Floating More Than an Idea
A primer on aircraft floats
By Greg Napert
July / August 1998
In several distinct areas of the country, and in locations around the world, float equipped planes, otherwise known as seaplanes, are a normal part of life. These aircraft are used for pleasure as well as utility and are a necessity in some remote areas of the world.
The floats, for the most part, are constructed much like the aircraft it is mounted on, but given the need for a structure to hold up to a great deal of impact and remain water tight, there are some distinct design differences which can make the maintenance of aircraft float fairly unique. In addition, the fact that many of these aircraft operate over salt water, presents some unique challenges to maintenance personnel. Floats aren't something you inspect and forget. Daily maintenance and care is required if these relatively expensive pieces of equipment are to last.
A bit of history
To give the technician in the field some of the basics about floats, AMT magazine visited the builder of what is generally considered to be the "Cadillac" of floats — Wipaire Inc., in MN. Wipaire, manufacturer of Wipline floats, says it builds over 70 percent of floats manufactured today.
According to Mark Mathisen, service center manager and chief pilot for Wipaire; the founder of the company, Ben Wiplinger was involved in converting WWII airplanes into corporate aircraft after the war. Someone came to him one time and asked him if he could correct problems on the floats people were currently using. After looking at the problem, he developed an STC to convert the electric floats to completely hydraulic floats with just electric indicating systems.
"This was very well received and later that same year, he made a decision to build his own floats. The company actually started as Wipline Floats. His first float that he built was an amphibious float for the Cessna 185 to compete with EDO, the dominant manufacturer at the time. His float business became so successful that he branched into float production and got away from aircraft conversions," says Mathisen.
Ben passed away in 1993, and his son, Bob ("Wip") Wiplinger took over operation of the company. Mathisen says that since Bob took over the company, there has been a very heavy emphasis on research and development. "We used to build floats for Cessna's 185, 206, and on up to aircraft as large as the Twin Otter. In the last three years; however, we've begun developing floats for small aircraft such as Super Cubs, PA-12s, Skyhawks, XPs, and we're now building a float for the brand new Cessna 182s. We plan to float the 180s, the Maules, and many of the current production airplanes," says Mathisen
"Right now," Mathisen continues, "the market is such that we probably have about 70 percent of the float market, our specialty being amphibious floats. But we also build straight floats for the same applications. In the last several years, there has also been a big demand for amphibious floats due to the versatility, ease of fueling, maintenance, etc. Other things that we do at our facility is related to our float business. We refurbish aircraft, for instance, with the intent of putting them on floats and do a good deal of factory reconditioning."
Mathisen says the company has also been adding service centers in the last couple of years for support due to the significant increase in its customer base. Its current service centers include one in Vancouver, BC, Anchorage, AK, One on the East coast called Telford Aviation in Waterville, ME, as well as one in Juneau, AK.
A good reason to keep them in good condition
You've probably heard rumors about the relatively high cost of floats and never really believed how expensive they are — well, believe the rumors.
Says Mathisen, "To give you an idea of float prices, the cheapest float that we build as a straight float is around $20,000 which is our Model 2100, which fits a SuperCub, PA12, Husky, Skyhawk. These floats as an amphibious float is $33,000. Our most expensive float, which fits on a Twin Otter, runs $482,000 a pair. With a water drop system for fire fighting, they are $650,000."
Mathisen explains the cost is justified through the rather extensive number of man-hours that go into building a float and the exceptional utility you get from them as an operator. "The material costs in these floats is fairly significant, but by far and away, the labor cost is the biggest contributing factor to the cost of the float. Everything is hand riveted and hand fitted and this can take some time. Much is misunderstood about the amount of material, number of bulkheads and number of rivets that go into building floats and the provisions that need to be made for attaching them to the aircraft.
Mathisen continues, "Many modifications may need to be made to the aircraft if it doesn't have a seaplane kit on it.
For instance, from the day a Cessna Caravan comes in the door, to the day it leaves, it takes us three weeks to configure the airframe for floats. Some of the modifications are structural, some are flight control related — like elevator down springs — or on the smaller ones, rudder centering systems to help prevent rudder lockup and to add stability in flight, finlets for yaw stability, etc. Typically the installation costs are set aside, but as an example, the Super Cub installation costs are around $4,000. The aircraft's hydraulics and electrical systems may need modification as well, but that varies from aircraft to aircraft. And, this is assuming that the aircraft has a sea plane kit already installed."
"If you start with a Ôgeneric' land airplane, you typically need to add a seaplane kit to include a propeller that can provide better static thrust (critical for taking off with floats), and hoisting rings, and corrosion proofing — so you've got to factor in these costs as well.
Mathisen adds, "There are extreme variations in the complexity of the installation from aircraft to aircraft. A Super Cub, for example, is fairly simple, requiring attach fittings to the lower longerons of the fuselage, whereas a Cessna 172 typically requires a beefing up of the fuselage by adding stringers or doublers."
Up goes the weight
Despite rumors to the contrary, Mathisen says that floats do add weight to the airplane. For example, he says, "On the Caravan, the float adds roughly 1,000 pounds to the aircraft. If you take away the weight of the original landing gear, you come up with the exchange weight. On this aircraft, the exchange weight works out to roughly 700 pounds."
This extra weight will take the Caravan from a useful load of around 3,600 pounds to around 3,000 pounds. So, you do have to take that into account. On a Super Cub, a straight float will reduce the useful load by around 170 pounds, depending on the float. The useful load ends up being around 350 to 400 pounds, and the gross weight goes from around 1,750 to around 2,000 lbs with the float installation."
Keeping it simple
Mathisen says that one of Wipaire's great successes with its floats has been the simplicity of the landing gear or hydraulic system. "Instead of the old single hydraulic actuator running a series of cables and pulleys to retract everything, we use simple dedicated actuators on their own, so it's all hydraulic, there's no mechanical linkage involved other than downlocks which are very simple. With our floats, if you lose hydraulics, the landing gear stays where it's at — it has a mechanical downlock and uplock so you don't have to worry about having a gear in transit."
Corrosion — the biggest challenge
Mathisen says that because of the nature of the float, corrosion is by far the most challenging aspect of float maintenance. This is compounded by the fact that most of its larger floats, such as on the Twin Otters and Caravans, 75 percent to 80 percent operate in warm/humid salt water.
He explains that Wipaire takes many corrosion prevention measures in-house, when it builds floats, that its customers have brought to light over the years.
Mathisen says, "There has been a big change in the way we manufacture the floats — all the way from the materials that we use to the coatings we put on the floats. We are using different alloys and plating processes that significantly improve corrosion protection. Additionally, for floats that operate in salt water environments, a zinc sacrificial anode is installed on the floats."
In addition to pre-priming the metal prior to manufacturing, the company takes small steps like installing Teflon® tape under stainless nut plates during installation to inhibit electrolysis or dissimilar metal corrosion. Additionally, it paints all rivets after assembly and coats all exposed hardware with a corrosion prevention compound.
Mathisen says that 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."
Mathisen says maintenance of the landing gear system is quite simple as it utilizes commonly available parts where possible. The wheel and tire and brake assemblies are available through most parts distributors. The main gear is custom designed by us.
He continues, "All nose landing gear systems are essentially the same on our floats with the only change being the size of the components. In addition, there is a change in the materials used for the nose gear strut on older floats. Larger nose gears utilize a 3M™ scotch ply, (composite) strut. The smaller used steel for the struts. All current floats; however, regardless of size, use the 3M™ scotch ply, (composite) struts.
For the main gear, the small floats utilize a track system and a neoprene biscuit, similar to a Mooney main gear, while the large floats use an oleo strut with dual main gears."
Due to a combination of the unusual loads and stresses placed on the floats, as well as the fact that they need to be water tight, Wipaire worked with 3M to develop a system years ago to bond the seams of the floats with a special heat-cured 3M adhesive. Many technicians in the field are unfamiliar with this system and are reluctant to make repairs. Mathisen says, "In actuality, the system allows total field repairs to chines and keels without having to debond anything. The main theme in repairing the floats is that the gunwall, chines, and keels, on the floats have an existing gap allowing for field repairs. All that's really needed is to remove the filler/sealant between gunwall, chines, or keels and the original skin; remove the damaged skin, and then place your patch into position and rivet it into place using the maintenance manual or approved sheet metal procedures as outlined in 43.13. The beauty of this existing gap is you don't have to remove the skin from the existing chines and keels and damage the integrity of the float. All you have to do is remove the damaged skin and insert new sheet metal in the space provided and seal it with PRC1422 or equivalent sealant."
Additionally, repair parts to include custom sized replacement cap splices for the keel, are readily available by calling the factory or service centers.
For any further information or assistance, contact Wipaire at 8520 River Road, Inver Grove Heights, MN, 55076, or call Mark Mathisen; (612) 451-1205 Fax (612) 457-7858. Web address is www.wipaire.com