'The Flight Safety Aspects of Covering Aircraft'.
by Michael Whitley


Can the use of aircraft covers lead to improved flight safety?
If you can't hanger your aircraft but consider aircraft covers to be a waste of money - read on. You may just change your mind…

It's a common sight. A walk around the light aircraft park at almost any GA airfield at any time of the year will reveal examples of aircraft from most of the major European and American manufacturers parked in open storage. A closer inspection will reveal the various design and configuration aspects favoured by different designers. Yet despite the characteristics of high wings, low wings, slab wings, tapered wings, cranked wings, slots, slats and a myriad of less obvious features these aircraft, perhaps surprisingly, have two vital things in common. The first is that they are designed to dance with angels and return the hapless crew to the safety of the ground relatively unscathed. The second is that they are all at the mercy of every meteorological condition that our inclement climate can invent. If there is a way into your aircraft the weather will find it and proceed to wreak joyous havoc! Contamination of whatever kind leading to damage and failures is at best annoying and expensive… at worst fuel or electrical contamination leading to an in-flight failure and subsequent forced landing is precursor to many of our most serious aviation incidents. Some of this contamination could be avoided by giving your aeroplane as much protection as possible. With hangar space at a premium, and the worst of Britains weather just around the corner, is it time to give aircraft covers a second look?

The aspects of aircraft design which lead to problems.

In broad terms aircraft are constructed from a combination of wood, fabric, metal and composite - materials chosen for their strength and lightness. Note that word 'combination', for it is this very aspect of aircraft building wherein lies many of the problems, and associated costs, of airframe maintenance. When we construct an aircraft we rivet, bolt, clamp or otherwise fasten together components made from any or all of the above and throw in a few bits of rubber and some oil and fuel for good measure. Then we sit the assembly outside in the middle of an open area of ground. In all weathers. Variations in temperature cause all of the pieces of different material to expand and contract at differing rates, opening up gaps which then let in the rain. If we ever manage to get the thing airborne we exacerbate all of these effects with vibration. We need go no further than our own garden gate to witness first hand the effect of weather on wood. Hands up the people who own a twenty year old item of clothing which is kept on the washing line when you're not wearing it? We all understand fabrics, then! The reason that aircraft using this type of construction are usually kept permanently under cover is clear. But what's the case for metal aircraft? The atmosphere seemingly contains marauding armies of 'metal worms' that will mercilessly attack your bank account whilst reducing your pride and joy to white powder and red rust. Differences in the metallurgy of metal components which are in contact with each other will, in the presence of water (or other electrolyte) create tiny electrical currents whose action decays one ('sacrificial') of the components. The problem is that both of those components form part of your aeroplane! Likewise plastic and composite components, clear or coloured, will suffer the ravages of ultra violet light causing clouding and crazing of transparent panels and the well known brittleness of plastic mouldings that is symptomatic of breakdown of the material structure. Which brings us to perhaps the most insidious individual in the plot against your aviation safety. Condensation. Due to the manner of its formation, condensation is pure water. As such it is far more electrically conductive, and therefore corrosive, than ordinary rain or tap water. It is the killer of anything electrical upon which it forms - magnetos and ignition systems, radios, navigation equipment and instruments. When we build an aircraft we build in with it the conspiracy that keeps firmly grounded!

Can Covers Really Help?

The answer to this question may lay in the arguments often levelled against covers, which seem to fall into three categories:

1. They damage the paint work and/or windscreen.
2. They cause condensation or make the aircraft 'sweat'.
3. They are expensive and not cost effective.

1. Nothing damages paint work quite as effectively as prolonged exposure to the elements. Heat, cold, frost and ultra violet light all play their part in destroying the coatings applied to the airframe to protect the structure. Apparent paint loss may well occur on aircraft whose paint has already been thus degraded, and a cover fitted over paint in this condition may quite quickly show paint traces on the inside as the old, damaged, paint surface powders off. In these cases the aircraft should be washed and polished before fitting the cover for the first time. If the paint is in good condition damage can only be caused by an ill-fitting or badly fitted cover, and occurs when the cover is allowed to move against the airframe over a period of time. The cure for this problem is to ensure that covers that are cut accurately to the shape of aircraft and that straps are so positioned that the cover pull down tightly without creating too many creases or wrinkles and without leaving any areas of fabric unrestrained. Any loose fabric will doubtless flap in the breeze and abrade the paint, as will loose strap ends, which should be folded up and tucked behind the strap or inside the covers' hem. It follows for windscreens and windows in that damage will occur only if the cover is allowed to flap or billow. Find out whether the manufacturer fits a soft lining in the windscreen - of particular importance with 'bubble' type canopies. And always clean the windows before fitting your cover! The best made cover in the world cannot prevent trapped dirt doing its' worst. When not on the aircraft the cover should be stored in a bag, which by rights should be waterproof and come with the cover. This will prevent soiling which may then be transferred to your aircraft.

2. Aircraft do not 'sweat' in that they cannot perspire - that is producing moisture of their own accord. It is often closer to the truth that, from wet feet and leaks in door, window and wing seals, aircraft are already wet inside the cockpit. Fitting a cover to an aircraft whose interior is in this condition will make this moisture more apparent in the short term as water contained in carpets and other cockpit fixtures begins to dry out. This is not normally apparent as, in the absence of a cover, the cockpit internal temperature will rise rapidly during the day allowing a much higher humidity than in the open air. A cover will stabilise the cockpit temperature and prevent further ingress of water. Air trapped in the confines of the cockpit may become temporarily saturated, but over a period of time the cockpit fixtures will dry out. For this reason covers should always be made from fabric which allows some passage of air. The internal condition of the cockpit upholstery and equipment will improve as this gradual drying out takes place. Here is a point worth mention. I heard recently of an owner of a PA28 Arrow who had decided to fit a new set of carpets. On taking out the old set he found so much water contained in the carpet and sound deadening material that he almost needed help to lift them out! How many of us take this extra weight into account on our weight and balance schedule? And how ever much we care for our aircraft externally how much unseen damage is all this hidden water causing? Worrying, isn't it?

3. Cost versus Cost Effectiveness. Time for some numbers. The average cost of a cockpit and cowling cover for a two or four seat single engine aircraft falls between £450.00 to £500.00. If the cover lasts five years before it need replacing the average cost is between UK£0.25 pence and UK£0.27 pence per day. A brief survey of hangar fees for the same aircraft reveals that the average cost at a UK airfield would be in the region of £5.00 per day. I'll let you make up your own mind as to whether or not the cover is expensive. When it comes to cost effectiveness then different criteria come into play. None of us enjoy paying for our flying and it is fair to say that most aircraft owners and shareholders will take all possible steps to keep operating costs to a minimum. But what price safety? It is of paramount importance that correct and timely repairs and maintenance are carried out, all of which are very costly. With hangarage costs rising it is tempting to move aircraft to outside parking, even with the associated risks of damage and corrosion. It would be easy to accuse engineering companies of profiteering but, like many of us, they have staff and overheads to pay. It is a point worth note that they have had to work for years to gain the qualifications that allow them to work on our aircraft. While ever legislation limits the amount of work that non-engineers can undertake there are few avenues open to reduce the cost of operating a light aircraft. One way that can be effective is to make the most efficient use of the time spent maintaining our aircraft. Knowing that we pay for every minute of engineers time are we happy for those minutes to be spent freeing screws and removing corrosion before maintenance can begin? Surely the time would be better spent tracing a fault, improving radio reliability or doing a little extra than the 'required' work. Saving a little cost on each service can help towards a new tyre or perhaps your engine and your peace of mind can enjoy the benefits of new spark plugs or an extra top end overhaul? Any hard-won funds resulting in a material improvement to your aircraft must surely contribute to flight safety - money in the bank rather than down the drain.

The Bottom Line..?

…For those of you that like numbers. It is generally accepted that the purchase of an aircraft is, next to home ownership, the largest single investment anyone is likely to make in their lifetime, and one which many owners would like to leave to their grandchildren. A very quick look at the prices of ten two and four seat aircraft advertised for sale in one of the UK's leading aviation magazines shows that the average entry level cost of aircraft ownership is £21,200¹. In an average year a privately owned aeroplane will fly in the region of one hundred hours. There are eight thousand seven hundred and sixty hours in one year, equating to an aircraft spending 98.75% of its life on the ground. Any engineer will impart the knowledge that an aircraft will experience the worst of this damage during the time that it is unused. If you buy a cover whose life span is five years the cost is 0.3% of the value of your aircraft per annum. A quick survey of engineering charges reveals that your cover is paid for if it's use saves two hours time per annum. That doesn't add up to much in the way of seized screws, peeling paint or avionics drying out time.

…and for those that try to avoid the numbers. Do you enjoy flying your aeroplane…or washing it? Are you annoyed by the scratches on your Perspex and paint that seem to appear from nowhere and the constant little leaks and unpleasant musty smell in the cabin? Do you like spending money on maintaining the condition of your hard won asset that could be spent on air under wing instead of ground under tyre? A sad indictment of 'the times we live in', surprising information that came to light as a consequence of preparing this article is the number and variety of incidents in which damage is done to parked aircraft. Some of the reports involve damage caused by people trying to see into aircraft cockpits, which are usually not reported and therefore the repair cost passes to the owner. We can all have accidents but if the cockpit is covered the curiosity factor is removed. Perhaps worst of all are acts of vandalism, which are not as uncommon as one would think and often involve multiple aircraft. It was often mentioned that covered aircraft were untouched when others suffered damage ranging from doors being forced open to having panel mounted equipment removed, or even having panels damaged and windows broken. The reasons why could be argued until the price of avgas doubles.The facts, apparently, remain.

Even in the hangar aircraft are not immune, suffering the dreaded 'Hangar Rash' and the attentions of birds and mice (that are ever more inventive in their nesting arrangements!) and drips from the roof. Obviously an aircraft will suffer less if covered. If, like me, you like to like to make well informed decisions I have a suggestion. The next time you see an aircraft cover being fitted or removed, take a moment to talk to the owner. It may be that the benefits of a good quality set of aircraft covers deserve further investigation. The proof of the pudding is, after all, in the eating.

¹. The average price of ten randomly selected aircraft in the Cessna 150 and Piper PA28 class - only two aircraft over £30,000 were included in order to attempt a reflection of first aircraft purchases.

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