The Pilot's Lounge #18:
They Put Skis on People, Don't They?
Come winter, many pilots hang up their flight bags and wait for warmer, sunnier flying weather. That's a shame, because they could be participating in one of the season's best flying activities: Skiplanes. Flying a small plane on skis is one of the most enjoyable ways to keep flying once snow covers the ground. AVweb's Rick Durden tells how.
As the snow piles up around the windows at the virtual airport and some of the resident airplanes grow funny appendages on their landing gear, I always ponder the fact that many pilots will spend a pile of money to put their feet into skis but few ever explore flying airplanes with skis attached. That's sad, as skiplane flying is one of those niches of flying, such as splashing around in seaplanes, that is a tremendous amount of fun but is only experienced by a few pilots. Plus, you don't need a new rating to fly on skis; you're still in a land airplane. As most pilots know, some of the most enjoyable vistas they have had in flight have been during the winter. Having skis on the airplane now allows you to more fully explore some of those remote spots you've looked at from afar for so many years.
A number of pilots here in the Lounge regularly fly on skis during the winter and, in fact, we have an annual get together in January where some of the regulars at the virtual airport converge on Cadillac, Mich. There we rent a J-3 and Super Cub and take dual from Derek DeRuiter at Northwoods Aviation. Because of events such as this over the years, I've had a chance to listen to and fly with the pilots who seem to know their way around airplanes on skis and I've tried to keep track of what they've said about ski operations. I've also read and reread the late Fred Potts' book, Guide to Bush Flying, Concepts and Techniques for the Pro, which has the best discussion of operation on skis and in cold weather of anything I've ever found in one place. He also developed a web site featuring a wealth of information and background on bush flying and Alaska.
Skis? On An Airplane?
There are skis available for most all tailwheel airplanes and even a few of the nosewheel variety. You can buy skis on the new or used market and have them installed on your airplane in fairly short order. There are even folks around that can give you instruction in the ways of flying off of snow and ice so that you don't turn a fun experience into a chilly and expensive ordeal. For, as with any endeavor involving airplanes, there are some things to learn before schussing off to the great white north. For example, last year a couple landed their airplane on a frozen lake, let it slide to a stop and then broke through the ice. Sadly, they did not survive their mistake.
What Kind Of Skis?
There are two general types of skis for airplanes: wheel replacement skis and wheel penetration skis. The wheel replacement skis are, as the name implies, put on once the wheels are removed. The skis attach to the axles themselves. Wheel penetration skis have cutouts in the side so that the landing gear tires can protrude some distance through the ski. Some versions have a hydraulic mechanism to extend and retract the ski as appropriate for the landing surface. Others have a lever mechanism activated at the ski itself to pry the ski down below the wheel. The ski may only be "extended" while on the ground. A spring release operated from the cockpit allows the ski to be retracted when landing on plowed runways. The limitation on extension of the ski is no big deal, because even with the ski retracted the airplane can be landed in snow as the wheel still sticks a few inches through the ski. On the first landing on snow after departing a plowed runway the pilot merely stops, shuts down, gets out and levers the individual skis down.
Tailwheel skis are also available. Most have a permanent cutout for the tailwheel to protrude a few inches. Opinions vary as to the value of tailwheel skis; however, for operations in deep powder a tailwheel ski is needed, otherwise the tail section sinks into the snow until the horizontal stabilizer and elevators are resting on the snow. Not good. Moving the airplane may require adding power and full down elevator to blow the tail into the air. Turning can be extremely difficult, as the tail must be blown into the air, the turn made and then completely stopped before lowering the tail, so as to avoid bending the aft fuselage.
I've observed that one of the best ways to get serious winter pilots into an argument is to innocently ask whether one should use a tailwheel ski. Then walk away. Come back in 10 or 15 minutes to see if there has been any bloodshed. Without a tailwheel ski it is easier to stop the airplane in a short distance by pulling back hard on the stick, plus it is easier to keep the airplane straight after landing because of the drag of the tailwheel in the snow. In any kind of deep snow, however, the tailwheel drag in any side load can potentially damage the aft portion of the fuselage. Even with a tailwheel ski, applying full up elevator during the slideout after landing will slow the airplane down relatively effectively. I do not take sides on the issue (hey, I rent, and take what is available), but I recognize that there are locations and conditions where a tailwheel ski is essential and others where it is a handicap.
Watching pilots learn to master tailwheel airplanes I've found that one of the best ways to begin a tailwheel checkout is to start on skis. The airplane is much easier to control on the ground on skis than on wheels. The pilot gets a feel for the attitude and behavior of the airplane on the ground before switching to the further joys of wheeled operations. I keep recalling the words of an experienced, old Alaskan bush pilot who liked flying his Cub on floats or skis but just couldn't tolerate "them casters."
Preflighting On Skis
Aircraft And Equipment
Preflight involves looking at the condition of the skis, the attaching hardware and the bungees that keep the skis positioned slightly nose up in flight. If the airplane is on wheel replacement skis or retractable skis that have been left extended on snow, there is a good chance the skis will have frozen to the surface. Most all skis have a high-density polyethylene surface bonded or riveted to the underside to help fight the problem of freeze down. Nevertheless, it does occur. Give the airplane a bit of a shove and see if it slides. If it doesn't move relatively easily, the skis have probably frozen down, so it's worthwhile to get them broken loose before you get in and start up. By going to the point where the wing strut attaches to the wing and pushing and pulling carefully, it is usually possible to break the skis loose. Sometimes using the handles on the rear fuselage in front of the horizontal stabilizer and working fore and aft as well as side to side will solve the problem.
That exercise should serve to remind you that when you stop for any length of time it is a good idea to retract skis that can be retracted or, better still, taxi onto some brush, 2 x 4s or even garbage bags to keep the skis out of the snow. (Most bush pilots carry 2 x 4s and garbage bags in the baggage compartment for just that purpose.)
If you are going anywhere more than a very short distance from civilization, make sure you have survival equipment easily accessible in the airplane. I have one of the kits put together by Dr. Brent Blueand I now just toss it in the baggage compartment and put a Leatherman tool with a knife I can open with one hand in my pocket. Just because I'm going out to have fun doesn't mean I'm going to forget that winter can be serious.
Operations And Planning
Before you get in and start up, look around the area. Are you going to blow snow all over someone else's airplane and potentially clog an air intake? If you are one of those that feels it necessary to start at half throttle and see 1,700 rpm on the tach immediately upon starter disengagement, I don't have a lot of sympathy for you, but I do feel sorry for your engine, particularly in the cold. On skis you are going to discover your habit will cause the airplane to start moving. Immediately. Is there a clear space ahead so you don't hit anything before you get your act together and the power back where it should be?
If the airplane does not have a starter it means you are going to have to figure out how to prop it. Standing on snow or ice in front of an airplane that has no brakes can make you a Darwin Award candidate. Most likely you will be dealing with a J-3; therefore you prop it from behind the propeller. Put a person at the controls you trust and you have briefed. You stand with your left hand holding the open door frame at the front and your right hand on the propeller. Wear gloves. Thick ones, as you are going to curve your fingers over the trailing edge of the blade. Keep your legs well aft of the plane of rotation of the propeller. In fact, you may want to stand on the ski so that if the airplane moves you stay with it in the same relative location. Pull the propeller downward, snap your wrist to get it through a couple of compression strokes and, assuming the engine is in a reciprocating mood, you're in business. As for startup procedures in the cold, you might want to review the column, "Pilot's Lounge #5: Yes, It's Winter," right here on AVweb.
Unless you are on ice, the airplane will normally stop when power is brought back to idle. The distance it will slide depends on the nature and depth of the snow, so it is a matter of judgment for the pilot when taxiing and when it's time to come to a halt. Naturally, you want to take care of everything on the pretakeoff checklist you can before you bring the power up to do the mag and carb heat checks. The airplane will be moving for those checks, so have a plan in mind as to where you want to be going and don't spend too long looking at things in the cockpit.
Making a takeoff may only require that you make sure you have enough open space ahead, point the airplane into the wind and open her up. If using a runway and there is a crosswind, normal crosswind aileron techniques work just fine. As with any tailwheel airplane, pick a spot at least a half mile ahead of the airplane and keep the airplane going straight for it. Initially keep the elevators about neutral or slightly nose down, to avoid causing the tailwheel to dig in and slow things down. The tailwheel drag will tend to help keep the airplane going straight so that is an advantage until the rudder is effective. With full power across the rudder, it should become effective early in the takeoff run. Shortly after going to full power, go ahead and raise the tail just out of the snow to minimize drag. Keep the tail low to maintain an upward thrust line to help reduce the weight on the main gear skis. If there are packed areas where skiplanes have been operating, stay in the packed tracks to maximize acceleration. Yes, you can do it. Tailwheel airplanes have plenty of rudder control. With a little practice you can keep the airplane in an established set of tracks. If flaps are approved for takeoff, use them. Snow adds drag, so you want to persuade the airplane to fly at the slowest speed possible. Ordinarily the taildragger's attitude during the takeoff run is perfect for lifting off at a slow speed — the airplane will simply fly off the snow. Sometimes you find that you have to do a little work to break free if the snow is sticky. There are times that pulling one ski off will allow the reduced drag of just one ski in the snow to let you accelerate that extra knot or two you need to free the remaining ski. This is similar to "unsticking" a seaplane's float when flying from calm water.
In cold weather the engine develops more than its rated horsepower, so, once you get into the air, the climb rate will be brisk. However, snow conditions will dictate how fast, and even whether you will get into the air. Normally one plans for a takeoff run at least 10 percent longer than on wheels. Wet or very deep powder snow will add to that distance. There may be times you have to make a run or two to pack the snow down so that you can accelerate to liftoff speed. If you do, watch the engine temperatures; even on a very cold day you can overheat the engine, particularly if a winterization kit is installed.
There is a condition known as "overflow" on frozen lakes where water flows over the ice and saturates the snow on top of the ice. This can create a situation in which it is impossible to get the airplane moving fast enough to takeoff. That is one of the reasons you check out an unknown landing area prior to making a full-stop landing. We'll talk about that a little later.
Taking off on glare ice can allow you to meet your minimum daily need for adventure. While launching directly into the wind won't even get your attention unless you slam on full power and discover the rudder won't yet correct for P factor, whistling down a glare ice runway on a crosswind takeoff will elevate heart rate radically. You get to explore the concept of crabbing while on the ground and discovering that the amount of crab needed changes as the speed builds. Seriously consider landing elsewhere under those conditions because you simply may not be able to keep the airplane on the runway.
Can We Land On That Pretty Lake Over There?
So, how do you determine if that lovely, inviting, pristine lake off the wingtip is suitable for a landing? Take your time and look it over. First, what is the wind direction? Is there room to fly a nearly normal traffic pattern? After all, you want all the familiar trappings you can get when doing something new. Is the area for landing and takeoff long enough? You generally want to figure on having at least four or five times the expected takeoff run available for a landing site. What about obstructions on the approach and departure paths? Trees surround most northern lakes. Many have power lines at or near the shore. If the lake is in a long valley there is a good chance some enterprising utility has strung high-tension cables across the valley and hasn't bothered to mark them. Is there an island with a cabin on it? If so, the chances are that there is a power line across the lake to the cabin. Don't land until you either spot the line (look for the poles, lines can be devilishly hard to spot) or can convince a healthy skeptic (that's you) that there isn't one. In general, look the entire area over carefully.
If you've done your homework, you know the general thickness of the ice in the area. Six inches of clear ice is generally considered the bare minimum for a Cub. Anything larger requires eight inches. I generally prefer 10 inches of ice for full stop landings. The ice will be thicker in the center than at the shore. If there is a creek or river flowing into or out of the lake it is very possible that junction will be ice-free or will have very thin ice. Stay well away from those areas.
Try to pick a spot for landing where you have some sort of visual reference; a shoreline is ideal. Stay about 100 yards out from shore to avoid thin shore ice and, if possible, land parallel to the shore, into the wind.
The first landing on an unfamiliar frozen lake will not be a full-stop affair. You need to get more information about the ice and the condition of the snow. Plan on making a modified touch and go, staying on the ground for about three times the normal takeoff run of the airplane holding a speed about 10 knots below liftoff speed, in a tail-low attitude. Fly final, flare and make a gentle landing. Full stall is fine or a wheel (ski?) landing with the tail low is also satisfactory, but be ready for wet sticky snow that wants to slow you rapidly and pitch the nose down abruptly. Bring the power in smoothly to a setting that will let you stay about 10 knots below liftoff speed so you have weight on the skis so you can pop back into the air quickly if you see or feel something you don't like. Track as nearly straight as you can because you are going to make your next landing in those tracks. Yes, you can do it.
During the run take some looks at the spray off the skis to see if it is snow or liquid. Naturally, snow is good, liquid isn't. Near the end of the run smoothly slide in full power and let the airplane fly off. Turn crosswind and then a close-in downwind at about 300 feet agl. Look at the tracks. If they turn black, then water has flowed into them. That makes your decision easy. Don't land. Either the ice is not thick enough or an overflow condition exists or there is something else that is allowing liquid water where you don't want it.
If the tracks stay white, things should be okay for a full-stop landing. Those tracks provide a number of good things for you. They provide contrast with the surrounding snow to help with depth perception; and being packed snow, they help avoid freeze-down when you stop; and during the subsequent takeoff run they allow faster acceleration than in untouched snow.
As you slide to a stop again look at the spray off the skis. If it is snow, it's a good indication all is well; if it is liquid, bring the power up and depart.
Once you stop, wait a bit before you shut down. Feel the airplane. If it is moving more than you expect it to in the wind that exists, you may be starting to break through the ice. Depart. Immediately. Smoothly firewall the throttle and take off along the tracks you made, keeping the tail slightly low.
If all is well, shut down, get out and do what you need to avoid freeze-down.
Once you decide to depart, fire up, do your pretakeoff checks and take off along the tracks you made earlier.
I generally do not land on open fields that I do not know well. Besides the obvious problem of trespassing on private property, the snow may simply be a light covering concealing something waiting patiently to reduce the airplane to flinders.
There is a nasty little phenomenon in winter flying known as "flat light" that particularly affects skiplane operations. It is akin to glassy water operations in seaplanes. It has been reported as the probable cause of a number of accidents in the north. Under the right conditions of snow cover, overcast and visibility, it is absolutely impossible to tell your height over the ground once you get below about 300 feet. The most dangerous part of the problem is that pilots who have not experienced it absolutely cannot believe it could ever happen to them. Even pilots who do not have binocular vision are affected. It is real and it is serious. Many pilots notice it the first time when landing on an airport with a snow-covered runway. They notice that there are snow piles on either side of the runway; however, the piles do not appear to stick up. They simply look flat. It is not until you get down between them do you realize the piles are 10 or 15 feet tall and the world suddenly springs back into three dimensions.
I'm spending some extra time on flat light because it has bitten a lot of very good pilots. One of the most famous accidents involved a B-17 flying across the Greenland ice cap. It literally was flown onto slowly rising snowy terrain, coming to a rather ignominious stop. The ensuing rescue operation headed by famed polar aviator, Bernt Balchen, took months and cost several lives.
By the way, flat light is sometimes erroneously called a white out. A white out is a blizzard condition involving almost complete loss of visibility.
The solution to the flat light problem is to get some sort of contrast in the snow. The shoreline of the lake will usually work. On an open field tall grasses sticking up through the snow will provide the contrast needed to judge your height on an approach. Another solution, used by bush pilots, is to carry a bunch of red shop rags in the baggage compartment. Simply fly over the intended landing site and pitch two or three out. The red rags on the snow will let you figure out when to flare. It is considered very poor form to not pick them up after you land.
Snowmobiles: Just Say No
One of the realities of winter flying operations near human habitation is that many of the lakes are also playgrounds for snowmobiles. They have the positive effect of packing snow so that you usually don't have to worry about bogging down. However, ice that is thick enough for snowmobiles is not necessarily thick enough for airplanes. The downside of snowmobiles is that a frightening percentage of snowmobile operators have been drinking and find skiplanes to be wonderful sources of amusement. Accordingly, it is wise to remain well clear of them if possible. Keep in mind that snowmobiles tend to be blisteringly fast; many will go faster than 70 miles per hour. They also have dynamic braking, something you do not possess on skis. Never, ever start a takeoff with snowmobiles nearby. Never let them try to race you on takeoff. They will out-accelerate you quickly. Once in front they have a tendency, I don't understand why, to swerve to a point directly in front of the airplane and then come off the power so that the dynamic brake takes effect. You simply don't need that much excitement in your life.
Give It A Try
One of the great joys of aeronautical life is that moment after you have closed the throttle just as your skis start to brush the snow on the surface of a frozen lake. There, away from the rest of the world, as the snow starts to fly from the skis, you realize once again, that this is why you fly.
See you next month.