A landplane accelerates down a smooth runway. A floatplane has to fight its way out of the water first, and water does not want to let go. The roar, the spray, the moment the nose drops and the aircraft suddenly skims like a speedboat: that is a seaplane getting "on the step", and it is the whole secret of flying off water.

This week Trans Maldivian Airways, which calls itself the world's largest seaplane operator, announced its 68th aircraft, every one of them a de Havilland Canada DHC-6 Twin Otter on floats. Each of its flights begins with the same four-phase dance across a lagoon. Here is how it works.

Floats, hulls and the step

Seaplanes come in two families. Flying boats float on their own hull. Floatplanes are landplanes fitted with separate floats, sometimes called pontoons, and amphibians add retractable wheels so they can use runways too. The FAA requires floats to have 80 percent more buoyancy than the aircraft's maximum weight demands, divided into at least four watertight compartments, so the aircraft stays afloat with any two of them flooded.

The crucial feature is easy to miss. Partway along each float, slightly behind the aircraft's centre of gravity, the bottom has a sharp break called the step. At speed, water breaks away from the float at the step instead of clinging all the way to the back, and that is what makes planing possible.

Four phases to flight

According to the FAA's seaplane handbook, a water takeoff has four phases. In the displacement phase the floats sit deep in the water, held up by buoyancy, and the aircraft moves slowly. As power comes in, the nose rises and the back of the floats digs in: this is plowing, and water drag climbs to a peak known as the hump, named after the shape of the drag curve.

With full power and back pressure on the elevator, the floats climb over the hump. Then the pilot relaxes the back pressure and the floats settle onto the step: most of the weight is now carried by hydrodynamic lift, like a water ski, and the rear of the floats is clear of the water. Drag falls away, the aircraft accelerates, and in the final phase the wings take over and it lifts off. There is a single attitude with the least drag, which pilots call the sweet spot.

AOPA's tips and techniques for learning to fly a seaplane.

Porpoising, glassy water and rough water

Getting it wrong on the step has a name: porpoising, a rhythmic pitching oscillation that, the FAA warns, grows with each cycle and can drive the bows of the floats under water. If it has not stopped by the second oscillation, the handbook says to cut the power and hold the elevator back, and never to chase the oscillations.

Glassy water is a trap of a different kind. A perfectly smooth surface clings to the floats, so a common technique is to lift one float out with aileron and accelerate on the other. Landing on glassy water is worse, because a pilot cannot judge height over a mirror; the FAA technique is to set up the landing attitude from about 200 feet and descend at no more than 150 feet per minute, flying onto the water without flaring. Rough water calls for the opposite: getting the weight onto the wings as soon as possible.

Amphibians add one more risk. A landing on water with the wheels down, the FAA says, is almost certain to capsize the aircraft, which is why many pilots say aloud before every water landing that the wheels are up.

Where seaplanes still rule

Floats cost weight and drag, so a floatplane carries less, climbs slower and cruises slower than the same aircraft on wheels. Where there are no runways, none of that matters. In the Maldives, Trans Maldivian Airways flies up to about 400 flights a day from Velana International Airport in Malé to more than 80 resorts across 16 atolls. In Canada, Harbour Air runs scheduled seaplane services from Vancouver Harbour, and in December 2019 its founder flew an all-electric DHC-2 Beaver, billed as the first flight of an all-electric commercial aircraft.

Sam Chui flies with the world's largest seaplane operation in the Maldives.

Harbour Air's footage of its all-electric seaplane test flight in December 2019.

Sources: FAA Seaplane, Skiplane, and Float/Ski Equipped Helicopter Operations Handbook (FAA-H-8083-23), AOPA, Trans Maldivian Airways, Harbour Air, Smithsonian Air & Space, AFP, Bloomberg, Wikipedia