Seaplanes & Salt Water:
To
Operate or Not to Operate?
That depends on
you!
The success of
companies such as Maldivian Air Taxi in the Maldives, and the longevity of
Turtle Airways in Fiji, proves that such seaplane transfer companies can
operate out of the most salt laden oceans of the world as long as proper
maintenance is performed routinely.
According
to Wipaire, 70-80% of larger commercial amphibious floatplanes, such as
the Caravan, are used in warm/humid salt-water environments. Naturally
aircraft owners of such expensive aircraft are reluctant to operate their
aircraft into such extreme conditions because of the inevitable effects of
corrosion. But, with most of the world’s water being tied up in the salt
laden seas and oceans it is near impossible to avoid operating into salt
water if you want to operate commercially.
More pointedly, water front property
anywhere in the world is in high demand for tourism and many of the best
hotels and resorts are built near the ocean or by lagoons. The market for
exclusive seaplane transfers from international airports of entry right to
the doorsteps of waterfront resorts and hotels often depend on being able
to operate in and out of salt water. The success of companies such as
Maldivian Air Taxi in the Maldives, and the longevity of Turtle Airways in
Fiji, proves that such seaplane transfer companies can operate out of the
most salt laden oceans of the world as long as proper maintenance is
performed routinely
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Corrosion
cannot be avoided. So you might as well learn how to prepare your aircraft. Then reward yourself with a day on this secluded island... |
Corrosion Control:
Corrosion cannot be avoided. That is a basic scientific
fact and an inevitable condition of mixing metals and electrolytes, i.e.,
seaplanes and saltwater. Seaplanes are the anodes and saltwater the great
conductor trying to return the seaplane to the sky, molecule by molecule.
According to Mark Mathisen of Wipaire, the solution is simple. The key to
preventing corrosion in the field is to seal any metal against exposure to
the elements. “If you can seal it from the environment, it will last
forever.” In other words, isolate the less noble anode and save it from
the evil electrolyte.
Unfortunately, although Mark is correct, the
answer is not so simple. Sealing depends on the surface coating preventing
moisture and oxygen from penetrating the surface and causing further
corrosion. The problem with corrosion resistant seals is that the
integrity of the seal is tenable. Cladding, for example, is the physical
act of placing a more anodic metal on the surface of a less anodic metal.
Ironically, a cladding coating will corrode (oxidize) first and create a
surface (oxide film) that will seal but not continue to penetrate.
Cladding depends on corrosion taking place in the first place and is thus
not perfectly stable. It’s like fighting fire with fire.
Stainless steel, on the other hand, works as
a seal in that it’s surface is not easily penetrated by moisture and
oxygen and thus will resist corrosion. (Unfortunately, it’s too heavy to
make seaplanes out of.) Electro-plating steel with chromium provides a
lighter corrosion resistance method of obtaining strength. Anodizing is an
electro-chemical way of cladding aluminum. Painting simply creates a
tenuous seal
by adhering to the metal surface and not allowing corrosive elements to
penetrate. Grease and wax work the same way except that they are very
temporary.
Seaplanes are built out of aluminum because
it gives the best strength to weight ratio, and has the most resistance to
fatigue. The airframe structure, however, is held together with stronger
steel bolts and hinges and bearing, and the amphibious landing gear is
made with steel to withstand the heavy loads. These dissimilar metals
generate an electrochemical process where the aluminum becomes the
reactive anode and the steel becomes the cathode, and it is all conducted
by the electrolyte water. The more salt in the water the better the
conductive properties. The idea of protecting the aircraft from corrosion,
therefore, would be to prevent the electrolyte from forming a conductive
path between the dissimilar metals.
The use of cladding, plating, anodizing,
paint, grease, and wax, however, to seal off the aircraft metals from all
catalyst is not a perfect art. Cladding and anodizing gets scored,
stainless steel contains impurities, plating and paint chips off, grease
oxidizes or absorbs water, and wax dissipates. The end result is that
electrolyte works its way into unprotected areas and starts to corrode the
bared metal.
Besides, the discontinuity of bolts,
fasteners, rivets, skin lap joints, bushing, bearings, hinges, and all the
other connectors that keep a thousand aircraft parts flying together in
tight formation makes sealing the entirety from the environment impossible
or impractical to begin with.
So that leaves the salt water operator with
a very proactive goal, and that is not to prevent corrosion, but to
control it by eliminating as many of the basic requirements for its
formation as possible.
1) Preventing the electrical potential
difference within the metal.
2) Insulating the conductive path between
areas of potential difference.
3) Eliminating any electrolyte that could
form a conductive path on the
surface of the metal.
4) Giving the electrical potential a
conductive path that is not detrimental
to the aircraft.
Aircraft Integrity: Was it Meant to Be In the Sea?
Much of the first control depends on the manufacturer of the aircraft or floats. To avoid undue corrosion, the manufacturer must avoid the improper use of metals for aircraft parts that are to be used in salt-water conditions. Using cladding on aluminum alloys, coating steel with cadmium or chromium plating and using stainless steel where practical, such as for control cables, are three methods of avoiding excessive corrosion on aircraft by preventing electrical potential within the metal itself. Wipaire has changed their methods of float building over the years to incorporate the expanding requirements of their clients, including using different alloys and plating processes that significantly improve corrosion control.Another method is to avoid using the improper metals to make aircraft parts out of to begin with. The Pilatus PC-6 was notorious for excessive corrosion when used in and around salt-water conditions simply because the manufacturer used so much magnesium in the airframe. Magnesium corrodes ferociously when exposed because it is the most anodic of all aircraft metals. In other words, magnesium turns to toast in no time at all, and an aircraft made of toast is sure to crumble. Wipaire did the same number to Caravan operators by supplying wheels made of magnesium alloy. One engineer claimed that he could witness the corrosion gnawing away at the wheels after the aircraft came out of salt water.
The second control that manufacturers, or remanufactures, have is to avoid dissimilar metal contact. When different metals are held in contact a galvanic corrosion can take place. Thus, some form of insulation must be provided. Placing strips of Teflon tape between dissimilar metals, and coating steel fasteners with zinc chromate primer before using them in an aluminum structure are the two most common methods of insulation. But, not common enough! Many manufacturers skip these steps when putting your aircraft together. This is also the main problem that operators suffer when they send their aircraft for rebuilding.
Many remanufactures have not had to deal with the extensive electrolytic process of having an aircraft immersed in salt water and therefore do not take any of the above preventative measures. They mostly scrape the old paint off with steel wire wheels and emery paper and spray on a fresh coat. It looks good for a while and the paint job most likely did not cost a great deal. But, the long-term costs are the extreme devaluation of the aircraft. No aircraft with extensive corrosion will be worth anywhere near it’s blue book value. Thus, in terms of pure economics (and ultimately maintenance related safety,) corrosion control is the most important issue facing seaplane operators for maintaining their aircraft’s true value. 1
Operator’s Integrity: What Part do You Play?
And that leads us to the third point in corrosion control, and this is the proactive part of the process. Eliminating any electrolyte that could form a conductive path on the surface of the metal. Naturally that means protecting the integrity of the aircraft’s paint, wax, and lubrication, but it means a bit more. It also means preventing the salt water from lingering or clinging onto any surface of the aircraft for any length of time. Electrolysis is a factor of conductivity plus time and the longer the salt water remains as a conductor, the more damaging the effect will be.This will take a bit of explaining. First of all, amphibious aircraft were not made to sit in the water. Like most seabirds, amphibs are seaworthy, but they are meant to roost on land. They simply have too many moveable components: brakes and oleos and actuators that depend on being corrosion free to work properly. The longer the aircraft sits in the electrolyte the more chance corrosion will find a conductive path.
One sunny warm and humid day in the rainforest of the Niger River Delta I anchored my C206 amphib near the mouth of fresh water tributary. The tide was coming in bringing with it the salty ocean waters. I was enjoying the sound of the birds singing deep in the forest, which I could easily hear on this windless day, when I noticed a distinct “crackling” static background noise. I could not place where it was coming from. Just then an old “papa” from one of the nearby villages came paddling up in his dugout canoe.
Even though he could not readily speak English, I questioned him about the unusual noise coming from my aircraft. He nodded his head knowingly and pointed to his canoe. “Same, same,” he said in broken English. “No,” I said, and knocked on his canoe and then on the aluminum floats, “your canoe is wood, my plane is metal.” He patiently patted on the inside on his canoe and pointed to his ear, “Same same.” I looked to where he patted and saw an aluminum patch nailed inside the wooden frame. I awarded the “papa” with a bottle of fresh drinking water, and took a closer inspection on my airplane.
Out of the pits in the prop, and out of every nick in the paint where I could see bare metal, I found salt crystals growing right before my very eyes. The entire aircraft frame had become conductive, as the salt water slid past the floats on it’s way inland.
Floatplanes, without the wheels, actuators, oleos and brakes, do not have such a fierce problem as amphibians. Over the long term corrosion will “eat away” a good set of aluminum floats, but the process will not result in an inoperative gear. Just a leaky set of floats. Eventually the floats will have to be rebuilt, but the implications are less risky. Amphibs need to be removed from the water at the end of each day. That is proactive decision because operators have to take this into account when planning their infrastructure. We will discuss more about proactive decisions shortly, but first lets move on to the forth point in the control program.
That takes us to the forth and final point: giving the electrical potential a conductive path that is not detrimental to the aircraft. After listening to my aircraft being eaten away that day on the river I decided to let the corrosion have something else to dine on. I went to the marine shop of our company and “borrowed” several zinc anodes they used on the aluminum boats. I used the machine shop and fashioned two anodes, which I bolted to bare metal on the floats. After that I never heard the crackling static again. The electrical potential was being dissipated through the sacrificial elements of the less noble zinc.
When we began operating the Caravan amphibs, we again started having corrosion problems. It was not until Wipaire finally got smart and devised a zinc anode to attach to the floats near the gear, that our problems came under control. There are three points to remember here. One, make sure the anode is in direct contact with bare metal. Two, remove the anode at every 100-hour inspection and brighten the metal under the anode. Finally, change out the anode whenever it shrinks down to 1/3 its original size to insure the protective properties are maximized.
So, with all these potential corrosion problems, what allows an amphib to be able to operate in salt water at all? Like I said earlier. Proactive corrosion control. A clean polished and protected surface cannot provide a place for corrosion to start. Dirt, old grease, exhaust emissions, oxidized paint film can all contribute to holding electrolyte in contact with the surface of the exposed metal and promote corrosion.
Aircraft should be kept clean, and painted surfaces should be kept waxed and polished. A good aqueous or emulsion type cleaner will remove dirt and industrial contaminants that serve to adhere moisture to the surface. Rinsing the salt off the aircraft with large volumes of water is imperative, but in itself is not enough. Salt can permeate the slightest dirt clinging to the aircraft and can even be absorbed along with water into the best aviation greases. Therefore, cleaning and polishing the surface (and refreshing the grease during inspections) will allow all moisture to drain off the aircraft as soon as possible and prevent further moisture-induced electrolysis.
Some of the most corrosive elements can be found in aircraft exhaust so it is important to emphasize that all traces of exhaust emissions should be removed from the aircraft daily. A good product for cleaning the exhaust will also work as a protectorate to expedite further cleaning. Any type of cleaner that uses micro-abrasive particles for polishing will scratch the surface leaving it porous and prone to absorbing water and thus holding an electrolyte potential. Make sure the polish is a recommended product for aircraft surfaces.
In addition, the interior of the aircraft should be washed out regularly. Specifically, the floor, the belly, and the tail sections should be flushed and washed out to prevent salt water from accumulating inside the aircraft structure. All drain holes should be kept free. (This can be done along with the 100 or 200 hour float inspection.)
Asking and Acting: Maintaining Your Integrity
I have had operators ask me two basic questions that relate the misunderstanding of how corrosion takes place. 1) If the aircraft is flown from the sea
to the airport and it is dry when it arrives, would it be more sensible to
not wash the aircraft instead of leaving it wet for the night?
2) If the aircraft has not been washed the
night before, does it make sense to wash it just before flying it back to
the salt-water environment?
For question one, the answer is simply that
washing the aircraft will remove the salts that make the water or moisture
so conductive. Even if it is dry, salt and contaminants will act as an
electrolyte and promote corrosion by attracting moisture out of the air.
That is how land planes suffer corrosion when they never touch salt water.
Furthermore, even if it is wet after the washing there is little
electrolysis action in fresh water and thus minimum corrosive effects.
Secondly, if the day’s dirt and contaminants are washed off the fresh
water remaining will run off easily and the remaining water will dry
quickly leaving a corrosion free environment.
The answer to question two is just a variation of
the answer to the question one. If you wash off the dirt and contaminants
that bind the salts to the surface of the aircraft, even when it is moored
directly in the sea, there is less chance of corrosion taking effect.
Plus, for both question one and two, after the
aircraft is washed the lubrication is refreshed and that will help seal
out the salt water. Some of the recommended lubricants, such as WD40 and
CorrosionX are water dispersants as well and can be sprayed directly on
wet, freshly washed, parts. CorrosionX will actually bypass any
contaminants and chemically bind directly to the metal to prevent
corrosion even if the corrosion has already started. It is an excellent
product to use in the hard to reach places, such as inside the aircraft
floats, gear well, the aircraft belly, and inside all parts of the tail
section.
Thus, if these conditions are met:
1) The aircraft is built or rebuilt to meet
corrosion control standards;
2) The aircraft is washed and sealed nightly with
proper lubrications;
3) A sacrificial anode is attached to the floats
and kept conductive;
corrosion from the daily excursions into the salt water will remain controllable with a minimum of effect on the long-term value of the aircraft.
One of the biggest mistakes I see operators
make is to buy a well used aircraft that has “only a trace” of corrosion
because it has been operated in fresh or brackish water and then
immediately begin operating it in ocean conditions. The trace corrosions
will suddenly blossom to almost uncontrollable proportions. The reason is
simple. Fresh or brackish water will not conduct as effectively as salty
water. The aircraft was able to continue in the previous environment with
minimum corrosion because of the minimum conductivity, but the integrity
of the paint and exposed metal were breached over time regardless.
When exposed to a warm moist and salty
environment, suddenly the corrosion explodes into all the breached areas
and starts taking hold. The only answer is to take the aircraft off line
and completely strip, de-corrode, alodine, prime, and repaint the entire
aircraft. In fact, buying the used aircraft was not a mistake, but not
preparing it properly for it's new environment is the mistake. That should
have been done to begin with.
Moreover, like I mentioned earlier, the paint
shop must realize the importance of doing the job right to begin with. A
land aircraft used in Arizona can get away with a quick strip and spray,
but an amphibious seaplane to be flown in the ocean cannot. Corrosion
control measures must be in place.
proactive stance:
Finally we come to the part about taking proactive decisions and
maintaining the integrity of the aircraft, the operations, and the owner.
That can all be realized by careful planning. It only makes sense that if
your aircraft are maintained they will be ready to fly when your customers
are ready. And if your customers are taken care of then your
business will do well. Then start by washing and lubricating your plane.
So many operators who have not worked with seaplanes will scrimp with such
a simple process. And the end result is corrosion.
The reasons given are usually that spending that
much time at the end and beginning of the day on corrosion control is
excessive thus bringing the costs of operating up beyond the acceptable
level. I have had operators compare the time spent on corrosion control to
that of time spent on helicopter maintenance. But, there is a major flaw
in this argument.
There is little comparison to helicopter
maintenance when looking at labor costs, because the helicopter requires
experience licensed engineers. Washing and lubricating an amphib is labor
intensive, but the job does not take skilled maintenance workers. Any wage
earner can be trained to spend the time on aircraft care, as the job does
not require a signature for release.
The job is time consuming, but not expensive. As
the best time for doing the cleaning is at the end of the day, then a
sheltered location with strong lights and access to pressurized fresh
water is definitely required. Work stands and stable ladders are also
needed to reach the wings and the tail with little danger of slipping. The
work stands are used for the day maintenance as well so can serve double
purpose.
Moreover, do not withhold on the lubricants. Buy
the best and use them liberally. That point cannot be overstated. I have
seen too many good Caravans for sale because the operator did not want
to spend the money in the first place and now no one
will touch them. They were operated in salt water
with little or no protection. What you save in corrosion control, maybe
thousands, will cost you in aircraft integrity, possibly hundreds of
thousands. Keep your aircraft airworthy and they will continue to be an
asset on your books and not a liability.
The second part of being proactive is to maintain
an abundance of spare parts for the known parts on the landing gear and
the aircraft that will corrode the fastest. Have a supply of assemblies
ready to change out the entire wheel assembly, brake assembly, and landing
gear and drag link assembly. Clean off the corrosion and re-prime and
lubricate the fixed parts on the floats, and then bolt on the new or
refurbished assembly. Do this every 100 or 200 hours as the case may
require, and you will greatly lessen your chances of having a down time
brake or gear problem. In fact, change out the brake assembly every 50
hours if need be. Do what is necessary to keep flying.
That does not mean you have to replace the gear
every 100 hours or the brakes every 50 hours with new parts. What you do
is rebuild and refurbish the removed parts on the work bench during the
day when the aircraft are flying. That way the engineers are in effect
working on the aircraft while it continues to fly, keeping both the
revenue coming in and making the best out of the engineers valuable time.
In the meantime, whenever corrosion is found
during regular inspections, it must be removed immediately. For example,
corrosion blisters under paint should be removed, neutralized, treated,
primed, and repainted with care. The integrity of the painted surface
cannot be over emphasized. And the paint should be cared for with the
appropriate waxes or sealers to prevent oxidization or chalking.
To prevent the original paint job from looking a little tatty
after a few years make a concerted effort to know your paint brand and
specific blend and you can avoid having the occasional patch spayed with mismatched paints.
Along with a better looking result the operator can rest
easier knowing that that value of his aircraft has remained intact along
with the painted surfaces. That says a lot about selecting a simple paint
scheme. After all who looks at the paint job when there are so many
other distractions near the sea.
Article and
Images by John S Goulet
Note from the Editor. If you want to miss out on enjoying 90% of the earth's surface then certainly hold back. If not, let The Bush Pilot Company show you how to operate off the world's oceans. 1 (With the possible exception of a well trained pilot.)
Use the attitude indicator as your guide back to The
Bush Pilot Company.
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Last modified on
May 07, 2006 .
© Virtual Horizons, 1996.