a good idea, although their main purpose is to discourage birds from nesting in the spring when you remove that cover.
That’s it. Come back in spring.