As the leaves fall, and our northern seasons change from fall to winter, pilots are faced with many challenges—the need to pre-heat our aircraft engine, fitting of engine and wing covers if our pride and joy spends its nights outdoors, an eclectic collection of not very stylish clothing, and a proclivity toward hot beverages, as opposed to cold ones. But while some aviators may let a little thing like winter restrict their flying adventures, the dyed in the wool aviation junkie will just shift gears. Landing gear, that is--from wheels to skis.
For those aviators who haven’t experienced ski flying, you don’t know what you’re missing. Many of those scenic destinations that you couldn’t quite find time to visit last summer are even more spectacular when covered with a blanket of snow. More and more resorts and communities are promoting year round tourism and attractions of all sorts are available to the traveler willing to brave the cold. If you’re a fishing devotee, ice fishing is a great way to extend the season, and competition for good “holes” is almost non-existent with an airplane on skis. Finally, if you’re into masochism, there are the challenges of winter camping in wilderness scenery unmatched at any other season.
Most aviators are, by necessity, restricted to runways of some sort during the summer months, but winter offers a whole new range of possible "airports", with literally thousands of level landing sites within reach of many northern locales. These landing sites are often lakes and rivers, but with sufficient snow cover, ski-equipped aircraft can land virtually anywhere that’s relatively level, and even some places that are very definitely not level.
Lets take a look at what it takes to prepare for ski flying adventure:
Traditionally, most ski-equipped aircraft were tailwheel types, but this isn’t an absolute requirement. In fact, some tricycle gear types, like many of the Cessna singles, make very capable and versatile ski aircraft.
These days, the aspiring ski pilot would be well advised to research the type certificate for his or her aircraft and the available supplemental type certificates, to determine if ski approvals exist for that aircraft. Field approvals for ski installations used to be an everyday occurrence, but nowadays approvals may have to be either via the aircraft type certificate or a supplemental type certificate (stc). Nonetheless, many pilots may be surprised to discover the number of approvals there are for ski installations on many aircraft.
Ski manufacturing is coming back to life after a lull that roughly paralleled the slump in new aircraft production. While there aren’t a lot of new ski approvals out there, and few ski manufacturers, new ski models are on the horizon and manufacturers will be looking to expand their installation base by working on new supplemental type certificates.
Bellanca 8GCBC Scout equipped with Landes 2500 straight skis Photo: Mike VivionWhat are other characteristics that you should look for in a candidate ski aircraft?
Power is high on every ski pilot’s priority list. Lots of power and a propeller optimized for takeoff and climb are essential ingredients for a good ski plane. Thrust is the name of the game on skis. While ski conditions can occasionally make a pilot wish for a little more braking action, more often than not the issue is whether you have enough thrust to overcome the drag caused by the snow.
Good cabin heat and defrost capability rank high on every ski pilot’s list of priorities as well.
A high wing design is more flexible than a low wing for ski operations, but there are successful low wing ski planes out there, so I wouldn’t totally discount equipping a low wing plane with skis. Tasks like draining the wing sumps can be somewhat more “interesting” on a low wing plane in snow, though, and deep snow may make the low wing design a liability due to snow berms and other obstacles. That said, I’ve had the wings of a high wing aircraft in the snow on occasion-but that’s another story.
Enough cabin space to carry all the necessary appurtenances is important in winter flying, whether on skis or wheels. Wing and engine covers, a snow shovel, snowshoes, survival gear, and sleeping bags all add up to a lot of bulk, but not a lot of weight. More often than not, the ski pilot runs out of room as opposed to weight carrying capacity in winter. This is where those extended baggage compartments really pay off. A large aft compartment where light gear can be stowed really helps-particularly in the smaller, two seat aircraft. I recall wishing for more cabin space in the Cessna 185 that I operated on skis for many years, though.
Favorites of the ski flying world are most of the fixed gear single engine Cessnas, the Champion/Bellanca/American Champion models, deHavilland aircraft of several models, Aviat’s Husky, the Maules, the high wing Pipers, and most if not all the Taylorcraft models. I’ve certainly failed to mention a number of excellent ski planes in this list, but the point is, there are a number of choices out there for the ski flying enthusiast.
Many different ski models have been built over the years, for use on a wide variety of aircraft. Many of these skis are no longer in production and would be difficult to install legally nowadays, since they were often installed via field approvals. Nonetheless, many of the aircraft listed above have ski approvals noted in their type data. As an example, the type certificate for my airplane, a Cessna 170 B, lists several different models of straight and retractable skis that are approved on this aircraft. Find a set of the appropriate model skis, and get a mechanic to install them per the type certificate and good operating practices, and you’re ready for some ski flying adventure. Finding a set of Wesco straight skis might be a bit difficult, mind you, but there are still lots of AWB retractable wheel skis around if you look for them.
The ski types in common use today are straight skis, penetration skis, semi-retractable skis, and fully retractable wheel skis. To make things more interesting, there are skis made from metal, fiberglass, thermoplastics, some high tech carbon fiber composites, and the original composite material, wood. For years, F. Atlee Dodge in Anchorage built fully certified wooden skis. Let’s review the pros and cons of the various ski types:
Straight skis are, as the name implies, purely ski landing gear. No wheels, no actuator mechanisms, just plain basic skis. For performance in deep snow or challenging conditions, nothing can beat a good straight ski. They are also lighter than any other ski type, and they generally have more bottom area as well, affording maximum “float” in deep or difficult snow conditions. And, believe me, at times landing in very deep snow can provide a deeply moving religious experience for the unwary pilot. Maximum float is a good thing.
Straight skis are also the simplest skis to rig, and require very little recurring maintenance. Nowadays, virtually all ski bottoms are covered with one of the new generation of plastic materials, such as Ultra High Molecular Weight plastic (UHMW), to reduce friction, and prevent the skis from sticking down after being parked. UHMW is available in various thicknesses, and is typically cut to fit the ski bottoms, then riveted or bolted to the ski bottoms.
Straight skis are installed by removing the wheel and brake assemblies (in some installations the brakes are left in place) and replacing them with the skis, mounted on the same axles, with the same hardware as the wheel assemblies. The typical straight ski rigging consists of a rear limit cable, fastened between the rear of the ski and a point on the fuselage aft of the main gear attach point (the rear float fittings if installed, are often used). The aft limit cable prevents the front, or toe of the ski from rising too high in flight and the length of this cable determines the angle of incidence of the ski in flight. A forward limit cable, which is generally attached between the toe of the ski and a point near the firewall of the airplane, prevents the toe of the ski from going down in flight (which falls into the category of a VERY bad thing). A bungee or spring attached between the front of the ski and the same fuselage attach point as the forward limit cable takes up the slack in the forward limit cable. The bungee or spring keeps the toe of the ski at the proper landing attitude by keeping tension on the aft limit cable in flight. Skis are typically rigged just slightly nose up with reference to the wing’s angle of incidence (generally from + 1 to 5 or 6 degrees).
While straight skis have their advantages, they also have a number of disadvantages. With straight skis, your only option is to taxi, takeoff and land on snow or ice covered surfaces. If your home base doesn’t offer some snow-covered surface that remains useable for a substantial part of the season, straight skis may not be practical. If you are so fortunate as to operate from your own airstrip, and it can be groomed for skis, straight skis may be just the ticket. Hangaring a ski plane can be accomplished with handling wheels that are available on the market or can be made for nearly any ski arrangement. Fuel availability may also be an issue, though some marinas still offer auto fuel in winter months. If you park the plane outside, you’ll need to develop a technique for propping the skis up off the snow when parked to prevent them from freezing down.
Finally, the straight ski airplane becomes an almost exclusively off-airport machine for all intents. If you prefer to fly to and from airports, straight skis may not be the best choice.
Wheel penetration skis offer a bit more flexibility than do straight skis, in that they provide the ability to operate on either runways or snow covered surfaces. Penetration skis use the standard landing gear, complete with wheels and brakes, but install a ski with a cutout for the landing gear tire to run in. The skis themselves attach to the landing gear axles via axle extensions, or stub axles. They are rigged in similar fashion to straight skis, and the ski height is set such that on pavement or gravel surfaces, only the main tire is in contact with the surface. Generally, a small “tailwheel” is attached to the rear of the ski to prevent it from dragging on hard surfaces. The penetration skis’ biggest advantages are that they are reasonably priced and they offer the pilot the ability to use either snow covered or bare surfaces, a major advantage over straight skis.
For every silver lining there has to be a storm cloud around the corner though, and penetration skis are a compromise. The penetration ski’s primary disadvantages are that they are heavy and they don’t perform as well as straight skis, particularly in deep snow. They are heavy because you are now effectively carrying around two complete sets of landing gear; the skis and the wheels. The opening in the ski through which the wheel runs, the protruding tire itself, and the tailwheel, create a lot of drag in deep snow, so takeoff performance suffers significantly over straight skis. Nonetheless, many pilots opt for these skis because they are simple, cost effective, and they require minimal effort to install and remove. If most of what you do with a ski plane is fly to lakes with a foot or so of loose snow, these skis may work just fine for you. Or, if you are into winter camping and deep snow, penetration skis may offer you many opportunities to test your winter camping skills.
The "ultimate" ski setup for many pilots is the retractable wheel ski. These skis are "ultimate" in many ways, not the least of which are cost, weight and complexity. Nevertheless, retractable wheel skis offer the best of both worlds in many ways. Retractable wheel skis install either on a separate stub axle, or on an extension to the existing landing gear axle.
There are as many types of operating mechanisms for retractable wheel skis as there are models of this type ski. Some use purely hydraulic retraction systems, some use electro-hydraulic retraction systems, and at least one uses purely mechanical, spring actuated gear, which is capable only of extension, but not retraction.
With retractable ski gear, the skis have some sort of cutout through which the wheel protrudes during wheel operations. Prior to landing in snow, the extension system is activated, which extends the ski downward, thus “retracting” the wheels (though the wheels themselves never actually move).
The Wipaire hydraulic retractable wheel skis (previously manufactured by Fluidyne) use a hydraulic ram to cycle the skis down and under the tires, so that the tires actually rest on the top of the skis. Similarly, the out of production Federal AWA and AWB line of skis use a hydraulically activated ram to extend the ski downward. The ski is actually mounted on a long arm and the weight of the airplane comes to rest on this arm, with the tire above the ski.
The out of production Fli-Lite skis originally made by Federal Ski look similar to a wheel penetration ski, except that the ski is mounted on an axle with a u-shaped arm assembly, and a plate is driven under the tire in flight by a hydraulic actuator. As the plate moves, it contacts the tire, forcing the ski downward and eventually the tire rests on the plate, which covers the opening in the ski.
Landes Air Glas in Anchorage currently offers a hydraulic retractable composite ski that is STC'd for Cessna 170/180/185, Found Bush Hawk, Maule M-4, Piper PA-12/14/18, Husky A-1/A/B/C, CubCrafters Top Cub, among others, in various sizes for applicable weight.
Aero Ski makes a semi-retractable (for want of a better description) wheel ski, the R2800 ski. These skis look very much like the wheel penetration skis, except that they can be repositioned. For runways, the skis are in the “up” or wheels position, with the wheel protruding from an opening in the ski. The pilot lands in snow with the wheels protruding, which produces a bit quicker stop than other skis might. Once on the surface, the pilot exits the airplane, and extends the skis one at a time manually via a prybar lever. After a snow takeoff, the skis can then be retracted by pulling a handle inside the cockpit, which releases a catch on the skis, allowing spring pressure to raise the skis, effectively “lowering” the wheels through the openings for a runway landing.
The advantages of these skis are that they are simple and relatively inexpensive. The primary disadvantage is that they can’t be retracted in flight, so some snow takeoffs must be done with wheels extended (creating a lot of drag-see comments under penetration ski discussion above). Takeoff performance suffers as a consequence.
My assigned aircraft while operating as a pilot for the US Fish and Wildlife Service in Alaska was an Aviat Husky, operating on Rosti-Ferndandez 8001 retractable wheel skis, manufactured by Solo Aviation, in Germany. These skis were field approved before field approval procedures changed a year ago, with the approval based on the JAA approved Supplemental Type Certificate. While it would be difficult to achieve a field approval on skis in this country today, these skis would be a great option for those many experimental aircraft flying in northern climes these days. Got a homebuilt Super Cub? These may be the skis for you. At present, these skis are by far the lightest skis on the market, and they perform as well or better than straight skis in the bargain.
As you can imagine, the geometry of the ski rigging can be a serious issue with any retractable ski gear, and the manufacturers have addressed this issue with a variety of engineering solutions including hydraulic cable retraction systems, hydraulic riggers and just plain clever application of geometry.
More and different types of materials are available these days for manufacture of aircraft parts, and this is becoming true for skis as well.
Originally, most skis were made from the original composite material: wood. F. Atlee Dodge Aircraft Services in Anchorage, Alaska manufactured a very nice set of wood straight skis until a few years ago and many sets of these skis are in service around the country.
Manufacturers gradually transitioned to metal skis, as produced by the two big ski manufacturers of yore: Federal and Fluidyne. Both these manufacturers built straight and hydraulic retractable wheel skis. The Fluidyne type certificates are now owned by Wipaire, Inc, which has placed the retractable Fluidyne skis back into production.
Landes Airglas began manufacturing aircraft skis in the 1950’s, when Wes Landes used his boat building skills to manufacture aircraft skis from fiberglass. A long line of fiberglass skis followed, including literally thousands of sets of skis and tundra pads for helicopters, including most military heavy lift helicopters today. Landes has manufactured a great line of straight and wheel penetration skis for years. The late Gary Landes (Wes’ son) worked on certification of a line of retractable wheel skis made of high tech thermo plastic, the primary advantage of which is its extremely light weight. The Landes thermoplastic skis STC'd for Cessna 170 through 185 tailwheel aircraft weigh a mere 39 pounds each, which is close to the average weight of a Super Cub straight ski. These skis have revolutionized the world of wheel skis. Landes has gained supplemental type certification on these skis for a variety of aircraft, and you can run them today.
Burl's Aircraft Magnum 1 tail ski Photo: Hitchcock AviationThe tail wheel on an airplane can serve as a very effective anchor in certain snow conditions. On occasion, this can be a benefit, but mostly, it’s a hindrance. If your chosen ski steed possesses a tailwheel, seriously consider equipping it with a penetration tail ski. Burl’s in Anchorage manufactures a very nice penetration tail ski, called the “Magnum 1” tail ski. One of these will significantly prevent wear and tear on your tail surfaces and aft structure.
You tricycle gear guys and gals can contact Landes Airglas for a nose ski that will fit most tri gear ski planes.
So, start thinking about those ice fishing/cross country skiing/snowshoeing/winter camping adventures. A set of skis on your airplane may just open a lot of doors to winter adventure that you never dreamed of.
This is a living article in the Knowledge Base. If you have feedback on the accuracy or legitimacy of this entry, or would like to add more information, join the discussion below or email firstname.lastname@example.org to volunteer your input. Suggestions and changes will be incorporated readily.