Intro to Ski Flying Part 2: Operations

1.14.2014 - Initial article format
In the second part of this series, we cover operations on skis, and the unique challenges of safely landing and taking off on frozen surfaces, and most important of all: How to avoid getting stuck.
1.14.2014 - Initial article format
In the second part of this series, we cover operations on skis, and the unique challenges of safely landing and taking off on frozen surfaces, and most important of all: How to avoid getting stuck.

Part 2: Operations

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So, from Part 1 in this series, you’ve got your pride and joy outfitted with skis, your mechanic has rigged them just so, and now you're ready for some winter flying fun. Well, almost. Granted that you're already equipped with winter covers, knowledge of engine preheating, survival gear, and other winter flying considerations (right?), we’ll jump right to the flying. Before we get started, you should know that the winter ski pilot must always be prepared to spend the night wherever the airplane has landed, if necessary.

I strongly recommend that the novice ski pilot find an experienced skiplane instructor, preferably one who is also experienced in his or her aircraft type, and do some introductory ski training. In Switzerland, a pilot is required to complete a glacier flying course, and a series of instructional flights including 200 ski landings in different seasons prior to being licensed to fly onto the glaciers solo. While U.S. pilots aren’t required to complete any training or obtain a rating to fly skis, it's wise to remember that a few hours of instruction from an expert is the cheapest insurance policy you'll ever buy. That said, it can be difficult to find such an instructor, particularly if you live in a part of the country where ski flying is less common. Nowadays, there are a multitude of internet forums available which offer the ability to contact others who own and operate similar aircraft, and often, with a request via one or more of those web forums, one can come up with a qualified instructor. Right here in the BackcountryPilot.org tailwheel instructor list or forums is a great place to start, as well as TreetopFlyers.org, SuperCub.org, and Skywagons.org. As always, though, remember that sometimes you get precisely what you pay for on the Internet: nothing, so be careful. Use these sites as a starting point, and go from there.

Being prepped for cold-weather operations is paramount. Using an engine cover while shut down makes restarts much easier in cold temps.Photo: Brad Thornberg

Some considerations

Prior to diving in and giving ski flying a go, it's important to consider some of the unique challnges that the frozen winter environment poses. The snowy environment really is the main character in ski flying, and should be studied well before departing. Here are some things to consider:

Snow quality

As any skier will tell you, snow varies greatly in both texture and character. The ski pilot must learn to read snow conditions from the air prior to landing and understand their effects on the aircraft's performance... or suffer the consequences. Perhaps nowhere in the world of flying does the term "site evaluation" have more meaning and importance than in ski flying. Many people think of snow as being soft and smooth, but the truth is far different. Wind moves snow into drifts, and once snow "sets up," it can become incredibly solid, uneven, and irregular. At near flying speed, even crossing a snowmachine track that has set up can damage a ski plane.

There is an old saying that there are dozens of words that describe snow in the Inuit languages of northern native cultures. People who ski recreationally have a number of different names for snow as well: "powder snow," "corn snow," etc. These are descriptors for differing snow composition, which also affect the performance of your ski plane. In fact, the reason a Swiss Glacier Flying license requires landings in different seasons is because snow conditions in winter are much different than summer conditions on glaciers. Warm/wet snow increases drag considerably.

The hard-frozen wind-eroded snow condition known as sastrugi is about the last thing you want to ski across if you value your aircraft and your dental work. Photo: JasonJthomas @ Flickr

This sparkly, cold powder is about as good as it gets... as long as it's not too deep, as such cold snow can be hard to pack down. Photo: Mike Vivion

Visibility and light quality

Visibility is a serious issue to the ski pilot. And not just the effects on visibility caused by precipitation. Winter sun angles are low in the north and this, coupled with overcast weather and a white environment can create a condition commonly referred to as "flat light." It can be nearly impossible for even the most experienced ski pilot to determine height above the surface in flat light. This is different than "white out" conditions, which generally involve significant obscuration to visibility as well as flat light. Flat light can be dangerous, but combine flat light with white out conditions, and it is simply not safe to fly low level, let alone land on the snow.

We humans rely heavily on depth perception, which relies on fairly subtle cues. Shading and shadows are both high on that list of cues that we exercise without conscious thought. Enter into a world with no shadows, no color for contrast, and little shading, and it can be difficult or impossible to see obstacles that are partially masked by a blanket of snow. It is a policy of the glacier flying operations in the Alps that no glacier flying is permitted on any day with contiguous cloud cover, even if that ceiling is very high, say 25,000 ft. The European glacier pilots understand that flat light can be very dangerous.

Flat light is such a risk that obstacles around airports like this snow berm are often spray painted with day-glow paint to prevent taxi collisions. Photo: Mike Vivion

While most of the photos you'll see in this article are of ski flying on a beautiful, sunny day with great visibility, those photos were chosen for their photo quality. Winter weather is often not nearly as good, and the dangers of PVFR conditions are pervasive.

Temperature

Temperature affects our function, the performance of the airplane, and the performance of the skis in the snow. There are some fascinating physics involved in ski flying, and it’s important to understand at least some basics. When a ski rests on a snow surface, the pressure between the ski bottom/snow interface actually causes some physical changes at the near molecular level within the snow. This creates a very thin layer of water where the interface occurs. This is one reason skis slide well. When temperatures decrease, the snow eventually becomes too cold for this to occur, which means that ski performance will suffer significantly at very cold temperatures. It is important to understand that a spot where you operated yesterday may demand significantly different performance from your airplane to manage today, and much of this relates to temperatures.

On the other hand, reciprocating engines produce more horsepower in cold temperatures, due to the density of the induction air. If you are operating at –20° F, you don’t necessarily need to use all the power the engine is capable of generating at those temperatures, so keep the throttle back a bit to save wear and tear on the engine, unless you really need maximum power.

Above all, ski flying should be fun, at least if done for recreation. I generally cut off my personal flying at a minimum of –20, but at work, -40 is our cutoff. My definition of fun definitely does not include –40 F temperatures. Remember that at these cold temperatures, the atmosphere is generally very stable, and cold sinks can form in lakes and other low areas, creating pockets of very cold air. Most intelligent beings will likely limit their ski flying to temperatures around 0 F or warmer. Just remember, though, that very warm snow can severely and negatively affect aircraft performance. The longest I’ve ever been stuck on skis was due to cool temperatures that warmed to +40 during the day, while we were in deep snow. The airplane would not climb up out of this “corn snow”, so we shut down and waited till very early the next morning, when sub freezing temperatures allowed the snow to consolidate again, and us to escape.

As good as it gets: CAVU conditions and a cherry big-engined Cessna 170B. Photo: Brad Thornberg

Snow and ice stability

A condition called overflow can be dangerous, or at least inconvenient. Overflow is unfrozen water lying on top of ice, which may exist as free water under a crust, or may turn the snow nearest the ice into the consistency of a Slurpee. Overflow is common in areas where large amounts of snow have fallen on relatively thin ice, pushing the ice down and cracking it. It can also occur where water levels have continued to drop after freeze-up, allowing water from a stream to flow out on top of the ice, and settling in the low spot in the center of the waterbody.

I have found overflow in areas that have been subjected to –50° F and colder temperatures. The snow that lies on top of the water can insulate the water enough for it to remain unfrozen. Land your ski plane here, and you may be in for some real winter "fun." If the skis break through the top layer of snow and settle into the water, they will immediately start to freeze in. As the pilot steps out of the plane, soaking wet boots and feet may be another consequence, and there you are, with a stuck airplane, which is starting to become an integral part of the lake you landed on, and you have wet feet.

The tell-tale signs of overflow as seen from the air. Photo: Brad Thornberg

A typical scenario involving overflow follows: The pilot puts down tracks on a lake surface, and does not observe any dark areas in his tracks, where the skis broke through a crust into water. Had he waited another five minutes or so, he may have noted that the airplane’s tracks gradually darkened, as the free water soaks through the tracks to the surface. Sometimes it takes quite some time for overflow to show itself. The lake is of insufficient length to land, stop, and takeoff straight ahead, so a turnaround will be required. On landing, everything seems fine, until the pilot starts to turn, at which point the outboard ski (the one on the outside of the turn) breaks through the crust, which had prevented the airplane from breaking through earlier—barely. This swings the airplane around, so that it is now pointing at the shoreline, and one gear leg is “knee deep” in slush. And, there you are: stuck, and maybe with wet feet. Hopefully, not at -40 degrees.

Now the pilot has a job on his hands, because the longer he waits, the more “stuck” the airplane will become. But, there are several things the pilot can do to mitigate the danger, and maybe even get home today. First, I always carry several extra large plastic trash bags in the plane for just this eventuality. Slide the trash bags over the skis, and you will minimize the freezing of the skis to the lake. Cutting a long pole to lever each ski up out of the free water, then sliding spruce boughs, branches or anything else you can find under the skis, one at a time, will lift the skis above the water level, and once the water freezes, you may be able to escape. I also carry a long (100 feet) section of aircraft cable, a few ice screws and a small rope come along (winch). With these devices, I can carefully turn an airplane that is pointed in the wrong direction, or pull a stuck airplane out of a tight spot. Just be VERY careful as you turn the skis that you don’t apply significant torque to the ski axles, which are easily damaged with twisting loads.

Above all, don't risk your health to try to extricate the airplane. Remember the camping gear in the back of the plane. Don't get yourself so sweated up struggling with a stuck plane that you risk hypothermia. If it looks like a really big job, get the trash bags on the skis, do the preliminaries, then set up camp, get a fire going to dry yourself out, and tackle the problem a step at a time, with the camp and fire to back you up. Survival is your first priority.

Overflow can be relatively common in some winters, and virtually non-existent in others. Generally, in interior Alaska, overflow has frozen up by March, so it also tends to be seasonal, at least in Alaska. I don't worry much about overflow, but I sure look carefully for it prior to any ski landing. Any hint of grey or black showing up in my ski tracks will have me going somewhere else to look for a landing site.

A large spider hole, indicative of overflow.Photo: Mike Vivion

Getting underway

Now that you've got an understanding of the kinds of hazards you should expect when operating in this environment, let's talk about actual operation of the ski plane and how it can differ from operating on wheels.

Photo: Brad Thornberg

Taxiing

Your first exposure to ski flying will be taxiing. If you are on straight skis, taxiing can provide some of the more tense moments of a flying career. There are no brakes on these things, so pay attention, and give yourself lots of room to maneuver and stop. If you are operating on wheel skis, you will still have some braking action. Takeoff from a packed ski strip is pretty straightforward, and in fact, most tailwheel aircraft are much more forgiving on skis than on wheels.

Taxiing on skis is, like landing and takeoff, very dependent on the depth and consistency of the snow. The key is to do whatever is necessary to get the main skis in a pitch attitude that is just slightly nose up, and the tailwheel sort of "floating" behind the airplane. That isn't easy to do without using big blasts of throttle, and in fact, that's one of the best ways to turn around: sudden, full applications of all controls is the order of the day.

A common scenario is to turn around at the end of a "strip," in teardrop fashion, and re-enter the tracks you've just made to use them for takeoff since they're nicely packed now. First, slow down to almost a stop, then initiate a turn to the OPPOSITE of the direction you intend to turn around. Like a "crop duster turn" in flight, we're going to ease off to the right, for example, then bring the airplane all the way around to the left....remember, we're aiming to get back in those tracks, so a mere 180 degree turn won't get us there....we need some to the right, then a lot to the left, then some back to the right to align. Once we get the airplane moving right a bit, we apply a lot of nose down elevator, full left rudder, and a quick blast of power, to get the airplane turning to the left. The quick blast is essential, because if you keep the power on a tad too long, the airplane will accelerate, and if it does that, it's going to go straight ahead, not turn. The key is quick, short blasts of power, with nose down elevator applied and full rudder to kind of blast the airplane around. It is not a pretty process, but it's all that will work in deep snow. Again, too much nose down pitch, and the noses of the skis will dig in....won't cause you to get upside down, but it will create a huge amount of drag. Remember, those aft limit cables control the ski pitch attitude when you start to bring the tail up.

The tailwheel can be more like a tailhook in snow with any depth. A tail ski is a huge benefit, and will not compromise stopping ability as some would argue.Photo: Brad Thornberg

When taxiing on a really slick surface, a short loop of rope tossed over the nose of a ski can help to turn around in some situations but providing a little differential friction. But frankly, if you need that kind of crutch, you probably shouldn't be there anyway.

If you really get jammed up and can't turn around in the space available, dig out that long piece of rope you carry just for such events, tie it to your tailwheel, and tell your passenger to pull sideways on that line as you power around to the desired heading. No passenger? Dig out that come-along that you carry, and the long piece of light cable. Attach the cable to an ice screw (go to the ice climbing counter at your local outdoors store) that you've screwed into the ice surface. Use the come along to winch the tail around.

Yes, I have done all of these at one time or another. Be aware, however, that turning a ski plane around in snow exerts a HUGE amount of torque to your axles, and it is quite possible to break an axle turning a ski plane around manually, while exerting an external force like pushing or pulling on the airframe. All things in moderation: turn it SLOWLY, and a little bit at a time. Using a come-along, I winch just a bit, stop and walk to the toe of the ski, give it a gentle kick sideways to release pressure on it, then winch some more, etc.

And finally, sometimes you just get properly stuck, which leads us into the next topic.

There's the struggle to taxi in deep snow, and then there is just plain stuck. Warm temps and deceptive snow depth spelled trouble for this Maule. Photo: George Thomas

Site evaluation and landing

As you approach your destination and start looking for a place to land, your first priority should be to find a useable landing zone near your destination. While that sounds pretty intuitive, there are times when conditions dictate a landing site farther from your actual destination than you’d like. Rough snow may preclude landing on the south side of a lake, but landing on the northeast side of the lake, then taxiing to the south side or snow-shoeing there may be an option. Your favorite cabin may be near the mouth of a small stream, and the ice in front of the stream may be either thin or overflowed. The place to make these determinations is while you’re still in the air, not after you are on the surface. In Alaska’s interior, the prevailing winter winds are from the northeast, so I can almost guarantee that if there is a smooth landing surface on a given lake, it’s likely to be on the northeast side of the lake.

What lurks down there on that water's edge? Is it a sandy bar or receding ice? Photo: Brad Thornberg

Next, you need to determine the wind direction. A wind circle, or wind eight is the best means for determining wind direction and approximate velocity. Initiate a constant rate turn over a selected point, fly a complete circle, and the drift of the airplane will tell you which direction and the approximate velocity of the wind. If your airplane is GPS equipped, you can of course, simply fly the four cardinal directions and watch your groundspeed on the GPS.

Once you find a likely landing area, it’s time to start taking a close look at your proposed landing zone. One of the characteristics of any off-airport operation is that there are no convenient white stripes indicating the centerline of the "runway." So, the pilot must fix in his or her mind, precisely where the runway is. That decided, your task is to determine if that surface is in fact suitable for landing. By flying very low over it, parallel and just slightly offset from it, you can clearly inspect the surface. On this pass, you’re looking for obstacles of any kind, drifts, beaver houses, wet spots, and if there’s any question as to whether its long enough, time it to determine the length of the landing surface (at 60 knots over the ground, you are moving at 100 feet per second). If you find no apparent obstacles, your next pass may include a touch-and-go to start feeling out the surface.

This first touchdown pass should include just a light touch of the skis on the surface. On this pass you are looking for large anomalies—things that could break the airplane if you put more weight on the skis. If, in the middle of your proposed touchdown zone, you feel a solid bump, you’ll need to do a bit more investigating to determine exactly what that was. There may be a snow covered log or a swale that is difficult to see. If so, you may have to re-align your “airport.”

Photo: Brad Thornberg

Traffic patterns in the bush are flown quite differently than at an airport. At an airport, typical patterns are flown at 800 to 1,000 feet AGL. In the bush, 200 feet is plenty high, and your downwind pass should be immediately adjacent to your proposed landing site, so that you can get a good look at it from different perspectives and sun angles. It’s amazing how often some very large objects are nearly impossible to see from one angle, but stick out like the proverbial sore thumb from another.

If you found no major anomalies in your first pass, it’s time to put more weight on the skis and test the snow more thoroughly. On downwind, look for any signs of free water from overflow, any anomalies in the tracks at all, and consider what they may mean. Evaluate your touchdown point, and determine if you can shift it forward or back, depending upon your needs, and any obstacles.

On this pass, you'll be slowing the airplane somewhat while on the surface. As you slow, you should be feeling for significant drag, bumps, and so forth that you may have missed on your "light" pass earlier. If the landing zone passes this test, and there are no real doubts remaining in your mind, the next pass may conclude in a landing. That is assuming you are comfortable with how deep the snow is, and how safe the conditions are.

These guys are obviously doing a formation low pass, but observation of the surface prior to landing will require similar. Photo: Brad Thornberg

Let's say you'e still a little suspicious. Maybe there's been a lot of overflow around recently, or perhaps the snow at home seems a lot deeper than it feels here. On your next pass, you'll want to land the aircraft and slow down even more. In fact, you may slow down to a virtual walking pace, then power up and go again. If there's overflow there, or if the snow is very deep, you should have put sufficient weight on the skis to reveal those characteristics. Keeping the airplane moving even at a slow speed may be just enough to keep you from getting stuck.

While powder is the most supple and soft form of snow, it can also be a lot of work if it has any depth. Notice the snowshoe tracks to pack down the runway; lots of work. Photo: Mike Vivion

If the snow is more than about knee deep, you may want to put down several sets of tracks with the airplane, along the length of the "runway" you intend to use. As these tracks set up over the next hour or so, they will offer much less drag on your subsequent takeoff attempt. When I say that snow "sets up," Im describing the phenomenon wherein the skis compact the snow by knocking much of the air out of it. Cold temperatures also change the consistency of the snow to very hard and compacted. If you land in an area where multiple passes aren’t practical, you can pack down a runway ahead of the airplane with snowshoes after you land. A few hours after this treatment, you can walk on the crust that is created, even in very deep snow. In warm temperatures, though, snow doesn’t set up, so be careful of landing in deep snow in temperatures close to the freezing point.

While pilots may pride themselves on making one very precise set of tracks with their skis on multiple passes thus exhibiting their superb airmanship, I prefer to make several sets of overlapping tracks. If you've ever been in a ski plane in VERY deep snow, you will know why. Once a single set of tracks set up they act almost like railroad tracks, composed of compacted snow instead of steel. You may land on those tracks, but as you slow, or during your takeoff attempt, your skis will tend to slide off the edge of them, into deeper snow. Another great way to get royally stuck. Had you instead made several sets of overlapping tracks during your site preparation, the likelihood of sliding off the tracks is much lower. Pack a runway, not a single track.

Touching down

Everything seems fine, and you've decided to land. You need to decide what you will do with the airplane after you land. Is the landing zone (LZ) long enough for you to land, stop, then takeoff straight ahead? Or will you have to turn around at one end of the LZ and either taxi back or take off in the opposite direction? These are all decisions that should be made in the air.

Once you've convinced yourself that this is a viable landing zone, set up for a soft field landing, and generally land short if practical, to extend the tracks you've made and to use the least amount of LZ. Touch down tail low; this is a real soft field landing, so follow the guidance of your airplane's manufacturer, or in lieu of that, land tail low at minimum controllable forward speed.

The elegant lines of a DHC-3 Otter on skis touching down Photo: Rich Hulina

As the airplane touches, let the tail come down to assist in braking. Remember, there are no brakes on most skis. Once you've stopped, either 1) run to the end of your prepped area and turn around, or 2) turn around now and run back to the start of your tracks and park in the tracks. Believe it or not, even in -30 F weather, skis warm up from the friction of running in snow, during the landing and taxi. Before you park the plane, stop it short of your parking place, and wait for a moment to allow the skis to cool. Now, taxi slowly forward the length of the skis, stop and again allow them to cool. Finally, taxi the length of the skis once more, and park the plane. You'd be surprised at how much frost a warm ski will generate if parked right after landing and taxi.

After parking in deep snow, I dig out the snow from under the ski, leaving the plane resting on a narrow pedestal of snow, directly under the axles, plus a small pedestal front and back ends of the ski. This permits air to circulate under the skis while parked. Some folks taxi up onto some pine boughs, and that works fine, but those aren't always available.

A note on equipment: With UHMW wear strips in place of the steel strips that come on most skis, this technique will almost always allow you to give the toe of each ski a light sideways kick when you're ready to go, and those skis won't freeze to the surface. Whatever you do, though, get rid of those steel wear strips....they WILL get you stuck if parked on snow. UHMW is essentially the same polyethylene as the PTEX that's used in the snow skis you strap to your feet.

I would venture that a large percentage of the incidents involving pilots getting stuck on skis occurred when the pilot attempted a turn. Turning puts significant side loads on the skis. The edges of the skis may tend to dig in and cut through the surface crust. In any case, as you turn around, you will put more weight on the outside ski than on the inside ski. If you must turn around, do so, but if it’s not really necessary, I always prefer to land, stop, and takeoff in the same direction.

Once you've come to a stop where you intend to park, stop the airplane and wait for a few seconds. Then power the airplane forward, about a ski length, and wait a bit. Power forward yet another ski length and shut down. Skis generate heat as they slide across snow. By allowing the skis to cool off in contact with the snow prior to parking, you’ll find that the skis stick down much less than if you’d just stopped and shut down in place.

Takeoff

The key to any snow takeoff is to get the airplane into the ideal pitch attitude which will permit the plane to climb up onto the snow surface, and accelerate, without dragging the heels of the skis (or the tailwheel) and without pitching nose forward far enough to reach the limit of the aft ski limit cables, thus pressing the nose of the skis down into the snow. Any of these conditions will create huge amounts of drag, and overcoming drag is what ski takeoffs are all about.

Notice the tailwheel just barely floated off the surface to keep it from dragging in the snow. This attitude will provide the shortest takeoff. Photo: Brad Thornberg

In most ski planes, I set the elevator trim to approximately neutral, power up to max power, and try to find that sweet spot in elevator travel where the airplane accelerates. Most often, unless loaded really heavy aft, that winds up sort of "floating" the stick or yoke in the neutral position. With a tail ski, that ski will float the plane's tail up out of the deepest snow, without forcing the nose of the skis into the snow, which a lot of forward elevator can and will do. This is a fairly subtle "feel" event, and takes some practice to get it right. Again, tail skis really help in deep snow, even in airplanes with light tails. It's really easy to push a little too hard on that stick/yoke and force the nose of those skis into the snow.

While tricycle gear ski planes aren't very common, they have some distinct advantages in the realms of turning around and takeoff in deep snow. For one thing, they have three LARGE skis, instead of two large skis and one little bitty one on the tail. Second, the pilot can yard back pretty hard on the yoke, apply some power, and if you've loaded the airplane right, you can lighten that nose ski up enough to help facilitate turning. The same application of nose up elevator helps reduce the drag for takeoff. A Cessna 206 on skis with a competent pilot at the controls can go just about anywhere a 185 ski plane can, and maybe some it can't.

Not nearly as common as tailwheel aircraft for ski flying, but trikes like this 172 work just fine and actually have some advantages. Photo: Jim Whiting

Sticking around

As mentioned earlier, you're flying into a winter environment, and getting stuck can be a serious deal if you have to overnight. Even if you're not presented with a survival situation, even the most rudimentary of stops and time spent on the ground require a little more preparation. If you're planning for a short visit on the ground at your destination, maybe for ice fishing or for a "biological" break, throw the engine cover on to keep the engine warm, and conduct your business. If, on the other hand, you plan to spend the night, you may want to try to minimize the amount of ski bottom that remains in contact with the snow, to prevent frosting of the skis. This can be done by digging snow from under the skis, by propping the skis up on spruce boughs, or by sliding plastic garbage bags over the toes of the skis, and moving the aircraft forward till the bags cover the skis. The idea is to prevent the ski bottoms from freezing down, or growing frost while parked.

In the morning, apply the appropriate flavor and duration of engine preheat, pull the covers off, and start another day in paradise: Ski flying, USA.

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 knowledge@backcountrypilot.org to volunteer your input.

  • Text by Michael Vivion
  • Photos by Michael Vivion, Brad Thornberg
  • Some text initially published in Pilot Getaways Jan/Feb 2005
  • Leading photo by Brad Thornberg

1.15.2014 - Format article - Z. Jacobson

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Disclaimer

While this knowledge base is a compilation of information from various sources, some official in nature, it is not a recognized or acredited source of aviation training information, and thus should be considered entertainment. Please consult a FAA-certificated flight instructor or mechanic prior to putting any information found here into practice.