Saturday, April 18, 2015

Float Plane Differences from Wheeled Planes

Are you a wheeled plane pilots who says, “One of these days, I'll get rated on floats?” Or perhaps you are a traveler who watches seaplanes take off and land?  Either way, below is a primer about some differences between planes on floats vs wheels.  (Our Piper has skis, floats, and wheels). 

Sleeping under the summer midnight moon
The most distinctive aspect of a seaplane (or float plane) is obviously the undercarriage. The floats (or pontoons) look like huge, bloated, Ronald McDonald shoes compared to puffy tundra tires and dainty tarmac tires. On our little Piper PA-20, the float assembly weighs 150 pounds more than wheels – the weight of an adult, cutting down on payload and fuel efficiency, and introducing additional drag. Other, less obvious differences are that floats are mounted directly to the fuselage with no suspension system, and that float planes have no brakes. They can rely on the friction of the water to slow down and stop.

Pre-flight checks of the floats underscore the fact that they are designed to function like boats, so many of the terms and design features are similar. In fact, we secure our float plane to an angled dock with a boat winch. In this position, most of each float is elevated above the water line, so we can inspect the keel (the bottom of the float) before sliding the plane down into the water and maneuvering it with a tow rope over to the adjacent boat dock, where we conduct the other pre-flight checks. Internally, the floats' bulkheads are divided into six watertight compartments, which must be “sumped out” with a portable bilge pump to remove any accumulated water (rain from above or seepage from below). Another chore is to check the retractable water rudder at the stern of each float. Some float planes also have a fin added under their tails for extra stability.
The ducks coming out to say "good morning"
to their floating friend

Unlike a wheeled plane that can be stationary when idling, a float plane is always moving once unmoored (because it has no brakes). There is no leisurely period to "warm up the engine with a foot on the brake while briefing passengers."  So as soon as the plane is unmoored, the pilot has to be acutely aware of wind, waves, currents, debris, overhanging trees, distance to shore, and vehicles or people in the surrounding water. (Many an air taxi pilot has told chatty passengers to "shut up.") 

If my husband is flying alone, I push the plane away from the lake shore, toward the center of the lake so he can turn into the wind, usually S or SW. However, if I am accompanying him, departure is a bit trickier on the move, particularly since the Piper has only one front access door, on the starboard side. Virtually all summer at our lake, when the plane is pointed south into the wind, that door is on the side away from the dock. So as soon as I push the plane out into the water, I have to tiptoe (usually in waders) as fast as possible across a tight wire strung between the fronts of the floats, around the propeller, step onto the far float, swing under the strut and then up into the passenger seat before Bryan can start the ignition. (Usually while he is yelling at me to move faster!)

Progress through the water occurs in three stages: idling (or displacement), plowing, and planing (or step position). When taxiing in water, a seaplane is regarded as a water vehicle, so it abides by Coast Guard rules for boats. At less than 6 or 7 knots per hour, the pilot keeps the elevators all the way back to protect the propeller from water spray, which can pit the blades like gravel. At higher speed, the center of buoyancy naturally shifts back, pushing more of the float into the water, increasing drag. This is called the plowing position. The plane is least stable at this point: it is vulnerable to weathervaning or capsizing as well as overheating (since water drag increases exponentially with speed), so this position is used briefly, during the run up before take off.

Take off:
To rise out of the water, the plane's attitude is nearly level, on the step part of the keel (so less drag), and the water rudders are up. With increasing speed, the weight of the plane becomes supported first by hydrodynamic lift (like water skiing) and then by aerodynamic lift. This stage of momentum is usually referred to as “being on the step.” During this time, an incorrect planing attitude can set off a series of oscillations evocatively called “porpoising.” Dangerously escalating amplitude can cause premature lift-off and a stall or a scary forward flip.
Taking off into the wind

The drag effect of the water means that a float plane's take off distance is MUCH longer than its landing distance. Many a new pilot has safely landed on a little lake that he cannot then leave! Glassy (flat) water is the worst, as it seems to suck at the floats. To reduce drag, some pilots roll the plane slightly, to lift one float off the water before the other. For wheeled pilots trained to maintain a level attitude on take-off, this can be a disconcerting maneuver to learn – just enough to raise a float but not enough for the opposite wing tip to touch water.

Once airborne, the weight and size of the floats reduce acceleration, speed, and fuel efficiency, compared to a wheeled plane of the same make. Despite these constraints, float planes departing from lakes have a big advantage over wheeled planes on a runway. The pilot can virtually always turn into the wind – whatever its direction. So float plane pilots can usually avoid cross wind take offs that bedevil wheeled plane pilots more often.

Listening for other traffic in the area
Floats decrease directional stability a bit, because they have more vertical surface in front of the airplane's center of gravity than behind it. This can exaggerate a plane's yaw. For this reason, many seaplanes have horizontal stabilizers on the floats and a fin added to the bottom of the tail. In our short coupled plane, without a fin, Bryan is constantly dancing on the rudder pedals to keep the nose straight.

For emergency landings, float planes have a higher rate of descent and a diminished power-off glide range than a comparable wheeled plane. On the other hand, they have more options. Although obviously designed for water, they can land on grass, dirt, snow, or even a smooth runway, in a dire situation, sometimes without damage to the float keels (or plane).

For intentional landings, prudent pilots fly low over the water first. One reason is to search for debris in remote lakes and to look for fishermen, watercraft, and swimmers in occupied ones. It is highly likely that motor boats and water skiers will not hear a plane overhead, so a low approach alerts them to your imminent landing. A second reason is to scrutinize wave and current action, which can differ from the prevailing wind and thus present some tricky landing variables. The orientation of water fowl offers an important clue. Glassy, reflective water is dangerous as it is very difficult for a pilot to gauge altitude (and flat water presages a hard landing). Water level reference points like a dock, buoy, or water lilies can be very helpful during descent.
Landing and stopping quickly

For touch down, the goal is to reduce speed as low as possible and “land on the step” - the same position as for take -off - in order to present the least amount of float surface to the water at the lowest speed. This reduces the drag effect, so energy can dissipate gently. With too much float surface at too much speed, the pilot and passengers are in for a quite a jolt, since the plane lacks a suspension system. Once the floats settle into the water (into plowing position), the pilot raises the nose to elevate the propeller above water spray and taxis in.

Docked behind the Fireweed and other wild flowers
Some places we fly for a $100 hamburger have docks but most of our destinations do not. So I wear waders on almost every flight and prepare to jump out before we stop! (While wheeled plane rules require seat belts the whole time the plane is running, seaplanes rules allow for freer movement when on the water.) As we approach the shore, I open the passenger door and dangle my feet out over the float. As soon as Bryan turns off the propeller, I slip down, grab the tow line, and either jump to the dock, if there is one, or plop into what I hope is shallow water, to haul the plane into the shallows, looking like Humphrey Bogart and his African Queen. Usually we tie up to the ubiquitous willows that line lakes in South Central Alaska. (In one location, a pair of swans, which can be mean and dangerous, despite their loveliness, hovered near the cove we needed. Did they have a nest in the grasses nearby? Only their fear of the engine noise enabled us to land and depart.

When we fly home, Bryan aims for the angled dock he built and revs the engine to drive us up onto the plastic slip strips. We tie the mooring lines to cleats and winch a wire line to the plane's spreader bar. If anyone else lived here, we'd call out, “Honey, we're home!”


Want to learn more about float plane flying?  Certification on floats requires 10 hours of instructor accompanied plane practice.

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