aircraft snow
Technical

Post winter TLC for aeroplanes

Engineer Jerry Parr gives you some pointers for the essential tender loving care an aircraft might need after winter…

Most aircraft owners and operators always have the intention to carry on flying throughout the year, regardless of the season. Few consciously decide to lay up their aircraft for the winter months and take all possible precautions before it is too late. Then the weather arrives and often, even if the weather is game, the runway may not.

In any case, whatever the season, if an aircraft is left alone for any length of time it is good to apply a little extra tender loving care (TLC), and carry out some extra fettling… which is especially true as we come out of the gloom of winter.

As far as pre-flying-season fettling is concerned, it will depend on your aircraft’s certification status and your experience as to how much you can do, are allowed to do, and where you need an engineer or inspector’s assistance and signature. No upkeep should be carried out without the proper tools, maintenance manuals and knowledge. If the aircraft has been sitting still for any length of time, it will require a really thorough look over.

An ideal place to start is the pre-flight inspection. Dig out the Flight Manual and follow it to the letter. It may be that over time, you have become so familiar with the pre-flight routine that the odd point is being overlooked. Using the published inspection regime will be good for the aircraft and perhaps stir up the grey matter. Many aircraft maintenance manuals have a few paragraphs on returning the aircraft to life after long- or short-term storage.

Fuel systems

Possibly the first thing to check before anything else, and certainly before the aircraft is moved, is the fuel system for contamination. Any substantial movement of the aircraft may disperse water and other contaminants away from the fuel drain points.

Ideally, the fuel tanks should always be left full to avoid condensation forming but this often isn’t possible due to weight-and-balance considerations. Switching the fuel off on some installations prevents any chance of fuel cross-feeding to a lower tank. Fuel cap seals should be maintained in good condition and if at all possible be kept covered. Water ingress can occur through fuel cap locking mechanisms, not just via poor seals.

Remember that the fuel tank drains are normally located at the lowest point of the fuel tank or fuel system when the aircraft is in a level attitude (longitudinally and laterally). It isn’t always just the obvious drain points either. For instance, some Cessnas have extra fuel system drain points in the fuselage that are not fitted to similar models of an earlier vintage. The later ‘restart’ Cessna singles have five drain valves in each wing-tank to ensure water collects near at least one of the drains, regardless of the aircraft attitude.

Aircraft fitted with bag-tanks on the other hand may have water caught in folds in the tank, so keep checking the drains even after the aircraft has been moved.

One of the attributes of avgas is its ability to remain ‘within spec’ for a fair length of time. With the advent of mogas used in many aircraft, care should be taken to ensure that the fuel is still usable and has not ‘expired’. With the use of fuels containing a percentage ethanol which is allowed in some aircraft, extra caution should be taken due to the ability of ethanol to absorb water under certain conditions and for that water to then reform later and sit in the tank in liquid form, when there is a change in ambient conditions.

cover
Some aircraft covers are better than others...

Airframe

Apart from the normal check of the airframe, particular attention must be paid to the lower surfaces of the fuselage, wings and empennage to check that all the drain holes are clear – it is all too easy to concentrate on the parts that are easier to see. Sod’s Law dictates that, by and large, the upper surfaces of aircraft are not watertight and water will flow to the lower points of the airframe, carrying with them dirt and crud, which promptly block the small drain holes.

Obviously, high-wing aircraft should not have the upper surfaces forgotten at inspection time, just because they tend to be more difficult to access. It may well require a stepladder or trestle – having a look from the wing strut-mounted steps on a Cessna, for instance, is not sufficient.

Pitot-static systems are particularly prone to blockage either by water, ice or insects. They find the ports extremely attractive places to occupy, something that many airliners suffered from after their Covid-19 lockdown-induced periods of inactivity. Not only should all the ports be securely blanked off whenever the aircraft isn’t flying, but if system drains are fitted, ensure these are checked prior to flight. Incidentally, warning flags on blanks are good for raising attention, but if left to their own devices they can really hammer the aircraft’s exterior finish after a time. If more than normal blanks have been fitted (such as static port blanks) make sure they have all been removed.

Flying controls should be checked carefully for water accumulation. The Piper PA-28 (among many other types) has an all-moving stabiliser with drain holes in the lower surface to prevent the build-up of water. In the winter, water freezing here could cause a serious control surface imbalance. Other places prone to ice forming are inside propeller spinners or tail fairings, where there are fair-sized holes to allow the rain in, but only very small drain holes to let it out again, which are easily blocked by debris.

‘Birds are clever and will find their nest again even if the aircraft has been flying’

Airframe covers help keep the paintwork protected and the weather out, if the aircraft is outdoors. Modern covers are available in breathable materials, which help keep the airframe dry underneath. An added bonus of a fully covered aircraft, if flying in winter conditions, is that there is no ice to remove before you can go flying.

Engine inlets are commonly blanked off, but outlets rarely are. As spring approaches, a careful inspection should be carried out on anywhere that birds may decide to nest. A cursory glance on top of the engine is not enough. Inside the lower cowling, beneath the engine as well as inside, induction and oil cooler inlets need checking, as do inside tail fairings and up inside the undercarriage bays of aircraft with retractable wheels. Birds are clever and will find their nest again even if the aircraft has been flying, so keep checking.

Rodent attack is possibly the worst enemy of an aircraft, and it does not matter if the aircraft is parked indoors or outside, the little blighters can break in. Supposedly, mice can get through a hole no bigger than a biro!

Mice can get inside the airframe via the undercarriage legs or cover straps that are left hanging down. Some people have had success fixing tall metal boxes around undercarriage legs to stop their ascent as mice can easily climb up inside PA-28 spats and leg fairings – and from there it’s straight into the wings and then the fuselage. Once inside, they will make nests out of seat cushions, seat belts and carpets. If they decide to nibble on the wiring loom then there are potentially going to be some serious issues. Keep an eye out for droppings as well as loose materials. Lift up seat cushions and have a look inside the rear fuselage as well, if possible. Have a chat with your engineer if in doubt as to what systems and spaces exist in your aircraft, which might be susceptible to rodent attack or nest-building.

Control locks should always be fitted when the aircraft is left outdoors and ideally when parked in a hangar, too. Some controls are linked to other systems such as the rudder to nosewheel steering, and ailerons to the rudder circuit. If the aircraft is to be moved with locks in place, then it’s a good idea to put a warning notice inside the windscreen alerting people to the fact.

Aircraft covers are good devices to help prevent flying control damage due to excessive winds. A careful inspection is required of the control system as far as possible. Operating links may have rod end-bearings attached with a threaded bar that is a potential weak point in the system if the control is moved in an incorrect or excessive manner. Rudders can take a real hammering in strong winds, although some are connected to the nose or tailwheel steering mechanism providing a form of self-locking but if not they, (as with any flying control), should be locked at a strong point – ideally at the point at which the control is normally moved, be it by cable or actuating rod.

Regardless of where an aircraft is kept, the systems must be kept lubricated. The only guide for what needs lubricating and with what lubricant is the aircraft manufacturers’ manuals, although it isn’t always as obvious as it might be. For instance, Piper PA-28 aileron piano hinges are sheathed in a Teflon sleeve and ordinary oils may well harm this sleeve. One often sees grease liberally applied to rod end-bearings and hinges. This can actually attract and retain dirt, and as the excess grease hardens and cracks allows water to pool exactly where you do not want it to, and therefore it can actually promote wear and corrosion.

 

airframe
C'mon, give it some love...

Undercarriage

Undercarriages need a lot of looking after all through the year – and that isn’t just the tyre pressures. Ideally, if stored outside, aircraft should be parked on paving slabs rather than grass as this helps to prevent corrosion of the wheel, wheel bearings and brake components, which can be accelerated by sitting in damp, ever-growing grass for extended periods. If the aircraft is fitted with wheel spats, make sure these are removed to allow proper inspection of the tyre – and to ensure that the spat does not become full of mud and other stuff during winter operations on grass.

If the wheels have been sitting around in the wet, it may be prudent to have the wheels off and the wheel-bearings inspected and repacked. A bit of preventative maintenance now will pay dividends in later months. Undercarriages work hard and should be lubricated regularly, especially post-winter – this is especially true for retractable gear. Tailwheels need looking after too, including the pivot mechanism. Over recent years, the castering nosewheel has become more common as it’s a simple mechanism for aircraft designers to incorporate. They are broadly similar to tailwheels in that the design does not necessarily lend itself to easy ‘relubing’, but do not assume the pivot shaft doesn’t need looking after. Remember that this type of undercarriage isn’t just the domain of the LSA and homebuilt end of the market – Diamond DA40s and Cirrus have castering nosewheels, too.

‘Aircraft should be parked on paving slabs rather than grass’

If the fescalised portion (the shiny bit) of an undercarriage oleo loses its smooth finish, then as the oleo does its thing, the seals may be damaged and require replacing. Make sure the fescalised portion is kept clean and polished as the seals can suffer from anything from rock-hard impacted flies to corrosion pits. The oleos may appear to be at the correct extension at a first glance, but this may be due to stiction in the seals. Care should be exercised when checking oleo extension and it might not be apparent until taxying the aircraft that there is a problem. Maintenance manuals will quote a pressure the oleo should be charged to or, more usefully for a pre-flight inspection, a measurement of exposed fescalised portion. This is normally quoted at maximum static load (empty weight plus full fuel), so some allowance would normally be required for the given fuel state. Remember oleos contain air and hydraulic fluid so a leak of either medium requires investigation as the shock absorption characteristics will be altered.

Tyres will develop flat spots when not turned for a while. Normally this isn’t an issue and once back in use they will resume their normal shape. Aircraft tyres have a relatively small total volume compared with car tyres and a small leakage can make a big difference on the pressure. This in turn adversely affects the ground handling and in particular the effective drag – not helpful on the take-off run.

One of the attackers of tyres is UV light, and if the wheels aren’t fitted with spats then it might be wise to have some wheel covers made. Some inner tubes definitely hold air pressure better than others – going cheap is not always the best way forward, and always fit proper, sealing dust caps on the valves.

Brake discs are prone to corrosion, but light surface corrosion should not be an issue once the aircraft is back in use – although deep pits, scores or corrosion at the disc/hub joint are a different matter. The brake caliper torque pins are to be kept lubricated and obviously there should be no signs of brake fluid leaks. Brake reservoirs should be checked for a correct fluid level but make sure they are topped up with the correct type of fluid. GA aircraft braking systems are normally filled with a red, mineral-based fluid that is not harmful to paint and other surfaces – unlike automotive brake fluid. Any signs of sticky fluid on the caliper or on the floor below should be investigated (another good reason for not parking directly on grass, where a leak can be difficult to spot).

undercarriage
Undercarriage legs must be kept clean and well-lubricated to function normally

Electrics

Light aircraft tend to have pretty basic electrical systems compared to those found in the modern car and they are not built to the same rugged, weatherproof, standard either. Modern and upgraded aircraft may have ‘state of the art’ avionics installations, but remember this is GA ‘state of the art’…

In normal use, our electrical systems can cope perfectly adequately but if left alone in a damp environment, problems can occur. Contact surfaces can corrode and water may pool, causing short circuits. Aircraft batteries take a real pounding in the cold and given that those found in certified aircraft tend not to be of the highest output capacity or latest technology, they need extra care. If in doubt as to the battery’s status, it should be removed and charged accordingly.

All too often in winter, aircraft engines are started on ground power due to the battery being discharged. This means the battery is being charged at a much higher rate by the aircraft electrical charging system than that of a battery charger. Wet cell batteries (still the most common type in certified GA aircraft) will start to vent out some acid during this high-power charging which will, in theory, find its way overboard through the battery vent lines.

Unfortunately, often the acid sits in the battery box or worse still, the aircraft structure, causing massive corrosion problems. Even when working correctly, battery drain tubes are not always the answer as they barely clear the airframe and allow the fluid to corrode the outside of the aircraft skin, especially in the airflow in flight. Battery terminal grease is available to prevent corrosion forming at the battery terminals themselves and will help prevent the efficiency suffering as a result.

‘Avionics are delicate flowers at the best of time’

When starting from cold, it is advisable to use all of the available battery power to turn the starter motor. No need for strobes, anti-collision and navigation lights blazing away needlessly, shouting ‘clear prop!’ still works. Avionics are delicate flowers at the best of times and as age progresses, some of the older gas discharge displays start to suffer from the cold and damp. Some people used to take their avionics boxes home with them for a winter’s hibernation in the airing cupboard. That’s all well and good but doesn’t protect the radio rack contacts, plugs and connectors from corrosion in the damp atmosphere and to stay legal, many navigation boxes require a test with an avionics engineer’s ground test equipment on reinstallation.

Once the engine has started and is running with the charging system doing its thing, only then is the time to warm the avionics and let their natural, internal operating warmth remove any dampness. Similarly, the flight and gyro instruments do not like sitting unused in a damp and cold atmosphere but should dry out when in use. It is not uncommon to see the instrument glasses mist up when first put back into use after a lay-off and there isn’t a great deal you can do about this except wait patiently for them to naturally dry out as the air temperature increases.

Dehumidifiers have become more common recently and will help keep the moisture levels down. Obviously, they need to be monitored and emptied as required. If you have the luxury of mains power near the aircraft, electric dehumidifiers (drained externally) do an excellent job of keeping the aircraft dry inside but also keep a small amount of airflow through the aircraft and the humidifier exit air is normally warmer than ambient.

avionics
Left alone in a damp environment, avionics can develop problems

Engine

There isn’t a great deal you can do with the engine apart from give it a good look-over for the aforementioned birds’ nests and corrosion. It is possible to fit desiccant bungs to inlets, exhausts and crankcase breathers (and even in place of spark plugs) to reduce the engine’s internal moisture level. Importantly, the engine should not be rotated by hand unless you are going flying, as this can wipe off any residual oil coating on the various surfaces and leave them prone to corrosion.

Check the external surfaces of the cylinders and other components for corrosion – especially the steel cylinder barrels. Keeping the cowlings nice and clean will show up any oil leaks that have appeared and give an indication as to where from a leak has occurred.

‘Regardless of where an aircraft is kept, systems must be kept lubricated’

The engine should not be run without the intention of flying as it is rarely possible to get an engine up to normal operating temperature on the ground to boil off any internal moisture. All ground running tends to do is to promote the formation of condensation as the engine cools again after the run.

It may be wise to change the engine oil as this can absorb moisture. In the ideal world, everyone would change the oil for inhibiting oil (or at least clean oil) prior to a winter lay-off as this will remove any of the harmful combustion process by-products and contaminants from the oil.

When it comes to going flying, carefully turn the engine over by hand – and listen. The vast majority of traditional aircraft engine aircraft have magnetos fitted with impulse couplings (many installations only have one impulse magneto fitted) to provide a fat spark for starting. The impulse coupling only engages at low rpm and can be heard as a loud ‘click’ as you pull the engine through. Impulse couplings are prone to sticking over time and it’s always wise to pull the engine through when leaving an aircraft for any extended period to ‘fire off’ the impulse coupling. This means that the impulse springs are not left under tension and more likely to work as they should, next time round. It goes without saying to always handle propellers with care… a hi-vis won’t be enough to save you!

Propellers

Aluminium blades are often down to bare metal along their leading-edges due to the life they lead beating their way through air, dust and rain and also through the frequent nick removal at the hands of the engineers. This does leave them prone to corrosion, which must be removed and, ideally, the surface finish restored.

Variable-pitch propellers have bearings in the hub and as with any other type of bearings, react badly to water ingress. If possible, leave these propellers horizontal to avoid water pooling on the blade seals. On twin-engine aircraft with feathering propellers, the feathering system may use air pressure, which can drop off with time so it would be wise to check the dome and / or accumulator pressure before flight.

Propeller hubs and blade root ferrules are also prone to corrosion and may require the spinner to be removed for a thorough inspection to be carried out. Some blades are wood sheathed in a composite covering and during the winter months, it is not unknown for the wood core to absorb moisture and then split the composite at the root end of the blade trailing edges.

It was always said that wooden propellers should be left horizontal to avoid damp ingress along the length of the blades. Unfortunately, Evra apparently says that its propellers should be left vertically, so that’s that theory blown out! Best way is to check what the manufacturer suggests.

Beware freezing conditions and the possibility of ice forming inside spinners – another good reason for leaving props with at least one blade pointing downwards.

Propeller care is a real necessity, regardless of the type, construction or number of blades. The best way, as with the rest of the aircraft, is to keep the propeller nice and snug in a cover and away from the weather.

aircraft parked
As Jerry says, "If the aircraft has been sitting still for any length of time, it will require a really thorough look over"

In summary

It’s a fact of life that none of us can fly as much as we want to – or indeed, intend to. When the weather intervenes, it makes a lot of sense to spend a bit of extra time fettling your aircraft before committing to flight again.

If in doubt, employ the services of an aircraft engineer – perhaps now is a good time of year to get the aircraft’s annual inspection carried out.

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