Section II: Flying Techniques
Part II, Chapter 20: Types of Skis
There are two basic types of skis that are used in professional bush flying:
Hydraulic wheel-skis, which can be used on both cleared runways and snow surfaces.
Board-skis, which are restricted to snow and ice only. The basic tradeoffs are versatility, which is often a necessity, versus performance, cost, and weight. The bush pilot naturally prefers the marked performance advantages of pure skis when operating out of difficult terrain, and usually keeps both types on hand so as to better contend with changing conditions.
Hydraulic wheel-skis. When you are forced by circumstances to operate from both maintained airports and snow fields, you must put up with the many disadvantages of wheel-skis in relation to board-skis and do the best you can with what you have. These disadvantages are:
Cost, which is in the prohibitive range and approaches the cost of floats.
Weight, which runs approximately 70 to 100 pounds more than board-skis, depending on the size of the airplane.
Lack of efficiency caused by the many engineering compromises required by its dual-use configuration. These compromises, under difficult snow conditions on short strips, add dangerous, if not impossible, amounts to the required takeoff run.
Aerodynamic drag, which causes a greater loss of airspeed, fuel economy, and range than a set of floats that weigh approximately twice as much. In stark contrast to this is the situation with board skis, which cause only about a five knot penalty compared with wheels.
For the bush pilot who must operate off rough, rocky gravel bars with his wheel-skis in the wheels-down configuration -- a common situation during spring breakup and fall freezeup when operating between a snow-free low elevation and a deep-snow high elevation -- there is only one model on the market that will do the job. This is the Federal ``Airglide'' manufactured by Genaire Limited in St. Catharines, Ontario, Canada, and distributed in the United States by FluiDyne Engineering Corporation of Minneapolis, Minnesota.
The reason for this lies in the design of the skis. With the exception of the Airglide, all wheel-skis on the market are designed for pavement operations and raise the skis parallel to the runway and only an inch or two up when in the wheels-down configuration. As a result, this restricts the user of these skis -- which work quite well on snow -- to paved runways or dirt strips with rocks no larger than an inch or so in diameter.
The Airglide works differently, raising the front of the skis quite high, and, while on rough gravel bars with fair-sized rocks the heels do drag a bit, this is just part of the price you pay for their utility (see illustration 27).
Maintenance is basically limited to replacing the bottoms from time to time as they get damaged from rough ice and rocks, and the occasional replacement of the bungee cords.
Board-skis. These simple and inexpensive wheel-replacement skis are basically flat boards turned up at the front (and sometimes the rear as well). They are made out of wood, aluminum, or fiberglass with a pedestal to attach them to the axle. Bungee cords (or coil springs) with safety cables are used, as with the wheel-skis, to keep them in place while providing the necessary flexibility. The bottoms can be wood, metal, or fiberglass, but the preferred bottom, for anti-sticking reasons, is a thick piece of slick plastic attached with rivets or screws. There are a number of different makes and sizes available, and most bush pilots have distinct preferences based on their type of flying, snow-conditions, and experience level.
The very high performance obtainable from board-skis (see illustrations 14 and 17) comes from their light weight and the fact that, since they replace the wheels, no compromises in design have had to be made regarding optimum size, pedestal placement -- which is very important for proper action in deep snow -- or flexibility of action.
With the Super Cub, board-skis increase empty weight about 35 pounds; with the C-180/185, about 70 pounds.
Tail-skis. As a general rule, precipitation is quite light in arctic and subarctic regions due to the cold, dry airmass. As a result, the snow rarely gets deep enough to require the use of a tail-ski. The exception to this rule is along the southern coastal areas where, during spring, warm, moist air off the ocean overrides the colder air of the landmass and causes wet, heavy snow. Under these conditions, the snow will sometimes get so deep that the horizontal stabilizer ends up supporting the tail of the skiplane, and at this point the tail-ski will come in handy.
There are two types of tail-ski in current use:
The wheel-penetration tail-ski, where the tailwheel is left on the skiplane and penetrates through the ski. This model is used mostly in conjunction with wheel-skis, and is distributed by FluiDyne.
The wheel-replacement tail-ski, where the tailwheel is removed and the ski placed on the axle, just like board-skis. This ski is manufactured by Airglas Engineering of Anchorage, Alaska.
There are two valid reasons for not using a tail-ski unless absolutely necessary:
A tail-ski prevents you from using the tailwheel as a brake to slow the skiplane during the landing run (slide). This technique, in which you dig the tailwheel into the snow, is especially effective on bush strips that have been packed down with snowshoes or a snowmachine, and for this reason the bush pilot -- especially with the C-180/185 -- is hesitant to use a tail-ski unless absolutely forced to.
The tail-ski also becomes a negative factor on very narrow strips, as its use degrades the rudder-like steering authority of the tailwheel during that part of the ground run where there is not enough airflow over the rudder to provide positive directional control. This is particularly important on strips that have seen a lot of traffic and as a result have deep, hard-packed ruts.
The Cessna 206. Nothing has been said in this chapter about the C-206. The reason for this is that tricycle gear airplanes, for a multitude of reasons, make lousy skiplanes. Best to stick with wheels and/or floats with this machine, and use the C-180/185 or Super Cub for winter bush jobs.