Fibreglass Aircraft vs Aluminium Maintenance Costs

[Magic Fingers]

Hi everyone,

I have read many claims by manufacturers of new GA Fibreglass aircraft how cheap they are to maintain and how maintenance friendly they are. Typical examples like the Cirrus, Grob and Diamond all use no corrosion as a big selling point. I have also read many posts, often in drifting threads, suggesting some seem to be in the hangar for much longer than anticipated and stories of annual inspections that were supposed to take 1 day being extended to weeks. The question of how long they will last compared to metal aircraft that are still flying 50 years later is a long way from being answered but I would like to make some real comparisons to the older metal aircraft we are all familiar with.

I have no bias either way except to say that most of my flying is in Pipers. I would just like to see some honest figures for the hours of maintenance required and see if the claims are backed up by experience. The more responses the better and I promise to collate and post the results back for anyone interested.

[englishal]

Not quite sure that your results will give any useful statistics.

For example, there are many more metal aeroplanes flying, and so you'd have to dig through all of these and find out how many had corrosion problem / replacements and come up with an average.

I can tell you though that our rallye had to be totally rebuilt in 2005 due to corrosion in the wings. Required new wings / spars, and a total bare metal re-spray / anti corrosion which cost about £38,000 in total, and took 6 months or so.

[IO540]

I think the composite part will last for decades - that's true.

The possible issue is that on any powered plane (e.g. a Cirrus or a Diamond) there are many metal parts, and from what I hear and see these are often poorly made, with thin plating and they corrode. A while ago I spoke to a Diamond (DA40/42) service shop and he said they were replacing most of the metal fittings around the engine, on every Annual, because they were rusty.

Currently, nobody knows how the "plastic planes" will keep their values. Probably not well, if current depreciation rates are anything to go by. But that is a slightly separate issue from actual corrosion.

A traditional aluminium-hull plane should be OK for about 15 years before needing significant airframe parts. Obviously hangarage helps. These are the really pricey parts and this is why buying something newer is not such a bad deal - you get a free ride for a bit.

[BackPacker]

Newer planes are not "just" using composites instead of stressed aluminium fuselage/wings. They are also using other new technologies, such as glass cockpits, FADEC and diesel engines. That makes the comparison very complex: There is no single aircraft that comes with the option of an alu fuselage vs. fiberglass for instance, so that you can compare like for like.

As for fiberglass itself: it should last for at least 30 years if properly UV protected, even when kept outdoors. Fiberglass makes more complex shapes possible and has no rivets so it's easier to make optimum aerodynamic shapes. And that leads to better performance on less horsepower.

On the other hand - there seem to be less places capable of doing fiberglass repairs vs. aluminium. This may lead to longer repair times and costlier repairs. The sailplane community has lots of experience though with this so if this is ever an issue I'd look there for expertise.

And, as IO540 said, you cannot make an airplane out of 100% composites. There will always be metal parts, whether that's steel or aluminium. And these parts may have the same corrosion issues as comparable parts in aluminium aircraft.

[SNS3Guppy]

Corrosion is only a small part of the picture. The material with which the airframe is composed isn't really what determines how long the airplane will be around. Let's face it...We've still got wood and fabric airframes out there which have been around for three quarters of a century now. The chief determining factor, among many, is how the airplane is maintained and looked after.

The cost of maintaining one over the other is not a direct comparison. Many metal aircraft use fiberglass parts, and fiberglass comes in many forms; there are man composite derivatives. The use of a particular synthetic part and the stresses that particular part incurs play a crucial role in answering the question. The question can only be answered in generalities, however. The major portion of maintenance that an aircraft requires is typically not structural repair...but routine maintenance. Powerplants still cost the same. Tires and wheels still cost the same. Plexiglass still requires care, and interior materials still require care, cleaning, and attention. Wiring is wiring regardless of whether it's run through a carbon fiber duct or supported along an aluminum spar. Fuel systems still use time limited components that wear out, age, and become brittle and leak.

Many sailplanes spend a great deal of their lives in the sun,and flex constantly in flight...yet soldier on with many years and hours of service. Fabric airplanes do likewise, requiring regular inspection and occasional replacement of fabric. Metal aircraft fatigue and require attention, but can go a great many hours of long service without issue. I've flown aircraft with a hundred thousand hours on the airframe which were still in very active constant service. I've flown metal and composite airframes with less than 25 hours total...which suffered catastrophic failures. The opposite may be true of other airframes.

Burt Rutan used to have a section of fiberglass he'd layed up some 40 years ago or more. It's been continuously in the sun since he laid it up. It was outside his office in Mojave, California. Visitors were invited to jump on it, in it's mount, as they approached the front door. I've stood on it myself. His point was that while many considered fiberglass and composite structures to be perishable in the sun and with time, this is not necessarily so.

Which costs more to maintain? Neither, really. Not enough information is provided. Rather than worrying about which one is glass and which one is metal, look at the cost of replacement parts, and their availability. Loook at the mission each one flies. Look at the care each receives. Look at the storage and frequency of maintenance, the cleanliness, and the way the respective aircraft are flown. Look at their design. Then come to a determination after putting all the factors down and make an apples to apples comparison. Anything else is just spitting in the wind.

[A and C]

Plastic vs Metal

The biggest problem with GRP aircraft is damage, not a lot of people know how to repair GRP aircraft and most of the quality repair shops started in the glider business.

The other problem is that shoddy repairs are easy to cover up on GRP aircraft and so it is easy to buy a death trap and not know it!

Only time will tell if the current crop of GRP powerd aircraft will be up to the rough & tumble of the light aircraft business after all they will do more hours than the gliders, be subjected to more vibration and will be maintaned by people who untill now had mostly "metal" aircraft to look after.

[IO540]

Also, the jury is still out on the glass cockpit avionics.

The Garmin 1000 stuff seems to be holding up OK, but there are only a few years to go on.

The weakness with glass cockpits is that the maintenance/installation manuals are not (yet) floating freely on the internet (as the manuals for all the separates are) and only a few dealers can work on it. This has the potential for far more downtime than the old type avionics where you can just pop in an exchange altimeter etc etc. This wouldn't bother a renter but it might bother an owner. I don't want to be flying up to Gloucestershire every time there is an issue.

[SkyHawk-N]

This year I went through FAA NDT & Composite (including inspection and repair) classes and as a result would be much happier purchasing a metal used aircraft over a composite one.

[ProfChrisReed]

If it helps my glider was built in 1968.

It has never had any fatigue-related repairs to the fibreglass structure - all repairs have been due to impact damage of some kind. So far as I and my inspector can see, the fibrelass is as structurally sound as the day it was complete. Normally, the only structural maintenance on a glider is for metal parts (or where metal parts such as hinges have become delaminated from their attachment points because of repeated stressing).

UV damages coatings (paint or gel) but not the underlying structure so far as is known. There is the potential for damage if the coating is chipped allowing moisture in long term, but I think (though am not sure) that the main damage this causes is flaking off the surrounding coating.

For an aircraft which is not stored under cover gel coat is a no-no - a glider which has sat out for a year is a sorry sight. The right paint should be as good as on metal.

Skilled repairers can make repairs invisible - crashed gliders have been rebuilt from little more than shards.

The best comparison might be motorgliders like the Grob 109, some of which must be 30 years old. Many live outside, and seem to bear up well.

[edited to ask SkyHawk what there was on the course that put him off composite?]

[IO540]

This year I went through FAA NDT & Composite (including inspection and repair) classes and as a result would be much happier purchasing a metal used aircraft over a composite one.

I was at Socata's factory a couple of years ago, and they showed a small crack in the composite roof, near one of the door hinges, caused presumably by somebody allowing the door to be grabbed by a strong wind.

The man explained how the repair is done... in multiple layers, and it takes several days, and no doubt costs thousands.

With a metal hull, you just rivet a reinforcing plate in there.

Same issues with installing GPS and VHF aerials. If one has a uniform composite hull thickness of say 4mm, one can't just screw a big rod aerial into it (Vne 189kt). I looked into this once and there was simply no approved procedure. Socata provide two scallopped-out aerial locations and that's your lot.

Metal repairs can also be easily inspected.

[SkyHawk-N]

[edited to ask SkyHawk what there was on the course that put him off composite?]

The NDT part of the course gave us hands-on experience on the following techniques over several weeks;

Eddy currents
Magnetic particle testing
X-ray radiography
Infrared thermography
Liquid penetrant inspection
Ultrasonic testing
Acoustic emission

Not many are useful when inspecting for impact damage and delamination of composite materials, due to the very nature of their structure. Ultrasonic is by far the most reliable but not infalible. The FAA recommend the coin tap test, basically dropping a dime on the material and listen out for a difference in the sound, not too techical but suprisingly accurate. NDT of composites is still in it's infancy and there are other techniques we didn't look at in very close details, such as Laser Shearography but I don't believe these are in wide general use at the moment.

NDT of metal including aluminium is a proven science and many of the above methods come up with reliable results. Even an amateur like myself can detect subsurface cracks and flaws as well as the easier surface faults with reasonable accuracy, although obviously a more experienced NDT techician achieves a much greater accuracy.

Composite damage is cut out and filled with replacement material which may or may not be bonded correctly to the existing structure and then covered over with resin and filler making the repair no longer visible. It's difficult to detect whether the repair was done using the correct temperatures and timings, etc, important for the correct strength to be achieved.

The majority of repairs to aluminium are more easily inspected. Rivets can be counted and measured, and sizes and thicknesses of patches can be measured. As as I said above, metal integrity can be relatively easily tested using widely accessible NDT devices.

In summary, I personally believe that damage is generally easier to detect on aluminium aircraft and repairs generally easier to inspect due to the repair techniques used on aircraft. I imagine that a less professional composite repairer can get away with making very unsatisfactory repairs and still make them look good.

If there are any professional NDT people out there it would be interesting to hear what you think.

[englishal]

I'm sure the professionals can repair them just fine. After all professionals repair boats and F1 cars all the time.

You will probably find that composite is less prone to accident damage than metal. Metal can get a hole punched through it fairly easily, composite either doesn't or breaks obviously.

One of my planes(.....I seem to be collecting them ) has a hole in the wing, but no damage to the structural components like the spar. Yes it is *easily* repaired, but I reckon had the aeroplane been a Composite structure like the Diamond DA40, it wouldn't have got a hole in in the first place....They seem far tougher to me, as metal does to fabric.

[SkyHawk-N]

I'd rather have a hole in my aluminium aircraft, which I can see and get repaired easily than have a composite aircraft that has been bashed and has hidden delamination damage which is difficult to detect with sophisticated equipment, and that's if you know where to look for it.

[SNS3Guppy]

Try getting a repair done on a compound curve metal surface, vs. one on fiberglass, then determine which one is easier.

Fiberglass is far more tolerant of a sloppy repair, and in many cases easier to do, and easier to do with a good finish. Glass offers many advantages. It's also got disadvantages.

One of the chief inspection methods on many glass and substrate surfaces still remains the coin tap test.

[SkyHawk-N]

Try getting a repair done on a compound curve metal surface, vs. one on fiberglass, then determine which one is easier.

But how about a repair to carbon fibre which retains the components original strength?

[A and C]

GRP repair

Repair of GRP structure is well understood but the skills are not widespread, take the aircraft to a reputable repair company and you can be assured of a safe repair.

The problems come when un or semi-skilled people work on these aircraft simply becase you can't inspect the repair once it it finnished without NDT kit or taking thie repair apart. It is inspection that is the issue with used GRP aircraft NOT the airworthiness of properly conducted repairs.

[Jim59]

I have a GFRP/CFRP glider (EASA CofA) that is two years old. Based on fatigue tests carried out on wing spar sections it has a maximum life of 12,000 hours. This is subject to passing special inspections at 6,000 9,000, 10,000 and 11,000 hours. After that it is scrap.

Some gliders have somewhat shorter airframe lives of only 3,000 hours so it clearly depends on the design of the particular airframe.

My previous glass glider was moulded in 1972 and in its life has had little maintenance to the 'plastic' parts - but a few repairs to metal fittings. However, this is exceptional because it had a very good gel coat. Later gliders used different gel coats that were prone to cracking after a few years (less than ten in some instances) and have had to be re-finished at significant cost (usually in Poland). So again it all depends. Incidentally the 'good' gel coats were alleged to be carcinogenic hence the change - but they were also more difficult to work so it may have been cost-cutting that used inferior gel coats. Some makers are going back to the earlier gel coats that seem to have indefinite life.

My current glider has a polyester finish on top of the gel coat (a factory only option at extra cost) - so hopefully that will avoid the need for it to be re-finished prematurely.

[ProfChrisReed]

Jim 59 wrote:

Some gliders have somewhat shorter airframe lives of only 3,000 hours so it clearly depends on the design of the particular airframe.

Nothing to do with the airframe design. When GRP was introduced no-one knew how it would stand up to the stresses of flying, so a 3,000 hour life limit was set. As gliders began to reach this limit, inspections indicated that there was no deterioration so limits were raised by the manufacturers, and most are now at 12,000 hours (and I expect them to be raised again once some old gliders reach that number).

Problems have arisen where the manufacturer went out of business before raising the hours limits. This happened with the Centrair Pegase, and I think most European gliding bodies had the power to raise the limit on a national basis, so did so. In the US, however, there is no such body, and of course no manufacturer to approve the raise.

I don't know of any GRP glider model which has been grounded due to running out of hours where there is some authority which has the power to raise the hours limit.

_______________________

SkyHawk's comments show how much familiarity plays a role in our perceptions of structural safety. I have many hours flying GRP gliders, rather less in wood and very few in metal. I feel most comfortable in GRP, because I think I could recognise a potential problem, less so in wood and metal because I know of structural failures in both these caused by hidden, unobservable (to me) defects. For major structural repairs to GRP I have to trust the repairer, but the UK system has meant that all repairs have to be undertaken by competent repairers, and there's no history of failures so I guess I trust the system.

SkyHawk's experience is the opposite, so he feels more comfortable with metal.

Repairs to GRP by gliding inspectors don't seem unduly costly. As an example, a bad ground loop can snap the fuselage just in front of the tail, and I think this kind of repair would normally cost £2 to £3k. GRP is pretty strong, thus minor knocks tend to cause no, or merely cosmetic, damage. If I punched my fuselage hard I'd break a knuckle but cause it no damage!

I'm not saying GRP is better than metal or wood, just that it has a very long track record for making structurally sound aircraft.

[Magic Fingers]

Thank you all for the discussion so far. I know it is not a simple question and didn't mean to get to the really complicated stuff but you have already answered many of my questions. I have found it interesting how the avionics packages have been a major cost on many new aircraft, not just the plastic ones. Glass cockpits are brilliant and when they become more prevalent will hopefully lead to more shops becoming capable of repairing them but I am also a mechanic and agree how it can be easier to swap out the older style instruments for repairs, particulary when flying in some of Australia's outback areas.

Again, speaking as a mechanic, I am used to being able to turn around the scheduled maintenance of smaller Cessna's and Pipers pretty quick, I just haven't touched many plastic aircraft yet and have heard a few nasty stories. The unscheduled side is always unknown whether its plastic or metal but as mentioned in several posts the detection of problems can be very different.

Looking forward to hearing more,

Thanks.

[englishal]

Theoretically instrumentation failures in a glass cockpit aeroplane such as the G1000 should be just as easy to fix - you open the box and swap the offending module - done in 2 minutes. The problem as IO mentioned was finding someone who can do this.

However the mean time betwee failures on the G1000 is over 2000 hrs, whereas on normal instrumentation it is running at about 800 hrs, so likely you will have less downtime due to failures (my turn coordination has just packed up )..

Composite technology must be pretty good these days if they are even building airliners out of them!